为什么这两个列表不相等？

Question

I was ask this java question in an interview:

    List a = new ArrayList();
    a.add(a);

    List b = new ArrayList();
    b.add(b);

    if (a.equals(b)) {
        System.out.println("equal");
    } else {
        System.out.println("non-equal");
    }

why the result is 'non-equal' ? thanks!

Answer 1

Your code (after the edit) results in StackOverflowException because a contains itself and b contains itself.

a.equals(b) will iterate the lists in parallel and compare each element, so a.get(0).equals(b.get(0)) , but a.get(0) is a and b.get(0) is b and comparing those will recurse the equals method, ad infinitum.

Answer 2

List a = new ArrayList();
a.add(a);

List b = new ArrayList();
a.add(b);

Look carefully. a has now two items (a and b) and b has no items. So a is not equal to b.

Answer 3

[THIS ANSWER REFERS TO THE POST BEFORE THE EDIT]

The .equals method compares the content of the two array lists. (Not if they are the same object)

If you look carefully after you use the method .add , the List a has two elements ( a and b ), instead of b which has nothing

The code doesn't go in java.lang.StackOverflowError , because the .equals method checks first the size of the two lists, and then the content.

In fact if you try this code:

    List a = new ArrayList();
    a.add(a);
    List b = new ArrayList();
    b.add(b); // <-watch carefully here. Now a and b are of the same size
    if (a.equals(b)) { // checks the size first, then the objects in the lists
        System.out.println("equal");
    } else {
        System.out.println("non-equal");
    }

you get the java.lang.StackOverflowError