How to return multiple objects from a Java method

Question

I want to return two objects from a Java method and was wondering what could be a good way of doing so   The possible ways I can think of are  return a HashMap  since the two Objects are related  or return an ArrayList of Object objects   To be more precise  the two objects I want to return are  a  List of objects and  b  comma separated names of the same   I want to return these two Objects from one method because I dont want to iterate through the list of objects to get the comma separated names  which I can do in the same loop in this method    Somehow  returning a HashMap does not look a very elegant way of doing so

User · Answer

I almost always end up defining n-Tuple classes when I code in Java. For instance:

public class Tuple2<T1,T2> {
  private T1 f1;
  private T2 f2;
  public Tuple2(T1 f1, T2 f2) {
    this.f1 = f1; this.f2 = f2;
  }
  public T1 getF1() {return f1;}
  public T2 getF2() {return f2;}
}

I know it's a bit ugly, but it works, and you just have to define your tuple types once. Tuples are something Java really lacks.

EDIT: David Hanak's example is more elegant, as it avoids defining getters and still keeps the object immutable.

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Regarding the issue about multiple return values in general I usually use a small helper class that wraps a single return value and is passed as parameter to the method   public class ReturnParameter lt T gt        private T value       public ReturnParameter     this value   null        public ReturnParameter T initialValue    this value   initialValue         public void set T value    this value   value        public T get     return this value         for primitive datatypes I use minor variations to directly store the value   A method that wants to return multiple values would then be declared as follows   public void methodThatReturnsTwoValues ReturnParameter lt ClassA gt  nameForFirstValueToReturn  ReturnParameter lt ClassB gt  nameForSecondValueToReturn                  nameForFirstValueToReturn set             nameForSecondValueToReturn set                       Maybe the major drawback is that the caller has to prepare the return objects in advance in case he wants to use them  and the method should check for null pointers   ReturnParameter lt ClassA gt  nameForFirstValue   new ReturnParameter lt ClassA gt     ReturnParameter lt ClassB gt  nameForSecondValue   new ReturnParameter lt ClassB gt     methodThatReturnsTwoValues nameForFirstValue  nameForSecondValue     Advantages  in comparison to other solutions proposed     You do not have to create a special class declaration for individual methods and its return types The parameters get a name and therefore are easier to differentiate when looking at the method signature Type safety for each parameter

User · Answer

Alternatively  in situations where I want to return a number of things from a method I will sometimes use a callback mechanism instead of a container  This works very well in situations where I cannot specify ahead of time just how many objects will be generated    With your particular problem  it would look something like this   public class ResultsConsumer implements ResultsGenerator ResultsCallback       public void handleResult  String name  Object value                               public class ResultsGenerator       public interface ResultsCallback               void handleResult  String aName  Object aValue               public void generateResults  ResultsGenerator ResultsCallback aCallback                 Object value   null          String name   null                        aCallback handleResult  name  value

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Before Java 5  I would kind of agree that the Map solution isn t ideal  It wouldn t give you compile time type checking so can cause issues at runtime  However  with Java 5  we have Generic Types   So your method could look like this   public Map lt String  MyType gt  doStuff      MyType of course being the type of object you are returning   Basically I think that returning a Map is the right solution in this case because that s exactly what you want to return - a mapping of a string to an object

User · Answer

As I see it there are really three choices here and the solution depends on the context   You can choose to implement the construction of the name in the method that produces the list   This is the choice you ve chosen  but I don t think it is the best one   You are creating a coupling in the producer method to the consuming method that doesn t need to exist   Other callers may not need the extra information and you would be calculating extra information for these callers   Alternatively  you could have the calling method calculate the name   If there is only one caller that needs this information  you can stop there   You have no extra dependencies and while there is a little extra calculation involved  you ve avoided making your construction method too specific   This is a good trade-off   Lastly  you could have the list itself be responsible for creating the name   This is the route I would go if the calculation needs to be done by more than one caller   I think this puts the responsibility for the creation of the names with the class that is most closely related to the objects themselves   In the latter case  my solution would be to create a specialized List class that returns a comma-separated string of the names of objects that it contains  Make the class smart enough that it constructs the name string on the fly as objects are added and removed from it   Then return an instance of this list and call the name generation method as needed   Although it may be almost as efficient  and simpler  to simply delay calculation of the names until the first time the method is called and store it then  lazy loading    If you add remove an object  you need only remove the calculated value and have it get recalculated on the next call

User · Answer

I almost always end up defining n-Tuple classes when I code in Java. For instance:

public class Tuple2<T1,T2> {
  private T1 f1;
  private T2 f2;
  public Tuple2(T1 f1, T2 f2) {
    this.f1 = f1; this.f2 = f2;
  }
  public T1 getF1() {return f1;}
  public T2 getF2() {return f2;}
}

I know it's a bit ugly, but it works, and you just have to define your tuple types once. Tuples are something Java really lacks.

EDIT: David Hanak's example is more elegant, as it avoids defining getters and still keeps the object immutable.

User · Answer

Why not create a WhateverFunctionResult object that contains your results  and the logic required to parse these results  iterate over then etc  It seems to me that either    These results objects are intimately tied together related and belong together  or  they are unrelated  in which case your function isn t well defined in terms of what it s trying to do  i e  doing two different things    I see this sort of issue crop up again and again  Don t be afraid to create your own container result classes that contain the data and the associated functionality to handle this  If you simply pass the stuff around in a HashMap or similar  then your clients have to pull this map apart and grok the contents each time they want to use the results

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In C    STL  there is a pair class for bundling two objects  In Java Generics a pair class isn t available  although there is some demand for it  You could easily implement it yourself though   I agree however with some other answers that if you need to return two or more objects from a method  it would be better to encapsulate them in a class

User · Answer

In C  you would do it by passing pointers to placeholders for the results as arguments   void getShoeAndWaistSizes int  shoeSize  int  waistSize         shoeSize   36       waistSize   45        int shoeSize  waistSize  getShoeAndWaistSize  amp shoeSize   amp waistSize   int i   shoeSize   waistSize    Let s try something similar  in Java   void getShoeAndWaistSizes List lt Integer gt  shoeSize  List lt Integer gt  waistSize        shoeSize add 36       waistSize add 45         List lt Integer gt  shoeSize   new List lt  gt     List lt Integer gt  waistSize   new List lt  gt     getShoeAndWaistSizes shoeSize  waistSize   int i   shoeSize get 0    waistSize get 0

