How can I pass an Integer class correctly by reference?

Question

I am hoping that someone can clarify what is happening here for me. I dug around in the integer class for a bit but because integer is overriding the

+

operator I could not figure out what was going wrong. My problem is with this line:

Integer i = 0;

i = i + 1;  // ← I think that this is somehow creating a new object!

Here is my reasoning:
I know that java is pass by value (or pass by value of reference), so I think that in the following example the integer object should be incremented each time.

public class PassByReference {



    public static Integer inc(Integer i) {

        i = i+1;    // I think that this must be **sneakally** creating a new integer...  

        System.out.println("Inc: "+i);

        return i;

    }



    public static void main(String[] args) {

        Integer integer = new Integer(0);

        for (int i =0; i<10; i++){

            inc(integer);

            System.out.println("main: "+integer);

        }

    }

}

This is my expected output:



Inc: 1

main: 1

Inc: 2

main: 2

Inc: 3

main: 3

Inc: 4

main: 4

Inc: 5

main: 5

Inc: 6

main: 6

...

This is the actual output.



Inc: 1

main: 0

Inc: 1

main: 0

Inc: 1

main: 0

...

Why is it behaving like this?

Recommended for you: Get network issues from WhatsUp Gold. Not end users.

Answer 1

There are two problems:

Java is pass by value, not by reference. Changing the reference inside a method won't be reflected into the passed-in reference in the calling method.
Integer is immutable. There's no such method like Integer#set(i). You could otherwise just make use of it.

To get it to work, you need to reassign the return value of the inc() method.

integer = inc(integer);

To learn a bit more about passing by value, here's another example:

public static void main(String... args) {
    String[] strings = new String[] { "foo", "bar" };
    changeReference(strings);
    System.out.println(Arrays.toString(strings)); // still [foo, bar]
    changeValue(strings);
    System.out.println(Arrays.toString(strings)); // [foo, foo]
}
public static void changeReference(String[] strings) {
    strings = new String[] { "foo", "foo" };
}
public static void changeValue(String[] strings) {
    strings[1] = "foo";
}

How can I pass an Integer class correctly by reference?

Sign up