将Integer的引用传递给Java中的方法

Question

I've been reading this topic: Is Java "pass-by-reference" or "pass-by-value"? , and I've tried to run example from this answer: https://stackoverflow.com/a/73021/2838739 . In fact value of myDog has been changed outside this method. Then I have tried to do the same with an Integer. I was surprised because its value has not been changed. Could someone explain me why?

My test code is:

   package testtest;

class Dog {

    String name;

    public Dog(String name) {
        this.name=name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

public class Main {

    public static void main(String[] args) {

        Dog myDog = new Dog("Rover");
        System.out.println(myDog.getName());
        foo(myDog);
        System.out.println(myDog.getName());

        Integer i = 5;
        changeValue(i);
        System.out.println(i);
    }

    public static void changeValue(Integer i) {
        i = 50;
    }
    public static void foo(Dog someDog) {
        someDog.setName("Max");     // AAA
        someDog = new Dog("Fifi");  // BBB
        someDog.setName("Rowlf");   // CCC
    }

}

And output was:

Rover

Max

5

Thank you in advance.

Answer 1

Integer is not a mutable class. (There is no set method on it.) You can point an old variable to a different Integer, but that doesn't change the Integer that you passed to the method.

Answer 2

Integer is a primitive data type. You cannot change them inside methods.

If you pass an object to a method you can change its accessible member variables.

In Java there are special wrapper classes for primitive types like Integer for int, Character for char etc, and even though they are like objects you cannot change their values inside methods either. That's the only exception that I know.

Answer 3

As pointed out already primitive types are passed by value. But if you really need to pass primitives by reference you can use an array.

public class PassByRefMain {

    public static void main(String[] args) {
        PassByRefMain m = new PassByRefMain();
        int[] number = new int[]{5};
        System.out.println(number[0]);
        m.changevalue(number);
        System.out.println(number[0]);  
    }

    public void changevalue(int [] number){
        number[0] = 9;
    }
}

The output will be

5
9