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If you know you are going to return two objects  you can also use a generic pair   public class Pair lt A B gt        public final A a      public final B b       public Pair A a  B b            this a   a          this b   b             Edit A more fully formed implementation of the above    package util   public class Pair lt A B gt         public static  lt P  Q gt  Pair lt P  Q gt  makePair P p  Q q            return new Pair lt P  Q gt  p  q              public final A a      public final B b       public Pair A a  B b            this a   a          this b   b              Override     public int hashCode             final int prime   31          int result   1          result   prime   result     a    null    0   a hashCode             result   prime   result     b    null    0   b hashCode             return result              Override     public boolean equals Object obj            if  this    obj                return true                    if  obj    null                return false                    if  getClass      obj getClass                  return false                     SuppressWarnings  rawtypes           Pair other    Pair  obj          if  a    null                if  other a    null                    return false                          else if   a equals other a                 return false                    if  b    null                if  other b    null                    return false                          else if   b equals other b                 return false                    return true             public boolean isInstance Class lt   gt  classA  Class lt   gt  classB            return classA isInstance a   amp  amp  classB isInstance b               SuppressWarnings  unchecked       public static  lt P  Q gt  Pair lt P  Q gt  cast Pair lt      gt  pair  Class lt P gt  pClass  Class lt Q gt  qClass             if  pair isInstance pClass  qClass                 return  Pair lt P  Q gt   pair                     throw new ClassCastException                Notes  mainly around rustiness with Java  amp  generics    both a and b are immutable   makePair static method helps you with boiler plate typing  which the diamond operator in Java 7 will make less annoying  There s some work to make this really nice re  generics  but it should be ok-ish now   c f  PECS  hashcode and equals are generated by eclipse  the compile time casting in the cast method is ok  but doesn t seem quite right  I m not sure if the wildcards in isInstance are necessary  I ve just written this in response to comments  for illustration purposes only

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public class MultipleReturnValues        public MultipleReturnValues                public static void functionWithSeveralReturnValues final String   returnValues            returnValues 0     return value 1           returnValues 1     return value 2              public static void main String   args            String   returnValues   new String 2           functionWithSeveralReturnValues returnValues           System out println  returnValues 0        returnValues 0            System out println  returnValues 1        returnValues 1

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public class MultipleReturnValues        public MultipleReturnValues                public static void functionWithSeveralReturnValues final String   returnValues            returnValues 0     return value 1           returnValues 1     return value 2              public static void main String   args            String   returnValues   new String 2           functionWithSeveralReturnValues returnValues           System out println  returnValues 0        returnValues 0            System out println  returnValues 1        returnValues 1

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You can utilize a HashMap lt String  Object gt  as follows public HashMap lt String  Object gt  yourMethod              different logic here     HashMap lt String  Object gt  returnHashMap   new HashMap lt String  Object gt       returnHashMap put  quot objectA quot   objectAValue     returnHashMap put  quot myString quot   myStringValue     returnHashMap put  quot myBoolean quot   myBooleanValue      return returnHashMap     Then when calling the method in a different scope  you can cast each object back to its initial type     call the method HashMap lt String  Object gt  resultMap   yourMethod                        fetch the results and cast them ObjectA objectA    ObjectA  resultMap get  quot objectA quot    String myString    String  resultMap get  quot myString quot    Boolean myBoolean    Boolean  resultMap get  quot myBoolean quot

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As I see it there are really three choices here and the solution depends on the context   You can choose to implement the construction of the name in the method that produces the list   This is the choice you ve chosen  but I don t think it is the best one   You are creating a coupling in the producer method to the consuming method that doesn t need to exist   Other callers may not need the extra information and you would be calculating extra information for these callers   Alternatively  you could have the calling method calculate the name   If there is only one caller that needs this information  you can stop there   You have no extra dependencies and while there is a little extra calculation involved  you ve avoided making your construction method too specific   This is a good trade-off   Lastly  you could have the list itself be responsible for creating the name   This is the route I would go if the calculation needs to be done by more than one caller   I think this puts the responsibility for the creation of the names with the class that is most closely related to the objects themselves   In the latter case  my solution would be to create a specialized List class that returns a comma-separated string of the names of objects that it contains  Make the class smart enough that it constructs the name string on the fly as objects are added and removed from it   Then return an instance of this list and call the name generation method as needed   Although it may be almost as efficient  and simpler  to simply delay calculation of the names until the first time the method is called and store it then  lazy loading    If you add remove an object  you need only remove the calculated value and have it get recalculated on the next call

User · Answer

Alternatively  in situations where I want to return a number of things from a method I will sometimes use a callback mechanism instead of a container  This works very well in situations where I cannot specify ahead of time just how many objects will be generated    With your particular problem  it would look something like this   public class ResultsConsumer implements ResultsGenerator ResultsCallback       public void handleResult  String name  Object value                               public class ResultsGenerator       public interface ResultsCallback               void handleResult  String aName  Object aValue               public void generateResults  ResultsGenerator ResultsCallback aCallback                 Object value   null          String name   null                        aCallback handleResult  name  value

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If you know you are going to return two objects  you can also use a generic pair   public class Pair lt A B gt        public final A a      public final B b       public Pair A a  B b            this a   a          this b   b             Edit A more fully formed implementation of the above    package util   public class Pair lt A B gt         public static  lt P  Q gt  Pair lt P  Q gt  makePair P p  Q q            return new Pair lt P  Q gt  p  q              public final A a      public final B b       public Pair A a  B b            this a   a          this b   b              Override     public int hashCode             final int prime   31          int result   1          result   prime   result     a    null    0   a hashCode             result   prime   result     b    null    0   b hashCode             return result              Override     public boolean equals Object obj            if  this    obj                return true                    if  obj    null                return false                    if  getClass      obj getClass                  return false                     SuppressWarnings  rawtypes           Pair other    Pair  obj          if  a    null                if  other a    null                    return false                          else if   a equals other a                 return false                    if  b    null                if  other b    null                    return false                          else if   b equals other b                 return false                    return true             public boolean isInstance Class lt   gt  classA  Class lt   gt  classB            return classA isInstance a   amp  amp  classB isInstance b               SuppressWarnings  unchecked       public static  lt P  Q gt  Pair lt P  Q gt  cast Pair lt      gt  pair  Class lt P gt  pClass  Class lt Q gt  qClass             if  pair isInstance pClass  qClass                 return  Pair lt P  Q gt   pair                     throw new ClassCastException                Notes  mainly around rustiness with Java  amp  generics    both a and b are immutable   makePair static method helps you with boiler plate typing  which the diamond operator in Java 7 will make less annoying  There s some work to make this really nice re  generics  but it should be ok-ish now   c f  PECS  hashcode and equals are generated by eclipse  the compile time casting in the cast method is ok  but doesn t seem quite right  I m not sure if the wildcards in isInstance are necessary  I ve just written this in response to comments  for illustration purposes only

