How do I make the method return type generic

Question

Consider this example  typical in OOP books      I have an Animal class  where each Animal can have many friends  And subclasses like Dog  Duck  Mouse etc which add specific behavior like bark    quack   etc   Here s the Animal class   public class Animal       private Map lt String Animal gt  friends   new HashMap lt  gt          public void addFriend String name  Animal animal           friends put name animal              public Animal callFriend String name           return friends get name             And here s some code snippet with lots of typecasting   Mouse jerry   new Mouse    jerry addFriend  spike   new Dog     jerry addFriend  quacker   new Duck        Dog  jerry callFriend  spike    bark      Duck  jerry callFriend  quacker    quack      Is there any way I can use generics for the return type to get rid of the typecasting  so that I can say      jerry callFriend  spike   bark    jerry callFriend  quacker   quack      Here s some initial code with return type conveyed to the method as a parameter that s never used    public lt T extends Animal gt  T callFriend String name  T unusedTypeObj       return  T friends get name               Is there a way to figure out the return type at runtime without the extra parameter using instanceof  Or at least by passing a class of the type instead of a dummy instance  I understand generics are for compile time type-checking  but is there a workaround for this

User · Answer

Additionally you can ask the method to return the value in a given type this way   lt T gt  T methodName Class lt T gt  var     More examples here at Oracle Java documentation

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Here is the simpler version   public  lt T gt  T callFriend String name        return  T  friends get name     Casting to T not needed in this case but its a good practice to do     Fully working code       public class Test           public static class Animal               private Map lt String Animal gt  friends   new HashMap lt  gt                  public void addFriend String name  Animal animal                   friends put name animal                              public  lt T gt  T callFriend String name                   return  T  friends get name                                    public static class Dog extends Animal                public void bark                     System out println  i am dog                                     public static class Duck extends Animal                public void quack                     System out println  i am duck                                     public static void main String    args                Animal animals   new Animal                animals addFriend  dog   new Dog                 animals addFriend  duck   new Duck                  Dog dog   animals callFriend  dog                dog bark                 Duck duck   animals callFriend  duck                duck quack

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Here is the simpler version   public  lt T gt  T callFriend String name        return  T  friends get name     Casting to T not needed in this case but its a good practice to do     Fully working code       public class Test           public static class Animal               private Map lt String Animal gt  friends   new HashMap lt  gt                  public void addFriend String name  Animal animal                   friends put name animal                              public  lt T gt  T callFriend String name                   return  T  friends get name                                    public static class Dog extends Animal                public void bark                     System out println  i am dog                                     public static class Duck extends Animal                public void quack                     System out println  i am duck                                     public static void main String    args                Animal animals   new Animal                animals addFriend  dog   new Dog                 animals addFriend  duck   new Duck                  Dog dog   animals callFriend  dog                dog bark                 Duck duck   animals callFriend  duck                duck quack

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As the question is based in hypothetical data here is a good exemple returning a generic that extends Comparable interface  public class MaximumTest          find the max value using Comparable interface     public static  lt T extends Comparable lt T gt  gt  T maximum T x  T y  T z            T max   x     assume that x is initially the largest          if  y compareTo max   gt  0               max   y     y is the large now                   if  z compareTo max   gt  0               max   z     z is the large now                   return max     returns the maximum value                   testing with an ordinary main method     public static void main String args              System out printf  quot Maximum of  d   d and  d is  d n n quot   3  4  5  maximum 3  4  5            System out printf  quot Maximum of   1f    1f and   1f is   1f n n quot   6 6  8 8  7 7  maximum 6 6  8 8  7 7            System out printf  quot Maximum of  s   s and  s is  s n quot    quot strawberry quot    quot apple quot    quot orange quot                   maximum  quot strawberry quot    quot apple quot    quot orange quot

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No  The compiler can t know what type jerry callFriend  spike   would return  Also  your implementation just hides the cast in the method without any additional type safety  Consider this   jerry addFriend  quaker   new Duck     jerry callFriend  quaker      unused    new Dog        dies with illegal cast   In this specific case  creating an abstract talk   method and overriding it appropriately in the subclasses would serve you much better   Mouse jerry   new Mouse    jerry addFriend  spike   new Dog     jerry addFriend  quacker   new Duck      jerry callFriend  spike   talk    jerry callFriend  quacker   talk

