按两个参数自定义排序无法正常工作

Question

public List<List<Integer>> demo(int[][] buildings) {
    List<List<Integer>> tracker = new ArrayList<>();
    int pointer = 0;
    for (int i = 0; i < buildings.length; i++) {
        tracker.add(Arrays.asList(buildings[i][0], -buildings[i][2]));
        tracker.add(Arrays.asList(buildings[i][1], buildings[i][2]));
    }
    Collections.sort(tracker, (a, b) -> {
        if (a.get(0) != b.get(0)) {
            return a.get(0).compareTo(b.get(0));
        } else {
            return a.get(1).compareTo(b.get(1));
        }
    });
    return tracker;
}

When I input:

[[1,10001,10000],[2,10001,9999],[3,10001,9998]]

the output I get is:

[[1,-10000],[2,-9999],[3,-9998],[10001,10000],[10001,9999],[10001,9998]]

This is not the output that I expect, because my sorting rule is supposed to be when 10001 == 10001 then the ones with smaller second coordinates should be arranged at first.

Answer 1

I think the issue is Integer caching. For the low numbers, the Integer is cached so != works. For the large numbers, new Integer s are used. Use Integer.equals to check for equality.

Change:

if(a.get(0) != b.get(0))

Into:

if( a.get(0).equals(b.get(0)) )

Otherwise you're comparing the reference not the value.

Another way would be to use the helper methods of the Comparator class.

Comparator.<List<Integer>>comparing( list -> list.get(0) ).thenComparing(list-> list.get(1) );

Answer 2

Answer by matt is spot on.

Here is an alternative way to fix it. Don't compare values twice, ie don't use both equals() and compareTo() :

Collections.sort(tracker, (a, b) -> {
    int cmp = a.get(0).compareTo(b.get(0));
    if (cmp == 0) {
        cmp = a.get(1).compareTo(b.get(1));
    }
    return cmp;
} );

Even easier solution:

Collections.sort(tracker, Comparator.comparing(x -> x.get(0))
                                .thenComparing(x -> x.get(1)));

Depending on the version of the compiler, it may need a "hint" to know what x is. Either of these should do it, neither is better than the other.

Collections.sort(tracker, Comparator.<List<Integer>>comparing(x -> x.get(0))
                                               .thenComparing(x -> x.get(1)));

Collections.sort(tracker, Comparator.comparing((List<Integer> x) -> x.get(0))
                                .thenComparing(x -> x.get(1)));

Answer 3

When comparing object in Java always use equals or compareTo (accordingly).

Instead of:

   if(a.get(0) != b.get(0)){ // <---- THIS IS WRONG
        return a.get(0).compareTo(b.get(0));
    }

You can reused already the compareTo result of:

a.get(0).compareTo(b.get(0));

namely:

    int result = a.get(0).compareTo(b.get(0));
    return result == 0 ? a.get(1).compareTo(b.get(1)) : result;

Moreover instead of Collection.sort you can simply used tracker.sort and replaced the comparison logic with a Comparator chain:

tracker.sort(Comparator.comparingInt((List<Integer> a) -> a.get(0))
                       .thenComparingInt(a -> a.get(1)));

A running example:

public class A {

    public static List<List<Integer>> demo(int[][] buildings) {
        List<List<Integer>> tracker = new ArrayList<>();
        int pointer = 0;
        for (int[] building : buildings) {
            tracker.add(Arrays.asList(building[0], -building[2]));
            tracker.add(Arrays.asList(building[1], building[2]));
        }
        tracker.sort(Comparator.comparingInt((List<Integer> a) -> a.get(0))
               .thenComparingInt(a -> a.get(1)));
        return tracker;
    }


    public static void main(String[] args) {
        int[][] x = new int[][]{{1,10001,10000},{2,10001,9999},{3,10001,9998}};
        List<List<Integer>> demo = demo(x);
        demo.forEach(System.out::println);
    }
}

OUTPUT:

[1, -10000]
[2, -9999]
[3, -9998]
[10001, 9998]
[10001, 9999]
[10001, 10000]