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)
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