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Is there a way to figure out the return type at runtime without the extra parameter using instanceof    As an alternative solution you could utilise the Visitor pattern like this  Make Animal abstract and make it implement Visitable   abstract public class Animal implements Visitable     private Map lt String Animal gt  friends   new HashMap lt String Animal gt        public void addFriend String name  Animal animal         friends put name animal          public Animal callFriend String name         return friends get name           Visitable just means that an Animal implementation is willing to accept a visitor   public interface Visitable       void accept Visitor v       And a visitor implementation is able to visit all the subclasses of an animal   public interface Visitor       void visit Dog d       void visit Duck d       void visit Mouse m       So for example a Dog implementation would then look like this   public class Dog extends Animal       public void bark            Override     public void accept Visitor v    v visit this         The trick here is that as the Dog knows what type it is it can trigger the relevant overloaded visit method of the visitor v by passing  this  as a parameter  Other subclasses would implement accept   exactly the same way   The class that wants to call subclass specific methods must then implement the Visitor interface like this   public class Example implements Visitor        public void main             Mouse jerry   new Mouse            jerry addFriend  spike   new Dog             jerry addFriend  quacker   new Duck                 Used to be    Dog  jerry callFriend  spike    bark            jerry callFriend  spike   accept this               Used to be    Duck  jerry callFriend  quacker    quack            jerry callFriend  quacker   accept this                 This would fire on callFriend  spike   accept this       Override     public void visit Dog d    d bark              This would fire on callFriend  quacker   accept this       Override     public void visit Duck d    d quack            Override     public void visit Mouse m    m squeak          I know it s a lot more interfaces and methods than you bargained for  but it s a standard way to get a handle on every specific subtype with precisely zero instanceof checks and zero type casts  And it s all done in a standard language agnostic fashion so it s not just for Java but any OO language should work the same

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Not possible  How is the Map supposed to know which subclass of Animal it s going to get  given only a String key    The only way this would be possible is if each Animal accepted only one type of friend  then it could be a parameter of the Animal class   or of the callFriend   method got a type parameter  But it really looks like you re missing the point of inheritance  it s that you can only treat subclasses uniformly when using exclusively the superclass methods

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You could implement it like this    SuppressWarnings  unchecked   public  lt T extends Animal gt  T callFriend String name        return  T friends get name        Yes  this is legal code  see Java Generics  Generic type defined as return type only    The return type will be inferred from the caller  However  note the  SuppressWarnings annotation  that tells you that this code isn t typesafe  You have to verify it yourself  or you could get ClassCastExceptions at runtime   Unfortunately  the way you re using it  without assigning the return value to a temporary variable   the only way to make the compiler happy is to call it like this   jerry  lt Dog gt callFriend  spike   bark      While this may be a little nicer than casting  you are probably better off giving the Animal class an abstract talk   method  as David Schmitt said

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Not possible  How is the Map supposed to know which subclass of Animal it s going to get  given only a String key    The only way this would be possible is if each Animal accepted only one type of friend  then it could be a parameter of the Animal class   or of the callFriend   method got a type parameter  But it really looks like you re missing the point of inheritance  it s that you can only treat subclasses uniformly when using exclusively the superclass methods

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Not really  because as you say  the compiler only knows that callFriend   is returning an Animal  not a Dog or Duck   Can you not add an abstract makeNoise   method to Animal that would be implemented as a bark or quack by its subclasses

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You could implement it like this    SuppressWarnings  unchecked   public  lt T extends Animal gt  T callFriend String name        return  T friends get name        Yes  this is legal code  see Java Generics  Generic type defined as return type only    The return type will be inferred from the caller  However  note the  SuppressWarnings annotation  that tells you that this code isn t typesafe  You have to verify it yourself  or you could get ClassCastExceptions at runtime   Unfortunately  the way you re using it  without assigning the return value to a temporary variable   the only way to make the compiler happy is to call it like this   jerry  lt Dog gt callFriend  spike   bark      While this may be a little nicer than casting  you are probably better off giving the Animal class an abstract talk   method  as David Schmitt said

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Not really  because as you say  the compiler only knows that callFriend   is returning an Animal  not a Dog or Duck   Can you not add an abstract makeNoise   method to Animal that would be implemented as a bark or quack by its subclasses

