Java中Comparator的使用 (后续补充）

1、 s1.compareTo(s2) 实际上是比较的ascii码

public class CompareTest {
    public static void main(String[] args) {
        ArrayList<Integer> list= new ArrayList<Integer>();
        String s1 = "1";
        String s2 = "2";
        System.out.println(s1.compareTo(s2));//相当于s1-s2; }输出如下图
}

1）String s1 = "1"; String s2 = "2"; 长度一致时，输出-1；

当String s1 = "1"; String s2 = "3";长度一致时，输出-2；

分析可知，是用对应的ascii字符对比计算 

1对应的ascii 是49 ，3对应的是51 比较第一个字符

1.1  Collections.sort默认是升序排列

1.2 Comparator中的compareTo方法默认是与Collections.sort一致的，从小到大排列

1.3改写Comparator中的compareTo方法

变成：

return o2.compareTo(o1);  

输出结果为从大到小。[56, 23, 5, 4, 2, 1]

另外可以发现    o1.compareTo(o2) 中o1与o2在数组中的位置，o1表示后加进来的元素，o2表示原来的元素.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

public class CompareTest {
    public static void main(String[] args) {
        ArrayList<Integer> list= new ArrayList<Integer>();

  int [] numbers={1,4,2,5,23,56};
       int len = numbers.length;
        for(int i=0;i<len;i++){
            list.add(numbers[i]);
        }
/*       Collections.sort(list);
        for(int a: list)   // printing the sorted list of names
            System.out.println(a);*/

        Collections.sort(list, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o1.compareTo(o2);
            }
        });
        System.out.println(list);

        }

}

2、纠正这篇博客：Java方法学习之路：compareTo()方法

 

3、compareTo()函数说明翻译

Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this

object is less than, equal to, or greater than the specified object.
为排序将对象与指定的对象进行比较，根据比较的结果是小于，等于或者是大于而返回一个负数，零或者正数。

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 sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception

iff y.compareTo(x) throws an exception.)

在前述中，符号sgn(expression)指出：数学函数必须根据表达式expression的值是负数，0，或者正数而返回一个-1,0,或者是1。实现该接口

者必须确保对于任意的对象x,y都有x.compareTo(y)) == -sgn(y.compareTo(x)成立。（这暗示如果x.compareTo(y)会抛出异常的话，那么

x.compareTo(x)，也必须抛出一个异常。）

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

(z)>0
实现者必须确保关系是传递的：即(x.compareTo(y)>0 && y.compareTo(z)>0)意味着x.compareTo(z)>0成立。

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

z.

最后，实现者必须确保x.compareTo(y)==0可以得到对于任意的z可以使得sgn(x.compareTo(z)) == sgn(y.compareTo(z))成立

It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any

class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended

language is "Note: this class has a natural ordering that is inconsistent with equals."

虽然不是非常的需要，但是必须强调的是，(x.compareTo(y)==0) ==(x.equals(y))。一般说来，任何实现了Compareable接口，但是违背了这

种情况的类必须清楚的指出这一点。指出的语句如****释，该类具有和equals（）方法不同的自然排序方法。

4、Java中Comparator的使用

 

另外可以参考：java comparator 升序、降序、倒序从源码角度理解