Keeping a SortedSet of objects based on a property

Question

I have an object, Test, that has two properties, double x and double y. I want to add these objects to a SortedSet, keeping the set sorted in ASC order on x of Test. If two instances of Test have the same x values, I want them to be sorted within the set by their y values.

I thought the following would do the trick:

private SortedSet<Test> tests = new TreeSet<Test>(new Comparator<Test>() {

@Override
public int compare(Test o1, Test o2) {
    if (o1.getXpos() < o2.getXpos()) {
        return -1;
    }
    if (o1.getXpos() > o2.getXpos()) {
        return 1;
    }
    if (o1.getXpos() == o2.getXpos()) {
        if (o1.getYpos() < o2.getYpos()) {
            return -1;
        }
        if (o1.getYpos() > o2.getYpos()) {
            return 1;
        }
        if (o1.getYpos() == o2.getYpos()) {
            return 0;
        }
    }
    return 0;
}
});

Instead this orders the actual x and y values; i.e.

testA: x=200, y=200,

testB: x=200, y=400

After inserting into tests:

testA: x=200, y=200,

testB: x=400, y=200

Instead of the instances within tests.

Answer 1

your comparator is correct. you've got bigger problems, though, if adding your Test objects to the set changes their member variables, e.g. "testB: x=400, y=200" -> "testB: x=200, y=400". I would guess your problem lies in code you have not included (maybe a botched constructor?).

Answer 2

Have you tried with more than two elements? More than once I've simply sorted things backwards without realizing it until later.

Answer 3

My guess is that comparing the doubles for exact equality using == is potentially the issue. See What's wrong with using == to compare floats in Java?