User · Answer

Before Java 5  I would kind of agree that the Map solution isn t ideal  It wouldn t give you compile time type checking so can cause issues at runtime  However  with Java 5  we have Generic Types   So your method could look like this   public Map lt String  MyType gt  doStuff      MyType of course being the type of object you are returning   Basically I think that returning a Map is the right solution in this case because that s exactly what you want to return - a mapping of a string to an object

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Use of following Entry object Example    public Entry lt A B gt  methodname arg             return new AbstractMap simpleEntry lt A B gt  instanceOfA instanceOfB

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While in your case  the comment may be a good way to go  in Android  you can use Pair   Simply  return new Pair lt  gt  yourList  yourCommaSeparatedValues

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As I see it there are really three choices here and the solution depends on the context   You can choose to implement the construction of the name in the method that produces the list   This is the choice you ve chosen  but I don t think it is the best one   You are creating a coupling in the producer method to the consuming method that doesn t need to exist   Other callers may not need the extra information and you would be calculating extra information for these callers   Alternatively  you could have the calling method calculate the name   If there is only one caller that needs this information  you can stop there   You have no extra dependencies and while there is a little extra calculation involved  you ve avoided making your construction method too specific   This is a good trade-off   Lastly  you could have the list itself be responsible for creating the name   This is the route I would go if the calculation needs to be done by more than one caller   I think this puts the responsibility for the creation of the names with the class that is most closely related to the objects themselves   In the latter case  my solution would be to create a specialized List class that returns a comma-separated string of the names of objects that it contains  Make the class smart enough that it constructs the name string on the fly as objects are added and removed from it   Then return an instance of this list and call the name generation method as needed   Although it may be almost as efficient  and simpler  to simply delay calculation of the names until the first time the method is called and store it then  lazy loading    If you add remove an object  you need only remove the calculated value and have it get recalculated on the next call

User · Answer

Before Java 5  I would kind of agree that the Map solution isn t ideal  It wouldn t give you compile time type checking so can cause issues at runtime  However  with Java 5  we have Generic Types   So your method could look like this   public Map lt String  MyType gt  doStuff      MyType of course being the type of object you are returning   Basically I think that returning a Map is the right solution in this case because that s exactly what you want to return - a mapping of a string to an object

User · Answer

If you want to return two objects you usually want to return a single object that encapsulates the two objects instead   You could return a List of NamedObject objects like this   public class NamedObject lt T gt      public final String name    public final T object     public NamedObject String name  T object        this name   name      this object   object          Then you can easily return a List lt NamedObject lt WhateverTypeYouWant gt  gt    Also  Why would you want to return a comma-separated list of names instead of a List lt String gt   Or better yet  return a Map lt String TheObjectType gt  with the keys being the names and the values the objects  unless your objects have specified order  in which case a NavigableMap might be what you want

User · Answer

Alternatively  in situations where I want to return a number of things from a method I will sometimes use a callback mechanism instead of a container  This works very well in situations where I cannot specify ahead of time just how many objects will be generated    With your particular problem  it would look something like this   public class ResultsConsumer implements ResultsGenerator ResultsCallback       public void handleResult  String name  Object value                               public class ResultsGenerator       public interface ResultsCallback               void handleResult  String aName  Object aValue               public void generateResults  ResultsGenerator ResultsCallback aCallback                 Object value   null          String name   null                        aCallback handleResult  name  value

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In C    STL  there is a pair class for bundling two objects  In Java Generics a pair class isn t available  although there is some demand for it  You could easily implement it yourself though   I agree however with some other answers that if you need to return two or more objects from a method  it would be better to encapsulate them in a class

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You can utilize a HashMap lt String  Object gt  as follows public HashMap lt String  Object gt  yourMethod              different logic here     HashMap lt String  Object gt  returnHashMap   new HashMap lt String  Object gt       returnHashMap put  quot objectA quot   objectAValue     returnHashMap put  quot myString quot   myStringValue     returnHashMap put  quot myBoolean quot   myBooleanValue      return returnHashMap     Then when calling the method in a different scope  you can cast each object back to its initial type     call the method HashMap lt String  Object gt  resultMap   yourMethod                        fetch the results and cast them ObjectA objectA    ObjectA  resultMap get  quot objectA quot    String myString    String  resultMap get  quot myString quot    Boolean myBoolean    Boolean  resultMap get  quot myBoolean quot

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Use of following Entry object Example    public Entry lt A B gt  methodname arg             return new AbstractMap simpleEntry lt A B gt  instanceOfA instanceOfB

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All possible solutions will be a kludge  like container objects  your HashMap idea     multiple return values    as realized via arrays   I recommend regenerating the comma-separated list from the returned List  The code will end up being a lot cleaner

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Keep it simple and create a class for multiple result situation  This example accepts an ArrayList and a message text from a databasehelper getInfo   Where you call the routine that returns multiple values you code   multResult res   mydb getInfo       In the routine getInfo you code   ArrayList lt String gt  list  new ArrayList lt String gt     add values to the list    return new multResult  the message   list     and define a class multResult with   public class multResult       public String message     or create a getter if you don t like public     public ArrayList lt String gt  list      multResult String m  ArrayList lt String gt  l           message   m          list  l

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Can do some thing like a tuple in dynamic language  Python   public class Tuple   private Object   multiReturns   private Tuple Object    multiReturns        this multiReturns   multiReturns     public static Tuple  t Object    multiReturns       return new Tuple multiReturns      public  lt T gt  T at int index  Class lt T gt  someClass        return someClass cast multiReturns index          and use like this  public Tuple returnMultiValues       return Tuple  t new ArrayList   new HashMap        Tuple t   returnMultiValues    ArrayList list   t at 0 ArrayList class

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All possible solutions will be a kludge  like container objects  your HashMap idea     multiple return values    as realized via arrays   I recommend regenerating the comma-separated list from the returned List  The code will end up being a lot cleaner