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There are a lot of great answers here  but this is the approach I took for an Appium test where acting on a single element can result in going to different application states based on the user s settings  While it doesn t follow the conventions of OP s example  I hope it helps someone   public  lt T extends MobilePage gt  T tapSignInButton Class lt T gt  type  throws NoSuchMethodException  IllegalAccessException  InvocationTargetException  InstantiationException         signInButton click        return type getConstructor AppiumDriver class  newInstance appiumDriver        MobilePage is the super class that the type extends meaning you can use any of its children  duh  type getConstructor Param class  etc  allows you to interact with the constructor of the type  This constructor should be the same between all expected classes  newInstance takes a declared variable that you want to pass to the new objects constructor   If you don t want to throw the errors you can catch them like so   public  lt T extends MobilePage gt  T tapSignInButton Class lt T gt  type           signInButton click        T returnValue   null      try          returnValue   type getConstructor AppiumDriver class  newInstance appiumDriver         catch  Exception e            e printStackTrace              return returnValue

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There are a lot of great answers here  but this is the approach I took for an Appium test where acting on a single element can result in going to different application states based on the user s settings  While it doesn t follow the conventions of OP s example  I hope it helps someone   public  lt T extends MobilePage gt  T tapSignInButton Class lt T gt  type  throws NoSuchMethodException  IllegalAccessException  InvocationTargetException  InstantiationException         signInButton click        return type getConstructor AppiumDriver class  newInstance appiumDriver        MobilePage is the super class that the type extends meaning you can use any of its children  duh  type getConstructor Param class  etc  allows you to interact with the constructor of the type  This constructor should be the same between all expected classes  newInstance takes a declared variable that you want to pass to the new objects constructor   If you don t want to throw the errors you can catch them like so   public  lt T extends MobilePage gt  T tapSignInButton Class lt T gt  type           signInButton click        T returnValue   null      try          returnValue   type getConstructor AppiumDriver class  newInstance appiumDriver         catch  Exception e            e printStackTrace              return returnValue

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I did the following in my lib kontraktor   public class Actor lt SELF extends Actor gt        public SELF self     return  SELF  self        subclassing   public class MyHttpAppSession extends Actor lt MyHttpAppSession gt               at least this works inside the current class and when having a strong typed reference  Multiple inheritance works  but gets really tricky then

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This question is very similar to Item 29 in Effective Java -  Consider typesafe heterogeneous containers    Laz s answer is the closest to Bloch s solution   However  both put and get should use the Class literal for safety   The signatures would become   public  lt T extends Animal gt  void addFriend String name  Class lt T gt  type  T animal   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  type     Inside both methods you should check that the parameters are sane   See Effective Java and the Class javadoc for more info

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I ve written an article which contains a proof of concept  support classes and a test class which demonstrates how Super Type Tokens can be retrieved by your classes at runtime  In a nutshell  it allows you to delegate to alternative implementations depending on actual generic parameters passed by the caller  Example   TimeSeries lt Double gt  delegates to a private inner class which uses double   TimeSeries lt OHLC gt  delegates to a private inner class which uses ArrayList lt OHLC gt   See   Using TypeTokens to retrieve generic parameters  web archive  Using TypeTokens to retrieve generic parameters  blog   Thanks Richard Gomes - Blog

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There is another approach  you can narrow the return type when you override a method  In each subclass you would have to override callFriend to return that subclass  The cost would be the multiple declarations of callFriend  but you could isolate the common parts to a method called internally  This seems a lot simpler to me than the solutions mentioned above  and does not need an extra argument to determine the return type

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Based on the same idea as Super Type Tokens  you could create a typed id to use instead of a string   public abstract class TypedID lt T extends Animal gt      public final Type type    public final String id     protected TypedID String id        this id   id      Type superclass   getClass   getGenericSuperclass        if  superclass instanceof Class          throw new RuntimeException  Missing type parameter               this type     ParameterizedType  superclass  getActualTypeArguments   0           But I think this may defeat the purpose  since you now need to create new id objects for each string and hold on to them  or reconstruct them with the correct type information    Mouse jerry   new Mouse    TypedID lt Dog gt  spike   new TypedID lt Dog gt   spike       TypedID lt Duck gt  quacker   new TypedID lt Duck gt   quacker        jerry addFriend spike  new Dog     jerry addFriend quacker  new Duck       But you can now use the class in the way you originally wanted  without the casts   jerry callFriend spike  bark    jerry callFriend quacker  quack      This is just hiding the type parameter inside the id  although it does mean you can retrieve the type from the identifier later if you wish   You d need to implement the comparison and hashing methods of TypedID too if you want to be able to compare two identical instances of an id

