java 比较器在内部如何工作？

Question

class Checker implements Comparator<Player> {
    @Override
    public int compare(Player p1, Player p2) {
        if (p1.score == p2.score) {
            return p1.name.compareTo(p2.name);
        } else {
            return p2.score - p1.score;
        }
    }
}

how does p2.score - p1.score make it descend while p1.score - p2.score makes is ascend

how does a positive or negative integer or zero return sort the objects? whats going on internally

please help thanks

Answer 1

From the Comparator 's documentation :

A comparison function, which imposes a total ordering on some collection of objects. ...

And from Comparator:compare 's documentation :

...

Returns:

a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

Thus, the Comparator only defines the order. It is up to the sorting algorithm to use this information and actually sort the data structure. If we take a look how, eg, Quicksort and Merge Sort work, we see that these algorithms only need to know whether some element a is smaller, equal or larger than b , which is the information that a Comparator provides (by returning a value < 0 , = 0 or > 0 ).

Now let us explain how a - b makes it sort in ascending order (lets give this Comparator the name asc ), while b - a makes it sort in decreasing order ((lets give this Comparator the name desc ): We have to look at three different cases for the two Comparator s.

First, suppose that a > b . Then

• a - b > 0 , thus a is "larger" than b according to asc
• b - a < 0 , thus a is "smaller" than b according to desc

Next, suppose that a < b . Then

• a - b < 0 thus a is "smaller" than b according to asc
• b - a > 0 , thus a is "larger" than b according to desc

Finally, suppose that a == b . Then a - b == b - a == 0 and the elements are "equal" according to both Comparator s.