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Pass a list to your method and populate it  then return the String with the names  like this   public String buildList List lt   gt  list        list add 1       list add 2       list add 3       return  something something something dark side       Then call it like this   List lt   gt  values   new ArrayList lt   gt     String names   buildList values

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Keep it simple and create a class for multiple result situation  This example accepts an ArrayList and a message text from a databasehelper getInfo   Where you call the routine that returns multiple values you code   multResult res   mydb getInfo       In the routine getInfo you code   ArrayList lt String gt  list  new ArrayList lt String gt     add values to the list    return new multResult  the message   list     and define a class multResult with   public class multResult       public String message     or create a getter if you don t like public     public ArrayList lt String gt  list      multResult String m  ArrayList lt String gt  l           message   m          list  l

User · Answer

If you want to return two objects you usually want to return a single object that encapsulates the two objects instead   You could return a List of NamedObject objects like this   public class NamedObject lt T gt      public final String name    public final T object     public NamedObject String name  T object        this name   name      this object   object          Then you can easily return a List lt NamedObject lt WhateverTypeYouWant gt  gt    Also  Why would you want to return a comma-separated list of names instead of a List lt String gt   Or better yet  return a Map lt String TheObjectType gt  with the keys being the names and the values the objects  unless your objects have specified order  in which case a NavigableMap might be what you want

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Pass a list to your method and populate it  then return the String with the names  like this   public String buildList List lt   gt  list        list add 1       list add 2       list add 3       return  something something something dark side       Then call it like this   List lt   gt  values   new ArrayList lt   gt     String names   buildList values

User · Answer

Alternatively  in situations where I want to return a number of things from a method I will sometimes use a callback mechanism instead of a container  This works very well in situations where I cannot specify ahead of time just how many objects will be generated    With your particular problem  it would look something like this   public class ResultsConsumer implements ResultsGenerator ResultsCallback       public void handleResult  String name  Object value                               public class ResultsGenerator       public interface ResultsCallback               void handleResult  String aName  Object aValue               public void generateResults  ResultsGenerator ResultsCallback aCallback                 Object value   null          String name   null                        aCallback handleResult  name  value

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In the event the method you re calling is private  or called from one location  try  return new Object   value1  value2     The caller looks like   Object   temp myMethod parameters   Type1 value1  Type1 temp 0      For code clarity  temp 0  is not descriptive Type2 value2  Type2 temp 1     The Pair example by David Hanak has no syntactic benefit  and is limited to two values     return new Pair lt Type1 Type2 gt  value1  value2     And the caller looks like   Pair lt Type1  Type2 gt  temp myMethod parameters   Type1 value1 temp a     For code clarity  temp a is not descriptive Type2 value2 temp b

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If you know you are going to return two objects  you can also use a generic pair   public class Pair lt A B gt        public final A a      public final B b       public Pair A a  B b            this a   a          this b   b             Edit A more fully formed implementation of the above    package util   public class Pair lt A B gt         public static  lt P  Q gt  Pair lt P  Q gt  makePair P p  Q q            return new Pair lt P  Q gt  p  q              public final A a      public final B b       public Pair A a  B b            this a   a          this b   b              Override     public int hashCode             final int prime   31          int result   1          result   prime   result     a    null    0   a hashCode             result   prime   result     b    null    0   b hashCode             return result              Override     public boolean equals Object obj            if  this    obj                return true                    if  obj    null                return false                    if  getClass      obj getClass                  return false                     SuppressWarnings  rawtypes           Pair other    Pair  obj          if  a    null                if  other a    null                    return false                          else if   a equals other a                 return false                    if  b    null                if  other b    null                    return false                          else if   b equals other b                 return false                    return true             public boolean isInstance Class lt   gt  classA  Class lt   gt  classB            return classA isInstance a   amp  amp  classB isInstance b               SuppressWarnings  unchecked       public static  lt P  Q gt  Pair lt P  Q gt  cast Pair lt      gt  pair  Class lt P gt  pClass  Class lt Q gt  qClass             if  pair isInstance pClass  qClass                 return  Pair lt P  Q gt   pair                     throw new ClassCastException                Notes  mainly around rustiness with Java  amp  generics    both a and b are immutable   makePair static method helps you with boiler plate typing  which the diamond operator in Java 7 will make less annoying  There s some work to make this really nice re  generics  but it should be ok-ish now   c f  PECS  hashcode and equals are generated by eclipse  the compile time casting in the cast method is ok  but doesn t seem quite right  I m not sure if the wildcards in isInstance are necessary  I ve just written this in response to comments  for illustration purposes only

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I followed a similar approach than the described in the other answers with a few tweaks based on the requirement I had  basically I created the following classes Just in case  everything is Java    public class Pair lt L  R gt        final L left      final R right       public Pair L left  R right            this left   left          this right   right             public  lt T gt  T get Class lt T gt  param            return  T   param    this left getClass     this left   this right              public static  lt L  R gt  Pair lt L  R gt  of L left  R right            return new Pair lt L  R gt  left  right             Then  my requirement was simple  in the repository Class that reaches the DB  for the Get Methods than retrieve data from the DB  I need to check if it failed or succeed  then  if succeed  I needed to play with the returning list  if failed  stop the execution and notify the error   So  for example  my methods are like this   public Pair lt ResultMessage  List lt Customer gt  gt  getCustomers         List lt Customer gt  list   new ArrayList lt Customer gt         try                Do some work to get the list of Customers from the DB                catch  SQLException e            return Pair of                         new ResultMessage e getErrorCode    e getMessage        Left                         null       Right           return Pair of                     new ResultMessage 0   SUCCESS       Left                     list      Right     Where ResultMessage is just a class with two fields  code message  and Customer is any class with a bunch of fields that comes from the DB   Then  to check the result I just do this   void doSomething        Pair lt ResultMessage  List lt Customer gt  gt  customerResult    repository getCustomers        if  customerResult get ResultMessage class  getCode      0            List lt Customer gt  listOfCustomers   customerResult get List class           System out println  do SOMETHING with the list             else           System out println  Raised Error    do nothing

