java - CompareTo 和 collections.sort() 错误

Question

我在使用 collections.sort 时遇到问题，因为它没有对任何内容进行排序。我有两个类，其中一个类有一个比较日期的compareTo 方法，另一个类有一个对日期进行排序的Collections.sort 方法。我的代码编译并运行没有错误，但 ArrayList 未排序，有人可以告诉我我做错了什么以及为什么 Collections.sort 不起作用。

这是代码:

public class CalendarDate implements Comparable<CalendarDate>{ 

    private int month;
    private int day;

    //Constructor
    public CalendarDate(int month, int day){
        this.month = month;
        this.day = day;
    }

    //CompareTo method
    public int CompareTo(CalendarDate other){
        if(month != other.month){
             return month - other.month;
        } 
        else {
            return day - other.day;
        }
    }


    //Getters
    public int getMonth(){
        return month;
    }

    public int getDay(){
        return day;
    }

    //toString method
    public String toString(){
        return month + "/" + day;
    }

public int compareTo(CalendarDate other) {
    return 0;
}
}

<小时/>

import java.util.*;


public class CalendarDateSort {
public static void main(String[] args){

    ArrayList<CalendarDate> list = new ArrayList<CalendarDate>();

    list.add(new CalendarDate(1, 11)); 
    list.add(new CalendarDate(5, 15 )); 
    list.add(new CalendarDate(3, 8)); 
    list.add(new CalendarDate(7, 10)); 
    list.add(new CalendarDate(3, 16)); 


    System.out.println("dates before sorting = " + list);
    Collections.sort(list);
    System.out.println("dates after sorting = " + list);        
}

}

Answer 1

您进行比较的 CompareTo 方法有一个拼写错误，并且名称不同。 Java 标识符区分大小写。实际调用的 compareTo 始终返回 0(表示所有元素都相等)。

public class CalendarDate implements Comparable<CalendarDate>{

    // doesn't get called by sort
    public int CompareTo(CalendarDate other){
        if(month != other.month){
             return month - other.month;
        } 
        else {
            return day - other.day;
        }
    }

    // gets called by sort
    public int compareTo(CalendarDate other) {
        return 0;
    }
}

使用@Override注释。如果犯这种错误，将会产生编译错误。

另请参阅Overriding and Hiding Methods .

When overriding a method, you might want to use the @Override annotation that instructs the compiler that you intend to override a method in the superclass. If, for some reason, the compiler detects that the method does not exist in one of the superclasses, then it will generate an error.