Java Pass by Value or Pass by Reference

In Java, it is always pass by value. I have been refreshing Java concepts lately to become a better programmer by understanding the fundamentals and to write clean code. I am not a fan of writing one liner unless it is really required. My main intention is to make sure that my code is understandable for me after a few months. Lets dive in.

Passing a primitive

Consider the below code block where x is being passed to two methods: one with return and another one with void.

As Java is always pass by value – the void method will print 20 inside the method. But the method which has the return of int will print 30.

• modifyXWithReturn(x) ? returns a new value (3 * 10 = 30), but the caller must explicitly use or assign it.

This shows primitives are passed by value

int x = 10;
System.out.println("X " + x);
modifyX(x);
System.out.println("X " + x);
System.out.println(modifyXWithReturn(x));

 private static int modifyXWithReturn(int x) {
       return 3 * x;
    }

private static void modifyX(int x) {
      x = 2 * x;
      System.out.println(x);
}

Passing an Array (int[] arr1)

• Because arrays are objects, the reference value is copied into the method. Both caller and method point to the same array object.
• Mutations inside the method are visible outside:

int[] arr1 = {1, 2, 3};
System.out.println("Before arr1 in main " + Arrays.toString(arr1));
modifyArr1(arr1);
System.out.println("After arr1 in main " + Arrays.toString(arr1));

 private static void modifyArr1(int[] arr1) {
        for (int i = 0; i < arr1.length; ++i) {
            arr1[i] = 2 * arr1[i];
        }
        System.out.println("Arr1 in modifyArr1 " + Arrays.toString(arr1));
    }

Before arr1 in main [1, 2, 3]
Arr1 in modifyArr1 [2, 4, 6]
After arr1 in main [2, 4, 6]

This shows object references are passed by value (the reference itself is copied, but both copies point to the same object).

Passing an Array Element (arr2[0])

int[] arr2 = {7, 8, 9};
System.out.println("Before arr2 in main " + Arrays.toString(arr2));
int newArr2Element = modifyArr2Element(arr2[0]);
System.out.println("After arr2 in main " + Arrays.toString(arr2));
System.out.println(newArr2Element);

private static int modifyArr2Element(int n) {
        System.out.println("Arr2 0th element is " + (n * 2));
        return n * 2;
}

• Inside modifyArr2Element, you print 14 and return it.
• The original array remains unchanged:

Before arr2 in main [7, 8, 9]
Arr2 0th element is 14
After arr2 in main [7, 8, 9]
14

This shows passing a primitive extracted from an array does not mutate the array.

Key Takeaway

• Java is always pass by value.
• For primitives the actual value is copied.
• For objects the reference value is copied. Mutations are visible, but reassignment isn’t.