Question

I know that with the following, a reference is made

public class MyClass
{
  public Integer value;
}

public class Main
{
  public static void main( String[] args )
  {
    MyClass john = new MyClass();
    john.value = 10;
    MyClass bob = john;
    bob.value = 20;

    System.out.println(Integer.toString(john.value));  // Should print value of "20"
  }
}

But how do you do similar referencing with primitive data-types?

public class Main
{
  public static void main( String[] args )
  {
    Integer x = 30;
    Integer y = x;
    y = 40;
    System.out.println(Integer.toString(x));  // Prints "30". I want it to print "40"
  }
}

Answer 1

Simple answer: you don’t. Primitive values are always passed by value (i.e. they are copied).

Wrapper objects like Integer are also immutable, i.e. y = 40 will create a new Integer object with the value 40 and assign it to y.

To achieve what you want you need a container object whose value you can change.

You could, for example, use AtomicInteger:

AtomicInteger x = new AtomicInteger(30);
AtomicInteger y = x;
y.set( 40 );

System.out.println(x.get());