How to do the equivalent of pass by reference for primitives in Java

Question

This Java code   public class XYZ          public static void main              int toyNumber   5             XYZ temp   new XYZ              temp play toyNumber             System out println  Toy number in main     toyNumber                void play int toyNumber             System out println  Toy number in play     toyNumber              toyNumber              System out println  Toy number in play after increement     toyNumber                     will output this     Toy number in play 5   Toy number in play after increement 6   Toy number in main 5     In C   I can pass the toyNumber variable as pass by reference to avoid shadowing i e  creating a copy of the same variable as below   void main          int toyNumber   5        play toyNumber         cout  lt  lt   Toy number in main    lt  lt  toyNumber  lt  lt  endl       void play int  amp toyNumber         cout  lt  lt   Toy number in play    lt  lt  toyNumber  lt  lt  endl         toyNumber          cout  lt  lt   Toy number in play after increement    lt  lt  toyNumber  lt  lt  endl          and the C   output will be this    Toy number in play 5   Toy number in play after increement 6   Toy number in main 6     My question is - What s the equivalent code in Java to get the same output as the C   code  given that Java is pass by value rather than pass by reference

User · Answer

Java is not call by reference it is call by value only  But all variables of object type are actually pointers   So if you use a Mutable Object you will see the behavior you want  public class XYZ        public static void main String   arg            StringBuilder toyNumber   new StringBuilder  5            play toyNumber           System out println  Toy number in main     toyNumber              private static void play StringBuilder toyNumber            System out println  Toy number in play     toyNumber           toyNumber append     1            System out println  Toy number in play after increement     toyNumber             Output of this code   run  Toy number in play 5 Toy number in play after increement 5   1 Toy number in main 5   1 BUILD SUCCESSFUL  total time  0 seconds    You can see this behavior in Standard libraries too  For example Collections sort    Collections shuffle    These methods does not return a new list but modifies it s argument object       List lt Integer gt  mutableList   new ArrayList lt Integer gt          mutableList add 1       mutableList add 2       mutableList add 3       mutableList add 4       mutableList add 5        System out println mutableList        Collections shuffle mutableList        System out println mutableList        Collections sort mutableList        System out println mutableList     Output of this code   run   1  2  3  4  5   3  4  1  5  2   1  2  3  4  5  BUILD SUCCESSFUL  total time  0 seconds

User · Answer

You cannot pass primitives by reference in Java  All variables of object type are actually pointers  of course  but we call them  references   and they are also always passed by value   In a situation where you really need to pass a primitive by reference  what people will do sometimes is declare the parameter as an array of primitive type  and then pass a single-element array as the argument  So you pass a reference int 1   and in the method  you can change the contents of the array

User · Answer

Make a  class PassMeByRef   public int theValue      then pass a reference to an instance of it  Note that a method that mutates state through its arguments is best avoided  especially in parallel code

User · Answer

You have several choices   The one that makes the most sense really depends on what you re trying to do   Choice 1  make toyNumber a public member variable in a class  class MyToy     public int toyNumber      then pass a reference to a MyToy to your method   void play MyToy toy         System out println  Toy number in play     toy toyNumber          toy toyNumber          System out println  Toy number in play after increement     toy toyNumber          Choice 2  return the value instead of pass by reference  int play int toyNumber         System out println  Toy number in play     toyNumber          toyNumber          System out println  Toy number in play after increement     toyNumber          return toyNumber     This choice would require a small change to the callsite in main so that it reads  toyNumber   temp play toyNumber     Choice 3  make it a class or static variable  If the two functions are methods on the same class or class instance  you could convert toyNumber into a class member variable   Choice 4  Create a single element array of type int and pass that  This is considered a hack  but is sometimes employed to return values from inline class invocations   void play int    toyNumber         System out println  Toy number in play     toyNumber 0           toyNumber 0           System out println  Toy number in play after increement     toyNumber 0

User · Answer

public static void main String   args        int   toyNumber   new int    5       NewClass temp   new NewClass        temp play toyNumber       System out println  Toy number in main     toyNumber 0       void play int   toyNumber       System out println  Toy number in play     toyNumber 0        toyNumber 0         System out println  Toy number in play after increement     toyNumber 0

User · Answer

For a quick solution  you can use AtomicInteger or any of the atomic variables which will let you change  the value inside the method using the inbuilt methods  Here is sample code   import java util concurrent atomic AtomicInteger    public class PrimitivePassByReferenceSample                    param args             public static void main String   args             AtomicInteger myNumber   new AtomicInteger 0           System out println  MyNumber before method Call     myNumber get             PrimitivePassByReferenceSample temp   new PrimitivePassByReferenceSample             temp changeMyNumber myNumber           System out println  MyNumber After method Call     myNumber get                   void changeMyNumber AtomicInteger myNumber            myNumber getAndSet 100               Output   MyNumber before method Call 0  MyNumber After method Call 100

[java] How to do the equivalent of pass by reference for primitives in Java

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