比较器报错：Comparison method violates its general contract

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;
}
}
    
    

主题代码

    
    
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);
}

}

}
    
    

  

转自: https://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;
}
}
  
  

主题代码

  
  
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);
}

}

}