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Based on the same idea as Super Type Tokens  you could create a typed id to use instead of a string   public abstract class TypedID lt T extends Animal gt      public final Type type    public final String id     protected TypedID String id        this id   id      Type superclass   getClass   getGenericSuperclass        if  superclass instanceof Class          throw new RuntimeException  Missing type parameter               this type     ParameterizedType  superclass  getActualTypeArguments   0           But I think this may defeat the purpose  since you now need to create new id objects for each string and hold on to them  or reconstruct them with the correct type information    Mouse jerry   new Mouse    TypedID lt Dog gt  spike   new TypedID lt Dog gt   spike       TypedID lt Duck gt  quacker   new TypedID lt Duck gt   quacker        jerry addFriend spike  new Dog     jerry addFriend quacker  new Duck       But you can now use the class in the way you originally wanted  without the casts   jerry callFriend spike  bark    jerry callFriend quacker  quack      This is just hiding the type parameter inside the id  although it does mean you can retrieve the type from the identifier later if you wish   You d need to implement the comparison and hashing methods of TypedID too if you want to be able to compare two identical instances of an id

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No  The compiler can t know what type jerry callFriend  spike   would return  Also  your implementation just hides the cast in the method without any additional type safety  Consider this   jerry addFriend  quaker   new Duck     jerry callFriend  quaker      unused    new Dog        dies with illegal cast   In this specific case  creating an abstract talk   method and overriding it appropriately in the subclasses would serve you much better   Mouse jerry   new Mouse    jerry addFriend  spike   new Dog     jerry addFriend  quacker   new Duck      jerry callFriend  spike   talk    jerry callFriend  quacker   talk

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No  The compiler can t know what type jerry callFriend  spike   would return  Also  your implementation just hides the cast in the method without any additional type safety  Consider this   jerry addFriend  quaker   new Duck     jerry callFriend  quaker      unused    new Dog        dies with illegal cast   In this specific case  creating an abstract talk   method and overriding it appropriately in the subclasses would serve you much better   Mouse jerry   new Mouse    jerry addFriend  spike   new Dog     jerry addFriend  quacker   new Duck      jerry callFriend  spike   talk    jerry callFriend  quacker   talk

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I know this is a completely different thing that the one asked  Another way of resolving this would be reflection  I mean  this does not take the benefit from Generics  but it lets you emulate  in some way  the behavior you want to perform  make a dog bark  make a duck quack  etc   without taking care of type casting    import java lang reflect InvocationTargetException  import java util HashMap  import java util Map   abstract class AnimalExample       private Map lt String Class lt   gt  gt  friends   new HashMap lt String Class lt   gt  gt         private Map lt String Object gt  theFriends   new HashMap lt String Object gt          public void addFriend String name  Object friend           friends put name friend getClass             theFriends put name  friend              public void makeMyFriendSpeak String name           try               friends get name  getMethod  speak   invoke theFriends get name              catch  IllegalArgumentException e                e printStackTrace              catch  SecurityException e                e printStackTrace              catch  IllegalAccessException e                e printStackTrace              catch  InvocationTargetException e                e printStackTrace              catch  NoSuchMethodException e                e printStackTrace                          public abstract void speak         class Dog extends Animal       public void speak              System out println  woof              class Duck extends Animal       public void speak              System out println  quack              class Cat extends Animal       public void speak              System out println  miauu              public class AnimalExample        public static void main  String    args             Cat felix   new Cat             felix addFriend  Spike   new Dog             felix addFriend  Donald   new Duck             felix makeMyFriendSpeak  Spike            felix makeMyFriendSpeak  Donald

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Not possible  How is the Map supposed to know which subclass of Animal it s going to get  given only a String key    The only way this would be possible is if each Animal accepted only one type of friend  then it could be a parameter of the Animal class   or of the callFriend   method got a type parameter  But it really looks like you re missing the point of inheritance  it s that you can only treat subclasses uniformly when using exclusively the superclass methods