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As I see it there are really three choices here and the solution depends on the context   You can choose to implement the construction of the name in the method that produces the list   This is the choice you ve chosen  but I don t think it is the best one   You are creating a coupling in the producer method to the consuming method that doesn t need to exist   Other callers may not need the extra information and you would be calculating extra information for these callers   Alternatively  you could have the calling method calculate the name   If there is only one caller that needs this information  you can stop there   You have no extra dependencies and while there is a little extra calculation involved  you ve avoided making your construction method too specific   This is a good trade-off   Lastly  you could have the list itself be responsible for creating the name   This is the route I would go if the calculation needs to be done by more than one caller   I think this puts the responsibility for the creation of the names with the class that is most closely related to the objects themselves   In the latter case  my solution would be to create a specialized List class that returns a comma-separated string of the names of objects that it contains  Make the class smart enough that it constructs the name string on the fly as objects are added and removed from it   Then return an instance of this list and call the name generation method as needed   Although it may be almost as efficient  and simpler  to simply delay calculation of the names until the first time the method is called and store it then  lazy loading    If you add remove an object  you need only remove the calculated value and have it get recalculated on the next call

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This is not exactly answering the question  but since every of the solution given here has some drawbacks  I suggest to try to refactor your code a little bit so you need to return only one value   Case one   You need something inside as well as outside of your method  Why not calculate it outside and pass it to the method   Instead of    thingA  thingB    createThings           just a conceptual syntax of method returning two values  not valid in Java   Try   thingA   createThingA       thingB   createThingB thingA          This should cover most of your needs  since in most situations one value is created before the other and you can split creating them in two methods  The drawback is that method createThingsB has an extra parameter comparing to createThings  and possibly you are passing exactly the same list of parameters twice to different methods     Case two   Most obvious solution ever and a simplified version of case one  It s not always possible  but maybe both of the values can be created independently of each other   Instead of    thingA  thingB    createThings           see above   Try   thingA   createThingA       thingB   createThingB         To make it more useful  these two methods can share some common logic   public ThingA createThingA            doCommonThings       common logic        create thing A   public ThingB createThingB            doCommonThings       common logic        create thing B

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In C  you would do it by passing pointers to placeholders for the results as arguments   void getShoeAndWaistSizes int  shoeSize  int  waistSize         shoeSize   36       waistSize   45        int shoeSize  waistSize  getShoeAndWaistSize  amp shoeSize   amp waistSize   int i   shoeSize   waistSize    Let s try something similar  in Java   void getShoeAndWaistSizes List lt Integer gt  shoeSize  List lt Integer gt  waistSize        shoeSize add 36       waistSize add 45         List lt Integer gt  shoeSize   new List lt  gt     List lt Integer gt  waistSize   new List lt  gt     getShoeAndWaistSizes shoeSize  waistSize   int i   shoeSize get 0    waistSize get 0

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Regarding the issue about multiple return values in general I usually use a small helper class that wraps a single return value and is passed as parameter to the method   public class ReturnParameter lt T gt        private T value       public ReturnParameter     this value   null        public ReturnParameter T initialValue    this value   initialValue         public void set T value    this value   value        public T get     return this value         for primitive datatypes I use minor variations to directly store the value   A method that wants to return multiple values would then be declared as follows   public void methodThatReturnsTwoValues ReturnParameter lt ClassA gt  nameForFirstValueToReturn  ReturnParameter lt ClassB gt  nameForSecondValueToReturn                  nameForFirstValueToReturn set             nameForSecondValueToReturn set                       Maybe the major drawback is that the caller has to prepare the return objects in advance in case he wants to use them  and the method should check for null pointers   ReturnParameter lt ClassA gt  nameForFirstValue   new ReturnParameter lt ClassA gt     ReturnParameter lt ClassB gt  nameForSecondValue   new ReturnParameter lt ClassB gt     methodThatReturnsTwoValues nameForFirstValue  nameForSecondValue     Advantages  in comparison to other solutions proposed     You do not have to create a special class declaration for individual methods and its return types The parameters get a name and therefore are easier to differentiate when looking at the method signature Type safety for each parameter

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This is not exactly answering the question  but since every of the solution given here has some drawbacks  I suggest to try to refactor your code a little bit so you need to return only one value   Case one   You need something inside as well as outside of your method  Why not calculate it outside and pass it to the method   Instead of    thingA  thingB    createThings           just a conceptual syntax of method returning two values  not valid in Java   Try   thingA   createThingA       thingB   createThingB thingA          This should cover most of your needs  since in most situations one value is created before the other and you can split creating them in two methods  The drawback is that method createThingsB has an extra parameter comparing to createThings  and possibly you are passing exactly the same list of parameters twice to different methods     Case two   Most obvious solution ever and a simplified version of case one  It s not always possible  but maybe both of the values can be created independently of each other   Instead of    thingA  thingB    createThings           see above   Try   thingA   createThingA       thingB   createThingB         To make it more useful  these two methods can share some common logic   public ThingA createThingA            doCommonThings       common logic        create thing A   public ThingB createThingB            doCommonThings       common logic        create thing B

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If you know you are going to return two objects  you can also use a generic pair   public class Pair lt A B gt        public final A a      public final B b       public Pair A a  B b            this a   a          this b   b             Edit A more fully formed implementation of the above    package util   public class Pair lt A B gt         public static  lt P  Q gt  Pair lt P  Q gt  makePair P p  Q q            return new Pair lt P  Q gt  p  q              public final A a      public final B b       public Pair A a  B b            this a   a          this b   b              Override     public int hashCode             final int prime   31          int result   1          result   prime   result     a    null    0   a hashCode             result   prime   result     b    null    0   b hashCode             return result              Override     public boolean equals Object obj            if  this    obj                return true                    if  obj    null                return false                    if  getClass      obj getClass                  return false                     SuppressWarnings  rawtypes           Pair other    Pair  obj          if  a    null                if  other a    null                    return false                          else if   a equals other a                 return false                    if  b    null                if  other b    null                    return false                          else if   b equals other b                 return false                    return true             public boolean isInstance Class lt   gt  classA  Class lt   gt  classB            return classA isInstance a   amp  amp  classB isInstance b               SuppressWarnings  unchecked       public static  lt P  Q gt  Pair lt P  Q gt  cast Pair lt      gt  pair  Class lt P gt  pClass  Class lt Q gt  qClass             if  pair isInstance pClass  qClass                 return  Pair lt P  Q gt   pair                     throw new ClassCastException                Notes  mainly around rustiness with Java  amp  generics    both a and b are immutable   makePair static method helps you with boiler plate typing  which the diamond operator in Java 7 will make less annoying  There s some work to make this really nice re  generics  but it should be ok-ish now   c f  PECS  hashcode and equals are generated by eclipse  the compile time casting in the cast method is ok  but doesn t seem quite right  I m not sure if the wildcards in isInstance are necessary  I ve just written this in response to comments  for illustration purposes only

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I almost always end up defining n-Tuple classes when I code in Java. For instance:

public class Tuple2<T1,T2> {
  private T1 f1;
  private T2 f2;
  public Tuple2(T1 f1, T2 f2) {
    this.f1 = f1; this.f2 = f2;
  }
  public T1 getF1() {return f1;}
  public T2 getF2() {return f2;}
}

I know it's a bit ugly, but it works, and you just have to define your tuple types once. Tuples are something Java really lacks.

