如何按列值对 2D ArrayList 进行排序，如果值相等怎么办？

Question

I want to sort two-dimensional ArrayList by their specific column values and if the column values of two arrays are equal then compare the two arrays using another column value.

For example:

array = [[2, 3, 4, 5][0, 2, 3, 4][0, 1, 2, 3][1, 2, 3, 4]]

I want to compare them first with their index [0] , but since array[1][0] and array[2][0] have the same value 0 I want to compare these two arrays by their index [3] .

So, the new array would be:

[[0, 1, 2, 3][0, 2, 3, 4][1, 2, 3, 4][2, 3, 4, 5]]

I have this code

public ArrayList<List<Integer>> sortArray(ArrayList<List<Integer>> spaces) {
    final Comparator<List<Integer>> comparator = new Comparator<List<Integer>>() {
        @Override
        public int compare(List<Integer> sList1, List<Integer> sList2) {
            return sList1.get(0).compareTo(sList2.get(0));
        }
    };

    Collections.sort(spaces, comparator);

    return spaces;
}

However, this only compares the value by the index [0] , and no condition when the values are equal.

Answer 1

I think you can use the Java Collections.List's compareTo method (Let's abbreviate CT). Here are some example outputs of CT. 5.CT(7) -> -1; 7.CT(5) -> 1; 5.CT(5) -> 0; compareTo returns 0 when they're the same, so you simply don't swap their positions. I'm not sure if I'm answering the question correctly though.

Answer 2

You need to compare all items in sub lists. For example:

public int compare(List<Integer> sList1, List<Integer> sList2) {
    for (int i = 0; i < 4; i++) {
        int c = sList1.get(i).compareTo(sList2.get(i));
        if (c != 0)
            return c;
    }
    return 0;
}

Not suggested if you are looking for a fast algorithm.

Answer 3

You need to modify your Comparator to handle that index check something like this:

public int compare(List<Integer> sList1, List<Integer> sList2) {
    if (sList1.get(0) != sList2.get(0)) {
        return sList1.get(0).compareTo(sList2.get(0));
    } else {
        return sList1.get(3).compareTo(sList2.get(3));
    }
}

But if you are running Java 8 and above you can directly do something like this to sort your collection:

Collections.sort(spaces, (sList1, sList2) -> {
    if (sList1.get(0) != sList2.get(0)) {
        return sList1.get(0).compareTo(sList2.get(0));
    } else {
        return sList1.get(3).compareTo(sList2.get(3));
    }
});

For pre-Java 8 you can do similar to what you are already doing:

Collections.sort(spaces, new Comparator<List<Integer>>() {
    @Override
    public int compare(List<Integer> sList1, List<Integer> sList2) {
        if (sList1.get(0) != sList2.get(0)) {
            return sList1.get(0).compareTo(sList2.get(0));
        } else {
            return sList1.get(3).compareTo(sList2.get(3));
        }
    }
});

In both cases, the output will be:

[[0, 1, 2, 3], [0, 2, 3, 4], [1, 2, 3, 4], [2, 3, 4, 5]]