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I know this is a completely different thing that the one asked  Another way of resolving this would be reflection  I mean  this does not take the benefit from Generics  but it lets you emulate  in some way  the behavior you want to perform  make a dog bark  make a duck quack  etc   without taking care of type casting    import java lang reflect InvocationTargetException  import java util HashMap  import java util Map   abstract class AnimalExample       private Map lt String Class lt   gt  gt  friends   new HashMap lt String Class lt   gt  gt         private Map lt String Object gt  theFriends   new HashMap lt String Object gt          public void addFriend String name  Object friend           friends put name friend getClass             theFriends put name  friend              public void makeMyFriendSpeak String name           try               friends get name  getMethod  speak   invoke theFriends get name              catch  IllegalArgumentException e                e printStackTrace              catch  SecurityException e                e printStackTrace              catch  IllegalAccessException e                e printStackTrace              catch  InvocationTargetException e                e printStackTrace              catch  NoSuchMethodException e                e printStackTrace                          public abstract void speak         class Dog extends Animal       public void speak              System out println  woof              class Duck extends Animal       public void speak              System out println  quack              class Cat extends Animal       public void speak              System out println  miauu              public class AnimalExample        public static void main  String    args             Cat felix   new Cat             felix addFriend  Spike   new Dog             felix addFriend  Donald   new Duck             felix makeMyFriendSpeak  Spike            felix makeMyFriendSpeak  Donald

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This question is very similar to Item 29 in Effective Java -  Consider typesafe heterogeneous containers    Laz s answer is the closest to Bloch s solution   However  both put and get should use the Class literal for safety   The signatures would become   public  lt T extends Animal gt  void addFriend String name  Class lt T gt  type  T animal   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  type     Inside both methods you should check that the parameters are sane   See Effective Java and the Class javadoc for more info

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You could define callFriend this way   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  type        return type cast friends get name        Then call it as such   jerry callFriend  spike   Dog class  bark    jerry callFriend  quacker   Duck class  quack      This code has the benefit of not generating any compiler warnings  Of course this is really just an updated version of casting from the pre-generic days and doesn t add any additional safety

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Not possible  How is the Map supposed to know which subclass of Animal it s going to get  given only a String key    The only way this would be possible is if each Animal accepted only one type of friend  then it could be a parameter of the Animal class   or of the callFriend   method got a type parameter  But it really looks like you re missing the point of inheritance  it s that you can only treat subclasses uniformly when using exclusively the superclass methods

User · Answer

This question is very similar to Item 29 in Effective Java -  Consider typesafe heterogeneous containers    Laz s answer is the closest to Bloch s solution   However  both put and get should use the Class literal for safety   The signatures would become   public  lt T extends Animal gt  void addFriend String name  Class lt T gt  type  T animal   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  type     Inside both methods you should check that the parameters are sane   See Effective Java and the Class javadoc for more info

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As the question is based in hypothetical data here is a good exemple returning a generic that extends Comparable interface  public class MaximumTest          find the max value using Comparable interface     public static  lt T extends Comparable lt T gt  gt  T maximum T x  T y  T z            T max   x     assume that x is initially the largest          if  y compareTo max   gt  0               max   y     y is the large now                   if  z compareTo max   gt  0               max   z     z is the large now                   return max     returns the maximum value                   testing with an ordinary main method     public static void main String args              System out printf  quot Maximum of  d   d and  d is  d n n quot   3  4  5  maximum 3  4  5            System out printf  quot Maximum of   1f    1f and   1f is   1f n n quot   6 6  8 8  7 7  maximum 6 6  8 8  7 7            System out printf  quot Maximum of  s   s and  s is  s n quot    quot strawberry quot    quot apple quot    quot orange quot                   maximum  quot strawberry quot    quot apple quot    quot orange quot

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You could implement it like this    SuppressWarnings  unchecked   public  lt T extends Animal gt  T callFriend String name        return  T friends get name        Yes  this is legal code  see Java Generics  Generic type defined as return type only    The return type will be inferred from the caller  However  note the  SuppressWarnings annotation  that tells you that this code isn t typesafe  You have to verify it yourself  or you could get ClassCastExceptions at runtime   Unfortunately  the way you re using it  without assigning the return value to a temporary variable   the only way to make the compiler happy is to call it like this   jerry  lt Dog gt callFriend  spike   bark      While this may be a little nicer than casting  you are probably better off giving the Animal class an abstract talk   method  as David Schmitt said