EDIT: David Hanak's example is more elegant, as it avoids defining getters and still keeps the object immutable.

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Apache Commons has tuple and triple for this    ImmutablePair lt L R gt   An immutable pair consisting of two Object elements  ImmutableTriple lt L M R gt   An immutable triple consisting of three Object elements  MutablePair lt L R gt     A mutable pair consisting of two Object elements  MutableTriple lt L M R gt     A mutable triple consisting of three Object elements  Pair lt L R gt    A pair consisting of two elements  Triple lt L M R gt    A triple consisting of three elements    Source  https   commons apache org proper commons-lang apidocs org apache commons lang3 tuple package-summary html

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All possible solutions will be a kludge  like container objects  your HashMap idea     multiple return values    as realized via arrays   I recommend regenerating the comma-separated list from the returned List  The code will end up being a lot cleaner

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I noticed there is no no-custom class  n-length  no-cast  type-safe answers yet to returning multiple values  Here is my go  import java util Objects   public final class NTuple lt V  T extends  NTuple lt      gt  gt        private final V value      private final T next       private NTuple V value  T next            this value   value          this next   next             public static  lt V gt  NTuple lt V    gt  of V value            return new NTuple lt  gt  value  null              public static  lt V   T extends  NTuple lt      gt  gt  NTuple lt V  T gt  of V value  T next            return new NTuple lt  gt  value  next              public V value             return value             public T next             return next             public static  lt V gt  V unpack0 NTuple lt V    gt  tuple            return Objects requireNonNull tuple   quot 0 quot   value               public static  lt V  T extends NTuple lt V    gt  gt  V unpack1 NTuple lt    T gt  tuple            NTuple lt    T gt  tuple0   Objects requireNonNull tuple   quot 0 quot            NTuple lt V    gt  tuple1   Objects requireNonNull tuple0 next     quot 1 quot            return tuple1 value               public static  lt V  T extends NTuple lt    NTuple lt V    gt  gt  gt  V unpack2 NTuple lt    T gt  tuple            NTuple lt    T gt  tuple0   Objects requireNonNull tuple   quot 0 quot            NTuple lt    NTuple lt V    gt  gt  tuple1   Objects requireNonNull tuple0 next     quot 1 quot            NTuple lt V    gt  tuple2   Objects requireNonNull tuple1 next     quot 2 quot            return tuple2 value             Sample use  public static void main String   args           pre-java 10 without lombok - use lombok s var or java 10 s var if you can     NTuple lt String  NTuple lt Integer  NTuple lt Integer    gt  gt  gt  multiple   wordCount  quot hello world quot        String original   NTuple unpack0 multiple       Integer wordCount   NTuple unpack1 multiple       Integer characterCount    NTuple unpack2 multiple        System out println original    quot    quot    wordCount    quot  words  quot    characterCount    quot  chars quot       private static NTuple lt String  NTuple lt Integer  NTuple lt Integer    gt  gt  gt  wordCount String s        int nWords   s split  quot   quot   length      int nChars   s length        return NTuple of s  NTuple of nWords  NTuple of nChars        Pros   no-custom container class - no need to write a class just for a return type n-length - can handle any number of return values no-cast - no need to cast from Object type-safe - the types are checked via Java s generics  Cons   inefficient for large numbers of return values  according to my experience with python s multiple return values  this should not happen in practice   heavy type declarations  can be alleviated by lombok Java 10 var

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All possible solutions will be a kludge  like container objects  your HashMap idea     multiple return values    as realized via arrays   I recommend regenerating the comma-separated list from the returned List  The code will end up being a lot cleaner

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I followed a similar approach than the described in the other answers with a few tweaks based on the requirement I had  basically I created the following classes Just in case  everything is Java    public class Pair lt L  R gt        final L left      final R right       public Pair L left  R right            this left   left          this right   right             public  lt T gt  T get Class lt T gt  param            return  T   param    this left getClass     this left   this right              public static  lt L  R gt  Pair lt L  R gt  of L left  R right            return new Pair lt L  R gt  left  right             Then  my requirement was simple  in the repository Class that reaches the DB  for the Get Methods than retrieve data from the DB  I need to check if it failed or succeed  then  if succeed  I needed to play with the returning list  if failed  stop the execution and notify the error   So  for example  my methods are like this   public Pair lt ResultMessage  List lt Customer gt  gt  getCustomers         List lt Customer gt  list   new ArrayList lt Customer gt         try                Do some work to get the list of Customers from the DB                catch  SQLException e            return Pair of                         new ResultMessage e getErrorCode    e getMessage        Left                         null       Right           return Pair of                     new ResultMessage 0   SUCCESS       Left                     list      Right     Where ResultMessage is just a class with two fields  code message  and Customer is any class with a bunch of fields that comes from the DB   Then  to check the result I just do this   void doSomething        Pair lt ResultMessage  List lt Customer gt  gt  customerResult    repository getCustomers        if  customerResult get ResultMessage class  getCode      0            List lt Customer gt  listOfCustomers   customerResult get List class           System out println  do SOMETHING with the list             else           System out println  Raised Error    do nothing

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Why not create a WhateverFunctionResult object that contains your results  and the logic required to parse these results  iterate over then etc  It seems to me that either    These results objects are intimately tied together related and belong together  or  they are unrelated  in which case your function isn t well defined in terms of what it s trying to do  i e  doing two different things    I see this sort of issue crop up again and again  Don t be afraid to create your own container result classes that contain the data and the associated functionality to handle this  If you simply pass the stuff around in a HashMap or similar  then your clients have to pull this map apart and grok the contents each time they want to use the results

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PASS A HASH INTO THE METHOD AND POPULATE IT        public void buildResponse String data  Map response

