comparator接口与compare方法的实现

刷leetcodecode时看到一道题需要利用自定义的比较器进行排序，最开始一头雾水，看了API终于懂了~

Arrays.sort(T[] a，Comparator<? super T> c)可以根据比较器的compare方法对数组进行排序，compare方法的不同实现对应着不同的排序准则；

可以看到API中关于compare方法的解释如下：

int compare(T o1,T o2)

Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return one of -1, 0, or 1 according to whether the value of expression is negative, zero or positive.

The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x)) for all x and y. (This implies that compare(x, y) must throw an exception if and only if compare(y, x) throws an exception.)

The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.

Finally, the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.

It is generally the case, but not strictly required that (compare(x, y)==0) == (x.equals(y)). Generally speaking, any comparator that violates this condition should clearly indicate this fact. The recommended language is "Note: this comparator imposes orderings that are inconsistent with equals."

参数：

o1 - the first object to be compared.

o2 - the second object to be compared. 

也就是说compare方法根据其返回值确定比较对象的大小，如果返回值为正，认为o1>o2;返回值为负，认为o1<o2;返回值为0，认为两者相等；

public class SortByComparator{
    //升序排列
    public static void sortAscend(Integer[] a){
        Arrays.sort(a, new Comparator<Integer>() {
        public int compare(Integer a, Integer b) {
                    return a.compareTo(b);
                }
           });
    }
   //降序排列，如果b.compareTo(a)>0,则compare方法根据其返回值认为a>b,与自然比较结果相反
    public static void sortDescend(Integer[] a){
        Arrays.sort(a, new Comparator<Integer>() {
        public int compare(Integer a, Integer b) {
                    return b.compareTo(a);
                }
           });
    }
    public static void main(String[] args) {
        Integer[] a={1,2,3};
        //升序排列
        sortAscend(a);
        for(int num:a)
            System.out.println(num);
        //降序排列
        sortDescend(a);
        for(int num:a)
            System.out.println(num);

    }
}
            

输出

1
2
3
3
2
1

注：Arrays.sort(T[] a，Comparator<? super T> c)默认为升序排列