User · Answer

I did the following in my lib kontraktor   public class Actor lt SELF extends Actor gt        public SELF self     return  SELF  self        subclassing   public class MyHttpAppSession extends Actor lt MyHttpAppSession gt               at least this works inside the current class and when having a strong typed reference  Multiple inheritance works  but gets really tricky then

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What you re looking for here is abstraction  Code against interfaces more and you should have to do less casting   The example below is in C  but the concept remains the same   using System  using System Collections Generic  using System Reflection   namespace GenericsTest   class MainClass       public static void Main  string   args                 HasFriends jerry   new Mouse            jerry AddFriend  spike   new Dog             jerry AddFriend  quacker   new Duck              jerry CallFriend lt  Animal gt   spike   Speak            jerry CallFriend lt  Animal gt   quacker   Speak             interface  HasFriends       void AddFriend string name   Animal animal        T CallFriend lt T gt  string name  where T    Animal     interface  Animal       void Speak       abstract class AnimalBase    Animal   HasFriends       private Dictionary lt string   Animal gt  friends   new Dictionary lt string   Animal gt           public abstract void Speak         public void AddFriend string name   Animal animal                friends Add name  animal                 public T CallFriend lt T gt  string name  where T    Animal               return  T  friends name            class Mouse   AnimalBase       public override void Speak     Squeek           private void Squeek                 Console WriteLine   Squeek  Squeek              class Dog   AnimalBase       public override void Speak     Bark           private void Bark                 Console WriteLine   Woof              class Duck   AnimalBase       public override void Speak     Quack           private void Quack                 Console WriteLine   Quack  Quack

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As you said passing a class would be OK  you could write this   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  clazz       return  T  friends get name       And then use it like this   jerry callFriend  spike   Dog class  bark    jerry callFriend  quacker   Duck class  quack      Not perfect  but this is pretty much as far as you get with Java generics  There is a way to implement Typesafe Heterogenous Containers  THC  using Super Type Tokens  but that has its own problems again

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This question is very similar to Item 29 in Effective Java -  Consider typesafe heterogeneous containers    Laz s answer is the closest to Bloch s solution   However  both put and get should use the Class literal for safety   The signatures would become   public  lt T extends Animal gt  void addFriend String name  Class lt T gt  type  T animal   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  type     Inside both methods you should check that the parameters are sane   See Effective Java and the Class javadoc for more info

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what about public class Animal       private Map lt String  lt T extends Animal gt  gt  friends   new HashMap lt String  lt T extends Animal gt  gt          public  lt T extends Animal gt  void addFriend String name  T animal           friends put name animal              public  lt T extends Animal gt  T callFriend String name           return friends get name

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As you said passing a class would be OK  you could write this   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  clazz       return  T  friends get name       And then use it like this   jerry callFriend  spike   Dog class  bark    jerry callFriend  quacker   Duck class  quack      Not perfect  but this is pretty much as far as you get with Java generics  There is a way to implement Typesafe Heterogenous Containers  THC  using Super Type Tokens  but that has its own problems again

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There is another approach  you can narrow the return type when you override a method  In each subclass you would have to override callFriend to return that subclass  The cost would be the multiple declarations of callFriend  but you could isolate the common parts to a method called internally  This seems a lot simpler to me than the solutions mentioned above  and does not need an extra argument to determine the return type

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What you re looking for here is abstraction  Code against interfaces more and you should have to do less casting   The example below is in C  but the concept remains the same   using System  using System Collections Generic  using System Reflection   namespace GenericsTest   class MainClass       public static void Main  string   args                 HasFriends jerry   new Mouse            jerry AddFriend  spike   new Dog             jerry AddFriend  quacker   new Duck              jerry CallFriend lt  Animal gt   spike   Speak            jerry CallFriend lt  Animal gt   quacker   Speak             interface  HasFriends       void AddFriend string name   Animal animal        T CallFriend lt T gt  string name  where T    Animal     interface  Animal       void Speak       abstract class AnimalBase    Animal   HasFriends       private Dictionary lt string   Animal gt  friends   new Dictionary lt string   Animal gt           public abstract void Speak         public void AddFriend string name   Animal animal                friends Add name  animal                 public T CallFriend lt T gt  string name  where T    Animal               return  T  friends name            class Mouse   AnimalBase       public override void Speak     Squeek           private void Squeek                 Console WriteLine   Squeek  Squeek              class Dog   AnimalBase       public override void Speak     Bark           private void Bark                 Console WriteLine   Woof              class Duck   AnimalBase       public override void Speak     Quack           private void Quack                 Console WriteLine   Quack  Quack