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In C    STL  there is a pair class for bundling two objects  In Java Generics a pair class isn t available  although there is some demand for it  You could easily implement it yourself though   I agree however with some other answers that if you need to return two or more objects from a method  it would be better to encapsulate them in a class

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Can do some thing like a tuple in dynamic language  Python   public class Tuple   private Object   multiReturns   private Tuple Object    multiReturns        this multiReturns   multiReturns     public static Tuple  t Object    multiReturns       return new Tuple multiReturns      public  lt T gt  T at int index  Class lt T gt  someClass        return someClass cast multiReturns index          and use like this  public Tuple returnMultiValues       return Tuple  t new ArrayList   new HashMap        Tuple t   returnMultiValues    ArrayList list   t at 0 ArrayList class

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PASS A HASH INTO THE METHOD AND POPULATE IT        public void buildResponse String data  Map response

User · Answer

In C    STL  there is a pair class for bundling two objects  In Java Generics a pair class isn t available  although there is some demand for it  You could easily implement it yourself though   I agree however with some other answers that if you need to return two or more objects from a method  it would be better to encapsulate them in a class

User · Answer

Apache Commons has tuple and triple for this    ImmutablePair lt L R gt   An immutable pair consisting of two Object elements  ImmutableTriple lt L M R gt   An immutable triple consisting of three Object elements  MutablePair lt L R gt     A mutable pair consisting of two Object elements  MutableTriple lt L M R gt     A mutable triple consisting of three Object elements  Pair lt L R gt    A pair consisting of two elements  Triple lt L M R gt    A triple consisting of three elements    Source  https   commons apache org proper commons-lang apidocs org apache commons lang3 tuple package-summary html

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Why not create a WhateverFunctionResult object that contains your results  and the logic required to parse these results  iterate over then etc  It seems to me that either    These results objects are intimately tied together related and belong together  or  they are unrelated  in which case your function isn t well defined in terms of what it s trying to do  i e  doing two different things    I see this sort of issue crop up again and again  Don t be afraid to create your own container result classes that contain the data and the associated functionality to handle this  If you simply pass the stuff around in a HashMap or similar  then your clients have to pull this map apart and grok the contents each time they want to use the results

User · Answer

Why not create a WhateverFunctionResult object that contains your results  and the logic required to parse these results  iterate over then etc  It seems to me that either    These results objects are intimately tied together related and belong together  or  they are unrelated  in which case your function isn t well defined in terms of what it s trying to do  i e  doing two different things    I see this sort of issue crop up again and again  Don t be afraid to create your own container result classes that contain the data and the associated functionality to handle this  If you simply pass the stuff around in a HashMap or similar  then your clients have to pull this map apart and grok the contents each time they want to use the results

User · Answer

PASS A HASH INTO THE METHOD AND POPULATE IT        public void buildResponse String data  Map response

User · Answer

I noticed there is no no-custom class  n-length  no-cast  type-safe answers yet to returning multiple values  Here is my go  import java util Objects   public final class NTuple lt V  T extends  NTuple lt      gt  gt        private final V value      private final T next       private NTuple V value  T next            this value   value          this next   next             public static  lt V gt  NTuple lt V    gt  of V value            return new NTuple lt  gt  value  null              public static  lt V   T extends  NTuple lt      gt  gt  NTuple lt V  T gt  of V value  T next            return new NTuple lt  gt  value  next              public V value             return value             public T next             return next             public static  lt V gt  V unpack0 NTuple lt V    gt  tuple            return Objects requireNonNull tuple   quot 0 quot   value               public static  lt V  T extends NTuple lt V    gt  gt  V unpack1 NTuple lt    T gt  tuple            NTuple lt    T gt  tuple0   Objects requireNonNull tuple   quot 0 quot            NTuple lt V    gt  tuple1   Objects requireNonNull tuple0 next     quot 1 quot            return tuple1 value               public static  lt V  T extends NTuple lt    NTuple lt V    gt  gt  gt  V unpack2 NTuple lt    T gt  tuple            NTuple lt    T gt  tuple0   Objects requireNonNull tuple   quot 0 quot            NTuple lt    NTuple lt V    gt  gt  tuple1   Objects requireNonNull tuple0 next     quot 1 quot            NTuple lt V    gt  tuple2   Objects requireNonNull tuple1 next     quot 2 quot            return tuple2 value             Sample use  public static void main String   args           pre-java 10 without lombok - use lombok s var or java 10 s var if you can     NTuple lt String  NTuple lt Integer  NTuple lt Integer    gt  gt  gt  multiple   wordCount  quot hello world quot        String original   NTuple unpack0 multiple       Integer wordCount   NTuple unpack1 multiple       Integer characterCount    NTuple unpack2 multiple        System out println original    quot    quot    wordCount    quot  words  quot    characterCount    quot  chars quot       private static NTuple lt String  NTuple lt Integer  NTuple lt Integer    gt  gt  gt  wordCount String s        int nWords   s split  quot   quot   length      int nChars   s length        return NTuple of s  NTuple of nWords  NTuple of nChars        Pros   no-custom container class - no need to write a class just for a return type n-length - can handle any number of return values no-cast - no need to cast from Object type-safe - the types are checked via Java s generics  Cons   inefficient for large numbers of return values  according to my experience with python s multiple return values  this should not happen in practice   heavy type declarations  can be alleviated by lombok Java 10 var

