Java: object assign

Question

I though in java, when you do object1=object2, you will copy the reference of object2 to object1, so object1 and object2 point" to the same object. Is it right?

I wrote a function to sort a stack.

import java.util.Stack;

public class sortStack {

    public static void main(String[] str) {
        Stack<Integer> s = new Stack<Integer>();
        s.push(2);
        s.push(21);
        s.push(43);
        s.push(3);
        s.push(87);
        s.push(2);
        s.push(12);
        s.push(10);
        s.push(25);
        sortStack(s);

        while (!s.isEmpty()) {
            System.out.print(s.pop() + " ");
        }
    }

    public static void sortStack(Stack<Integer> src) {
        Stack<Integer> dst = new Stack<Integer>();
        Stack<Integer> buf = new Stack<Integer>();//buffer

        while (!src.isEmpty()) {
            int v = src.pop();
            if (dst.isEmpty()) {
                dst.push(v);
            }

            while (!dst.isEmpty() && dst.peek() > v) {
                buf.push(dst.pop());
            }

            dst.push(v);
            while (!buf.isEmpty()) {
                dst.push(buf.pop());
            }
        }
        src = dst;

        //Print:
        //while(!src.isEmpty()){
        //  System.out.print(src.pop()+" ");
        //}
    }
}

I couldn't get any output from the class. If I uncomment the print part, it words fine. I don't understand the s stack is empty after I call the function. I already assign dst to s, so s should point to the dst stack, right?

Please help! Thanks!

A stack is normally used for FILO operations (first-in-last-out). Why not use simple sortable collections like `ArrayList`? — Meno Hochschild, Jan 05 '14 at 06:06
Relevant: Java is not pass-by-reference. It's strictly pass by value, only some values happen to be object references. http://stackoverflow.com/questions/40480/is-java-pass-by-reference — Simplefish, Jan 05 '14 at 06:07
@MenoHochschild The question not allow to use ArrayList. It's like an Interview question. — fuiiii, Jan 05 '14 at 06:10

score 9 · Accepted Answer · answered Jan 05 '14 at 06:04

You are assigning a reference to a parameter, src, which is in fact a local variable, and hence you're not seeing any effect on the original Stack variable, s. Instead have the method return dst and assign the object returned. i.e.,

public static Stack<Integer> sortStack(Stack<Integer> src) {
    // ....

   return dst;
}

and

s = sortStack(s);

Jeff Bowman · Answer 2 · 2014-01-05T06:37:31.877

When you set src=dst you are only overwriting the local copy of the reference within the method. In the process of calling the method local copies are made of each parameter, such that within your method modifying src through its methods would have an effect outside the method but assigning src to a new reference like dst would not.

Your code will work if you return dst in sortStack and put s = sortStack(s) in your calling function.

Java: object assign

2 Answers2