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You could define callFriend this way   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  type        return type cast friends get name        Then call it as such   jerry callFriend  spike   Dog class  bark    jerry callFriend  quacker   Duck class  quack      This code has the benefit of not generating any compiler warnings  Of course this is really just an updated version of casting from the pre-generic days and doesn t add any additional safety

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Based on the same idea as Super Type Tokens  you could create a typed id to use instead of a string   public abstract class TypedID lt T extends Animal gt      public final Type type    public final String id     protected TypedID String id        this id   id      Type superclass   getClass   getGenericSuperclass        if  superclass instanceof Class          throw new RuntimeException  Missing type parameter               this type     ParameterizedType  superclass  getActualTypeArguments   0           But I think this may defeat the purpose  since you now need to create new id objects for each string and hold on to them  or reconstruct them with the correct type information    Mouse jerry   new Mouse    TypedID lt Dog gt  spike   new TypedID lt Dog gt   spike       TypedID lt Duck gt  quacker   new TypedID lt Duck gt   quacker        jerry addFriend spike  new Dog     jerry addFriend quacker  new Duck       But you can now use the class in the way you originally wanted  without the casts   jerry callFriend spike  bark    jerry callFriend quacker  quack      This is just hiding the type parameter inside the id  although it does mean you can retrieve the type from the identifier later if you wish   You d need to implement the comparison and hashing methods of TypedID too if you want to be able to compare two identical instances of an id

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You could define callFriend this way   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  type        return type cast friends get name        Then call it as such   jerry callFriend  spike   Dog class  bark    jerry callFriend  quacker   Duck class  quack      This code has the benefit of not generating any compiler warnings  Of course this is really just an updated version of casting from the pre-generic days and doesn t add any additional safety

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Is there a way to figure out the return type at runtime without the extra parameter using instanceof    As an alternative solution you could utilise the Visitor pattern like this  Make Animal abstract and make it implement Visitable   abstract public class Animal implements Visitable     private Map lt String Animal gt  friends   new HashMap lt String Animal gt        public void addFriend String name  Animal animal         friends put name animal          public Animal callFriend String name         return friends get name           Visitable just means that an Animal implementation is willing to accept a visitor   public interface Visitable       void accept Visitor v       And a visitor implementation is able to visit all the subclasses of an animal   public interface Visitor       void visit Dog d       void visit Duck d       void visit Mouse m       So for example a Dog implementation would then look like this   public class Dog extends Animal       public void bark            Override     public void accept Visitor v    v visit this         The trick here is that as the Dog knows what type it is it can trigger the relevant overloaded visit method of the visitor v by passing  this  as a parameter  Other subclasses would implement accept   exactly the same way   The class that wants to call subclass specific methods must then implement the Visitor interface like this   public class Example implements Visitor        public void main             Mouse jerry   new Mouse            jerry addFriend  spike   new Dog             jerry addFriend  quacker   new Duck                 Used to be    Dog  jerry callFriend  spike    bark            jerry callFriend  spike   accept this               Used to be    Duck  jerry callFriend  quacker    quack            jerry callFriend  quacker   accept this                 This would fire on callFriend  spike   accept this       Override     public void visit Dog d    d bark              This would fire on callFriend  quacker   accept this       Override     public void visit Duck d    d quack            Override     public void visit Mouse m    m squeak          I know it s a lot more interfaces and methods than you bargained for  but it s a standard way to get a handle on every specific subtype with precisely zero instanceof checks and zero type casts  And it s all done in a standard language agnostic fashion so it s not just for Java but any OO language should work the same

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No  The compiler can t know what type jerry callFriend  spike   would return  Also  your implementation just hides the cast in the method without any additional type safety  Consider this   jerry addFriend  quaker   new Duck     jerry callFriend  quaker      unused    new Dog        dies with illegal cast   In this specific case  creating an abstract talk   method and overriding it appropriately in the subclasses would serve you much better   Mouse jerry   new Mouse    jerry addFriend  spike   new Dog     jerry addFriend  quacker   new Duck      jerry callFriend  spike   talk    jerry callFriend  quacker   talk

