Question

Code

class Test {
    public static void main(String args[]) {
        StringBuffer a = new StringBuffer("A");
        StringBuffer b = new StringBuffer("B");
        modify(a, b);
        System.out.println(a + " " + b);
    }

    public static void modify(StringBuffer a, StringBuffer b) {
        a.append(b);
        a = b;
        System.out.println(a + " " + b);
    }
}

I understand the print statement in function modify and I also know StringBuffer class modifies String inplace therefore a.append(b) makes String refer to “AB”.

My question is how can String a be changed to “AB” outside the function modify but statement a=b has no impact outside function modify. Basically, when is variable passed by value, when by reference?

Answer 1

Here’s a simple picture:

main                            modify

StringBuffer <------a           a
     ^                          |
     +--------------------------+

StringBuffer <------b           b
     ^                          |
     +--------------------------+

In main, both a and b are references that point to separate StringBuffer instances. When main calls modify, it passes copies of the references a and b (pass by value). modify can change the contents of the StringBuffer instances, but if it changes the values of a and b, it operates only on its own local copies and does not affect what main‘s a and b point to.

The basic answer is that everything is passed by value, but when passing objects it’s the reference that is passed (by value), not the object itself.