If Java is pass by value, how does this swap method work?

Question

OK so I understand that Java is pass by value (including for References, it passes the Object by the VALUE of the reference - i.e. the address).

If so then why does the swap(int[] array) method - passing an array of primitives - work?

public class TestArraySwap {

    public static void swap(int a, int b) {
        int temp =b;
        b=a;
        a=temp;
    }

    public static void swap(int[]a) {
        int temp = a[1];
        a[1]=a[0];
        a[0]=temp;
    }

    public static void main(String[] args) {
        //Test Primitive swap
        int a=1;
        int b=2;
        System.out.println("a="+a +" b="+b);
        swap(a,b);
        System.out.println("a="+a +" b="+b);

        //Test array swap
        int[] array = new int[2];
        array[0]=1;
        array[1]=2;
        System.out.println("a[0]="+array[0] +" a[1]="+array[1]);
        swap(array);
        System.out.println("a[0]="+array[0] +" a[1]="+array[1]);
    }
}

The output is

a=1 b=2
a=1 b=2
a[0]=1 a[1]=2
a[0]=2 a[1]=1

The second swap works - I would expect it not to but perhaps I am missing something?

Answer 1

The main method has a reference to an array:

R1 ------> [1, 2]

Then it calls the swap() method. So a copy of the reference to the same array is created and used by the swap() method:

 R1 -----> [1, 2]
             ^
             |
 R2 ---------|

Then the swap() method changes the content of the array:

 R1 -----> [2, 1]
             ^
             |
 R2 ---------|

And it returns. R1 still references the same array, whose content has been changed.

Answer 2

The swap works because you pass the address (reference) of the array to the method. The method then operates on that array -- the same one the other method has a reference to.

I really don't like the statement, "Java is pass by value." I think it's really confusing.

---

 public void methodA(){
     int[] values = {1,2}; 
    // MethodA stack frame has a register that contains a 
    //   memory address (let's say it's 0x1F) that points to a contiguous section of 
    //   memory sized for an integerarray of size 2. 
     methodB(values); // Pass the **memory address** (aka REFERENCE) not the register address
     System.out.println(values[0]); // Prints "100"
 } 

 public void methodB(int[] ints){
     // MethodB stack frame now has a register with the memory address (0x1F)
     //    of 'ints' which is the same address that MethodA stack frame has.
     //    The same REFERENCE
     ints[0] = 100; // Alter the value in mem of 0x1F+0
     ints = new int[2]; // The local register overwrites the memory address of 
                        // the REFERENCE value that was there with 
                        // a new memory address -- say 0x5A
                        // since it's not storing it in the same register 
                        // as MethodA, MethodA will not reflect this.


 }

Answer 3

After some more searching I found this answer Array seems to be getting passed by reference in Java, how is this possible?

and of course the Java spec itself

In the Java programming language, arrays are objects

Answer 4

Everything in Java is passed by value (by copy).

swap(int[] a) works, because it passes (copies) only an address of the beginning of an array. Later it directly modifies memory under that address (a[0]) and the next one (a[1]).

swap(int a, int b) doesn't work, because it modifies copies of integers.

Answer 5

References in Java are something pointing to an object in heap. Arrays in Java are always allocated in heap. So arrays in Java are like normal objects.

References to objects in Java are stored in stack like primitive variables (int, long), it's a memory address stored in variable. So when they say that objects are passed by value, the literal meaning is that you copy address value into method parameters.

And for me this has always been confusing because though I do understand the process, I can't recall how it's properly named as

• there are no references in Java
• I'm always stuck when I hear the word 'value' applied to an object