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As you said passing a class would be OK  you could write this   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  clazz       return  T  friends get name       And then use it like this   jerry callFriend  spike   Dog class  bark    jerry callFriend  quacker   Duck class  quack      Not perfect  but this is pretty much as far as you get with Java generics  There is a way to implement Typesafe Heterogenous Containers  THC  using Super Type Tokens  but that has its own problems again

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As you said passing a class would be OK  you could write this   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  clazz       return  T  friends get name       And then use it like this   jerry callFriend  spike   Dog class  bark    jerry callFriend  quacker   Duck class  quack      Not perfect  but this is pretty much as far as you get with Java generics  There is a way to implement Typesafe Heterogenous Containers  THC  using Super Type Tokens  but that has its own problems again

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You could define callFriend this way   public  lt T extends Animal gt  T callFriend String name  Class lt T gt  type        return type cast friends get name        Then call it as such   jerry callFriend  spike   Dog class  bark    jerry callFriend  quacker   Duck class  quack      This code has the benefit of not generating any compiler warnings  Of course this is really just an updated version of casting from the pre-generic days and doesn t add any additional safety

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Not really  because as you say  the compiler only knows that callFriend   is returning an Animal  not a Dog or Duck   Can you not add an abstract makeNoise   method to Animal that would be implemented as a bark or quack by its subclasses

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You could implement it like this    SuppressWarnings  unchecked   public  lt T extends Animal gt  T callFriend String name        return  T friends get name        Yes  this is legal code  see Java Generics  Generic type defined as return type only    The return type will be inferred from the caller  However  note the  SuppressWarnings annotation  that tells you that this code isn t typesafe  You have to verify it yourself  or you could get ClassCastExceptions at runtime   Unfortunately  the way you re using it  without assigning the return value to a temporary variable   the only way to make the compiler happy is to call it like this   jerry  lt Dog gt callFriend  spike   bark      While this may be a little nicer than casting  you are probably better off giving the Animal class an abstract talk   method  as David Schmitt said

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what about public class Animal       private Map lt String  lt T extends Animal gt  gt  friends   new HashMap lt String  lt T extends Animal gt  gt          public  lt T extends Animal gt  void addFriend String name  T animal           friends put name animal              public  lt T extends Animal gt  T callFriend String name           return friends get name

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Not really  because as you say  the compiler only knows that callFriend   is returning an Animal  not a Dog or Duck   Can you not add an abstract makeNoise   method to Animal that would be implemented as a bark or quack by its subclasses

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Additionally you can ask the method to return the value in a given type this way   lt T gt  T methodName Class lt T gt  var     More examples here at Oracle Java documentation

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Based on the same idea as Super Type Tokens  you could create a typed id to use instead of a string   public abstract class TypedID lt T extends Animal gt      public final Type type    public final String id     protected TypedID String id        this id   id      Type superclass   getClass   getGenericSuperclass        if  superclass instanceof Class          throw new RuntimeException  Missing type parameter               this type     ParameterizedType  superclass  getActualTypeArguments   0           But I think this may defeat the purpose  since you now need to create new id objects for each string and hold on to them  or reconstruct them with the correct type information    Mouse jerry   new Mouse    TypedID lt Dog gt  spike   new TypedID lt Dog gt   spike       TypedID lt Duck gt  quacker   new TypedID lt Duck gt   quacker        jerry addFriend spike  new Dog     jerry addFriend quacker  new Duck       But you can now use the class in the way you originally wanted  without the casts   jerry callFriend spike  bark    jerry callFriend quacker  quack      This is just hiding the type parameter inside the id  although it does mean you can retrieve the type from the identifier later if you wish   You d need to implement the comparison and hashing methods of TypedID too if you want to be able to compare two identical instances of an id

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I ve written an article which contains a proof of concept  support classes and a test class which demonstrates how Super Type Tokens can be retrieved by your classes at runtime  In a nutshell  it allows you to delegate to alternative implementations depending on actual generic parameters passed by the caller  Example   TimeSeries lt Double gt  delegates to a private inner class which uses double   TimeSeries lt OHLC gt  delegates to a private inner class which uses ArrayList lt OHLC gt   See   Using TypeTokens to retrieve generic parameters  web archive  Using TypeTokens to retrieve generic parameters  blog   Thanks Richard Gomes - Blog

[java] How do I make the method return type generic?

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