[FIXED] How to Increment a class Integer references value in java from another method

Issue

package myintergertest;

/**
 *
 * @author Engineering
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        //this one does not increment 
        Integer n = new Integer(0);
        System.out.println("n=" + n);
        Increment(n);
        System.out.println("n=" + n);
        Increment(n);
        System.out.println("n=" + n);
        Increment(n);
        System.out.println("n=" + n);
        Increment(n);

        //this one will increment
        MyIntegerObj myInt = new MyIntegerObj(1);
        Increment(myInt);
        System.out.println("myint = " + myInt.get());
        Increment(myInt);
        System.out.println("myint = " + myInt.get());
        Increment(myInt);
        System.out.println("myint = " + myInt.get());

    }

    public static void Increment(Integer n) {
        //NO.  this doesn't work because a new reference is being assigned
        //and references are passed by value in java
        n++;
    }

    public static void Increment(MyIntegerObj n) {
        //this works because we're still operating on the same object
        //no new reference was assigned to n here.
        n.plusplus();   //I didn't know how to implement a ++ operator...
    }
}

The result for all of those is n=0. Integer n is an object and therefore passed by reference, so why isn't the increment reflected back in the caller method (main)? I expected output to be n=0 n=1 n=2 etc...

UPDATE: Notice I updated the code example above. If I'm understanding correctly, Jon Skeet answered the question of why myInt would increment and why n does not. It is because n is getting a new reference assigned in the Increment method. But myInt does NOT get assigned a new reference since it's calling a member function.

Does that sound like I understand correctly lol ?

Solution

No, objects aren't passed by reference. References are passed by value - there's a big difference. Integer is an immutable type, therefore you can't change the value within the method.

Your n++; statement is effectively

n = Integer.valueOf(n.intValue() + 1);

So, that assigns a different value to the variable n in Increment - but as Java only has pass-by-value, that doesn't affect the value of n in the calling method.

EDIT: To answer your update: that's right. Presumably your "MyIntegerObj" type is mutable, and changes its internal state when you call plusplus(). Oh, and don't bother looking around for how to implement an operator - Java doesn't support user-defined operators.

Answered By - Jon Skeet

Answer Checked By - Terry (PHPFixing Volunteer)