比较器报错：Comparison method violates

转载自：http://blog.csdn.net/fanzitao/article/details/8040201

Brother Zeng遇到的错误：
Java.lang.IllegalArgumentException: Comparison method violates its general contract!
网上查到一个解释：
Description: The sorting algorithm used by java.util.Arrays.sort and (indirectly) by java.util.Collections.sort has been replaced. The new sort implementation may throw an IllegalArgumentException if it detects a Comparable that violates the Comparable contract. The previous implementation silently ignored such a situation. If the previous behavior is desired, you can use the new system property, java.util.Arrays.useLegacyMergeSort, to restore previous mergesort behavior.
也就是说jdk 7的sort函数的实现变了，造成了这个问题，具体原因未知。
改一下系统设置，还是选择使用老版本的排序方法，在代码前面加上这么一句话：System.setProperty("java.util.Arrays.useLegacyMergeSort", "true");

另外复习一下比较器，一个简单的例子：
类代码：

public class Stu {
    public double age;
    public String name;
    public Stu(String name,double age)
    {
        this.name = name;
        this.age = age;
    }
}

测试

import java.util.Collections;
import java.util.Comparator;
import java.util.List;


public class CodeForBrotherZeng {

    /**
     * @param args
     */
    public static void main(String[] args) {
            Stu stu1=new Stu("foyd",22.1);
            Stu stu2=new Stu("teddy",19.1);
            Stu stu3=new Stu("dean",26.1);
            Stu stu4=new Stu("lucas",19.1);
            Stu stu5=new Stu("tina",26.1);
            
            List<Stu> list = new ArrayList<Stu>();
            list.add(stu1);
            list.add(stu2);
            list.add(stu3);
            list.add(stu4);
            list.add(stu5);
            
            Comparator<Stu> comparator = new Comparator<Stu>() {
                public int compare(Stu p1, Stu p2) {//return必须是int，而str.age是double,所以不能直接return (p1.age-p2.age)
                    if((p1.age-p2.age)<0) 
                        return -1;
                    else if((p1.age-p2.age)>0)
                        return 1;
                    else return 0;
                }
            };
            //jdk 7sort有可能报错，
            //加上这句话:System.setProperty("java.util.Arrays.useLegacyMergeSort", "true");
            //表示，使用以前版本的sort来排序
            Collections.sort(list,comparator);
            
            for(int i=0;i<list.size();i++)
            {
                System.out.println(list.get(i).age+"  "+list.get(i).name);
            }
            
    }

}