How to use an output parameter in Java

Question

Could someone please give me some sample code that uses an output parameter in function  I ve tried to Google it but just found it just in functions  I d like to use this output value in another function   The code I am developing intended to be run in Android

User · Answer

As a workaround a generic  ObjectHolder  can be used  See code example below    The sample output is   name  John Doe dob 1953-12-17 name  Jim Miller dob 1947-04-18   so the Person parameter has been modified since it s wrapped in the Holder which is passed by value - the generic param inside is a reference where the contents can be modified - so actually a different person is returned and the original stays as is          show work around for missing call by reference in java     public class OutparamTest             a test class to be used as parameter       public static class Person      public String name       public String dob       public void show           System out println  name    name   ndob   dob                     ObjectHolder  Generic ParameterWrapper        public static class ObjectHolder lt T gt        public ObjectHolder T param         this param param            public T param               ObjectHolder is substitute for missing  out  parameter       public static void setPersonData ObjectHolder lt Person gt  personHolder String name String dob           Holder needs to be dereferenced to get access to content     personHolder param new Person        personHolder param name name      personHolder param dob dob                  show how it works         public static void main String args          Person jim new Person        jim name  Jim Miller       jim dob  1947-04-18       ObjectHolder lt Person gt  testPersonHolder new ObjectHolder jim          modify the testPersonHolder person content by actually creating and returning        a new Person in the  out parameter      setPersonData testPersonHolder  John Doe   1953-12-17        testPersonHolder param show        jim show

User · Answer

Java does not support output parameters  You can use a return value  or pass in an object as a parameter and modify the object

User · Answer

You can either use    return X   this will return only one value    return object   will return a full object  For example your object might include X  Y  and Z values  pass array  arrays are passed by reference  i e  if you pass array of integers  modified the array inside the method  then the original code will see the changes    Example on passing Array   void methodOne      int    arr    1 2 3       methodTwo arr            print arr here   void methodTwo int    arr       for  int i 0  i lt arr length i              arr i   3            This will print out  4 5 6

User · Answer

Thank you  I use passing in an object as a parameter  My Android code is below      String oPerson  null      if  CheckAddress  5556   oPerson                 Toast makeText this                    It s Match      oPerson                                     Toast LENGTH LONG  show               private boolean CheckAddress String iAddress  String oPerson        Cursor cAddress   mDbHelper getAllContacts        String address                   if  cAddress getCount    gt  0            cAddress moveToFirst            while  cAddress isAfterLast      false                address   cAddress getString 2  toString                oPerson   cAddress getString 1  toString                 if iAddress indexOf address     -1                                Toast makeText this                            Person       oPerson                                            Toast LENGTH LONG  show                    System out println oPerson                   cAddress close                    return true                                                else cAddress moveToNext                        cAddress close        return false      The result is     Person   John      It s Match  null   Actually   It s Match  John   Please check my mistake

User · Answer

Java passes by value  there s no out parameter like in C    You can either use return  or mutate an object passed as a reference  by value    Related questions   Does Java have something like C  s ref and out keywords     NO   Is Java pass by reference   NO        Code sample  public class FunctionSample       static String fReturn             return  Hello              static void fArgNoWorkie String s            s    What am I doing         Doesn t  work   Java passes by value            static void fMutate StringBuilder sb            sb append  Here you go               public static void main String   args            String s   null           s   fReturn            System out println s      prints  Hello            fArgNoWorkie s           System out println s      prints  Hello            StringBuilder sb   new StringBuilder            fMutate sb           s   sb toString            System out println s      prints  Here you go              See also   What is meant by immutable  StringBuilder and StringBuffer in Java     As for the code that OP needs help with  here s a typical solution of using a special value  usually null for reference types  to indicate success failure   Instead of   String oPerson  null  if  CheckAddress  5556   oPerson        print oPerson      DOESN T  WORK   Java passes by value  String is immutable     private boolean CheckAddress String iAddress  String oPerson          on search succeeded     oPerson   something     DOESN T  WORK      return true             on search failed     return false      Use a String return type instead  with null to indicate failure   String person   checkAddress  5556    if  person    null       print person      private String checkAddress String address          on search succeeded     return something             on search failed     return null      This is how java io BufferedReader readLine   works  for example  it returns instanceof String  perhaps an empty string    until it returns null to indicate end of  search    This is not limited to a reference type return value  of course  The key is that there has to be some special value s  that is never a valid value  and you use that value for special purposes   Another classic example is String indexOf  it returns -1 to indicate search failure      Note  because Java doesn t have a concept of  input  and  output  parameters  using the i- and o- prefix  e g  iAddress  oPerson  is unnecessary and unidiomatic      A more general solution  If you need to return several values  usually they re related in some way  e g  x and y coordinates of a single Point   The best solution would be to encapsulate these values together  People have used an Object   or a List lt Object gt   or a generic Pair lt T1 T2 gt   but really  your own type would be best   For this problem  I recommend an immutable SearchResult type like this to encapsulate the boolean and String search results   public class SearchResult      public final String name     public final boolean isFound      public SearchResult String name  boolean isFound          this name   name        this isFound   isFound           Then in your search function  you do the following   private SearchResult checkAddress String address         on address search succeed   return new SearchResult foundName  true            on address search failed   return new SearchResult null  false       And then you use it like this   SearchResult sr   checkAddress  5556    if  sr isFound      String name   sr name              If you want  you can  and probably should  make the final immutable fields non-public  and use public getters instead

User · Answer

This is not accurate ---        pass array  arrays are passed by reference  i e  if you pass array of integers  modified the array inside the method   Every parameter type is passed by value in Java  Arrays are object  its object reference is passed by value   This includes an array of primitives  int  double     and objects   The integer value is changed by the methodTwo   but it is still the same arr object reference  the methodTwo   cannot add an array element or delete an array element  methodTwo   cannot also  create a new array then set this new array to arr  If you really can pass an array by reference  you can replace that arr with a brand new array of integers   Every object passed as parameter in Java is passed by value  no exceptions

User · Answer

Wrap the value passed in different classes that might be helpful doing the trick  check below for more real example      class Ref lt T gt        T s       public void set T value           s    value             public T get            return s             public Ref T value            s   value            class Out lt T gt        T s       public void set T value           s    value            public T get            return s             public Out              public static void doAndChangeRefs  Ref lt String gt  str  Ref lt Integer gt  i  Out lt String gt  str2         refs passed    set value     str set  def        i set 10          out param passed as null    instantiate and set      str2   new Out lt String gt         str2 set  hello world      public static void main String args              Ref lt Integer gt   iRef   new Ref lt Integer gt  11           Out lt String gt  strOut   null           doAndChangeRefs new Ref lt String gt   test    iRef  strOut           System out println iRef get             System out println strOut get

[java] How to use an output parameter in Java?

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