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You may use any of following ways   private static final int RETURN COUNT   2  private static final int VALUE A   0  private static final int VALUE B   1  private static final String A    a   private static final String B    b     1  Using Array  private static String   methodWithArrayResult                   return new String    valueA    valueB       private static void usingArrayResultTest         String   result   methodWithArrayResult        System out println        System out println  A       result VALUE A        System out println  B       result VALUE B        2  Using ArrayList  private static List lt String gt  methodWithListResult                   return Arrays asList  valueA    valueB       private static void usingListResultTest         List lt String gt  result   methodWithListResult        System out println        System out println  A       result get VALUE A        System out println  B       result get VALUE B        3  Using HashMap   private static Map lt String  String gt  methodWithMapResult         Map lt String  String gt  result   new HashMap lt  gt  RETURN COUNT       result put A   valueA        result put B   valueB                  return result     private static void usingMapResultTest         Map lt String  String gt  result   methodWithMapResult        System out println        System out println  A       result get A        System out println  B       result get B        4  Using your custom container class   private static class MyContainer lt M N gt        private final M first      private final N second       public MyContainer M first  N second            this first   first          this second   second             public M getFirst             return first             public N getSecond             return second                  hashcode  equals  toString if need    private static MyContainer lt String  String gt  methodWithContainerResult                   return new MyContainer  valueA    valueB       private static void usingContainerResultTest         MyContainer lt String  String gt  result   methodWithContainerResult        System out println        System out println  A       result getFirst         System out println  B       result getSecond         5  Using AbstractMap simpleEntry  private static AbstractMap SimpleEntry lt String  String gt  methodWithAbstractMapSimpleEntryResult                   return new AbstractMap SimpleEntry lt  gt   valueA    valueB       private static void usingAbstractMapSimpleResultTest         AbstractMap SimpleEntry lt String  String gt  result   methodWithAbstractMapSimpleEntryResult        System out println        System out println  A       result getKey         System out println  B       result getValue         6  Using Pair of Apache Commons   private static Pair lt String  String gt  methodWithPairResult                   return new ImmutablePair lt  gt   valueA    valueB       private static void usingPairResultTest         Pair lt String  String gt  result   methodWithPairResult        System out println        System out println  A       result getKey         System out println  B       result getValue

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If you want to return two objects you usually want to return a single object that encapsulates the two objects instead   You could return a List of NamedObject objects like this   public class NamedObject lt T gt      public final String name    public final T object     public NamedObject String name  T object        this name   name      this object   object          Then you can easily return a List lt NamedObject lt WhateverTypeYouWant gt  gt    Also  Why would you want to return a comma-separated list of names instead of a List lt String gt   Or better yet  return a Map lt String TheObjectType gt  with the keys being the names and the values the objects  unless your objects have specified order  in which case a NavigableMap might be what you want

User · Answer

I almost always end up defining n-Tuple classes when I code in Java. For instance:

public class Tuple2<T1,T2> {
  private T1 f1;
  private T2 f2;
  public Tuple2(T1 f1, T2 f2) {
    this.f1 = f1; this.f2 = f2;
  }
  public T1 getF1() {return f1;}
  public T2 getF2() {return f2;}
}

I know it's a bit ugly, but it works, and you just have to define your tuple types once. Tuples are something Java really lacks.

EDIT: David Hanak's example is more elegant, as it avoids defining getters and still keeps the object immutable.

User · Answer

PASS A HASH INTO THE METHOD AND POPULATE IT        public void buildResponse String data  Map response

User · Answer

If you want to return two objects you usually want to return a single object that encapsulates the two objects instead   You could return a List of NamedObject objects like this   public class NamedObject lt T gt      public final String name    public final T object     public NamedObject String name  T object        this name   name      this object   object          Then you can easily return a List lt NamedObject lt WhateverTypeYouWant gt  gt    Also  Why would you want to return a comma-separated list of names instead of a List lt String gt   Or better yet  return a Map lt String TheObjectType gt  with the keys being the names and the values the objects  unless your objects have specified order  in which case a NavigableMap might be what you want

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In the event the method you re calling is private  or called from one location  try  return new Object   value1  value2     The caller looks like   Object   temp myMethod parameters   Type1 value1  Type1 temp 0      For code clarity  temp 0  is not descriptive Type2 value2  Type2 temp 1     The Pair example by David Hanak has no syntactic benefit  and is limited to two values     return new Pair lt Type1 Type2 gt  value1  value2     And the caller looks like   Pair lt Type1  Type2 gt  temp myMethod parameters   Type1 value1 temp a     For code clarity  temp a is not descriptive Type2 value2 temp b

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Before Java 5  I would kind of agree that the Map solution isn t ideal  It wouldn t give you compile time type checking so can cause issues at runtime  However  with Java 5  we have Generic Types   So your method could look like this   public Map lt String  MyType gt  doStuff      MyType of course being the type of object you are returning   Basically I think that returning a Map is the right solution in this case because that s exactly what you want to return - a mapping of a string to an object

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While in your case  the comment may be a good way to go  in Android  you can use Pair   Simply  return new Pair lt  gt  yourList  yourCommaSeparatedValues

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You may use any of following ways   private static final int RETURN COUNT   2  private static final int VALUE A   0  private static final int VALUE B   1  private static final String A    a   private static final String B    b     1  Using Array  private static String   methodWithArrayResult                   return new String    valueA    valueB       private static void usingArrayResultTest         String   result   methodWithArrayResult        System out println        System out println  A       result VALUE A        System out println  B       result VALUE B        2  Using ArrayList  private static List lt String gt  methodWithListResult                   return Arrays asList  valueA    valueB       private static void usingListResultTest         List lt String gt  result   methodWithListResult        System out println        System out println  A       result get VALUE A        System out println  B       result get VALUE B        3  Using HashMap   private static Map lt String  String gt  methodWithMapResult         Map lt String  String gt  result   new HashMap lt  gt  RETURN COUNT       result put A   valueA        result put B   valueB                  return result     private static void usingMapResultTest         Map lt String  String gt  result   methodWithMapResult        System out println        System out println  A       result get A        System out println  B       result get B        4  Using your custom container class   private static class MyContainer lt M N gt        private final M first      private final N second       public MyContainer M first  N second            this first   first          this second   second             public M getFirst             return first             public N getSecond             return second                  hashcode  equals  toString if need    private static MyContainer lt String  String gt  methodWithContainerResult                   return new MyContainer  valueA    valueB       private static void usingContainerResultTest         MyContainer lt String  String gt  result   methodWithContainerResult        System out println        System out println  A       result getFirst         System out println  B       result getSecond         5  Using AbstractMap simpleEntry  private static AbstractMap SimpleEntry lt String  String gt  methodWithAbstractMapSimpleEntryResult                   return new AbstractMap SimpleEntry lt  gt   valueA    valueB       private static void usingAbstractMapSimpleResultTest         AbstractMap SimpleEntry lt String  String gt  result   methodWithAbstractMapSimpleEntryResult        System out println        System out println  A       result getKey         System out println  B       result getValue         6  Using Pair of Apache Commons   private static Pair lt String  String gt  methodWithPairResult                   return new ImmutablePair lt  gt   valueA    valueB       private static void usingPairResultTest         Pair lt String  String gt  result   methodWithPairResult        System out println        System out println  A       result getKey         System out println  B       result getValue

[java] How to return multiple objects from a Java method?

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