How to use references in Java?

Question

I want to use reference in Java but I don't know how! for example in C++ we write:

void sum(int& x)
{
    ...
}

but in Java & sign is a compiler error! please help me to understand references in Java.

Answer 1

Objects are passed by reference by default Objects are accessed by reference, but there is no way to create a reference to a primitive value (byte, short,int, long). You either have to create an object to wrap the integer or use a single element array.

public void sum(int[] i){
   i[0] = ...;
}

or

public void sum(MyInt i){
   i.value = ...;
}
public class MyInt{
   public int value; 
}

for your example something like the following could work

public int sum(int v){
   return ...;
}

or

public int sum(){
   return ...;
}

Update:

Additional/Better description of object references:

Java Objects are always accessed by a reference. Like the primitive types this reference is passed by value (e.g. copied). Since everything a programmer can access in java is passed by copying it (references, primitives) and there is no way to create a reference to a primitive type, any modification to a method parameter (references, primitives) only affects the local copy within the method. Objects can be modified within a method since both copies of the reference (local and other) still point to the same object instance.

example:

Modify a primitive within method, this only affects the internal copy of i and not the passed value.

void primitive(int i){
  i = 0;
}

Modify a reference within method, this only affects the internal copy of ref and not the passed value.

 void reference(Object ref){
    ref = new Object();//points to new Object() only within this method
 }

Modify an object, visible globally

void object(List l){
   l.add(new Object());//modifies the object instead of the reference
}

Both the array and MyInt above are based on the modification of an object.