sixtyfootersdude sixtyfootersdude - 2 months ago 7
Java Question

How can I pass an Integer class correctly by reference?

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?

Answer

There are two problems:

  1. 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.
  2. 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";
}