爲什麼我不能排序這個ArrayList？

Question

我從我的教科書中複製了一個例子，但它拒絕編譯。我在某處做了一個錯字嗎？出於某種原因，在客戶端代碼上，Collections.sort（單詞）不允許程序編譯。任何幫助表示讚賞。代碼從Stuart Reges和Marty Stepp的「Building Java Programs」第2版複製而來。我試圖通過複製來理解它。

該程序應該創造一個CalendarDate對象放入一個ArrayList。通過實現CalendarDate的Comparable接口，我可以使用Collections.sort在ArrayList中排序生日。但是，這不起作用b/c Collections.sort（日期）將不會運行。

客戶端代碼（包含發行）：

import java.util.*; 

// Short program that creates a list of birthdays of the 
// first 5 U.S. Presidents and that puts them into sorted order. 
// We can now use Collections.sort for ArrayList<CalendarDate> b/c CalendarDate implements the Comparable interface. 

public class CalendarDateTest { 
    public static void main(String[] args) { 
     ArrayList<CalendarDate> dates = new ArrayList<CalendarDate>(); // Creates a new ArrayList of 'CalendarDate' object type. 

     // adds a new CalendarDate object with month = 2 and day = 22 into an element of ArrayList dates, and etc. 
     dates.add(new CalendarDate(2, 22)); // Washington 
     dates.add(new CalendarDate(10, 30)); //Adams 
     dates.add(new CalendarDate(4, 13)); // Jefferson 
     dates.add(new CalendarDate(3, 16)); // Madison 
     dates.add(new CalendarDate(4, 28)); // Monroe 

     System.out.println("birthdays = " + dates); // Before sorting 
     Collections.sort(dates); // WHY WON'T THIS WORK? 
     System.out.println("birthdays = " + dates); // After Sorting 
    } 

} 

的CalendarDate對象類：

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; 
    } 

    // Compares this calendar date to another date 
    // Dates are compared by month and then by day 
    public int compareTo(CalendarDate other) { 
     if (month != other.month) { // If different months 
      return month - other.month; //negative, positive, or zero 
     } else { // If same months; compare days instead 
      return day - other.day; // negative, positive, or zero 
     } 
    } 

    // Accessor for month (b/c month is private) 
    public int getMonth() { 
     return this.month; 
    } 

    // Accessor for day (b/c day is private) 
    public int getDay() { 
     return this.day; 
    } 

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

可比接口：

public interface Comparable<T> { // T is the generic type (a placeholder for when other classes implement this) 
     public int compareTo(T other); // placeholder to be implemented; need more specific version into class implementing this. 
    } 

編譯器錯誤：

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    Bound mismatch: The generic method sort(List<T>) of type Collections is not applicable for the arguments (ArrayList<CalendarDate>). The inferred type CalendarDate is not a valid substitute for the bounded parameter <T extends Comparable<? super T>> 

    at CalendarDateExample.CalendarDateTest.main(CalendarDateTest.java:21) 

Answer 1

不要定義你自己的Comparable接口。您需要implementjava.lang.Comparable。

Answer 2

從你的問題，這些是主要部分：

However, that's not working b/c Collections.sort(dates) will not run.

Collections.sort(dates); // WHY WON'T THIS WORK? 

public interface Comparable<T> { // T is the generic type (a placeholder for when other classes implement this) 
    public int compareTo(T other); // placeholder to be implemented; need more specific version into class implementing this. 
} 

Collections#sort將在List<T>在哪裏工作T實現java.lang.Comparable。看起來你正在創建你自己的Comparable界面，這是行不通的。刪除界面Comparable的定義，然後重試。注意：您不需要導入java.lang.Comparable接口，因爲它屬於java.lang包，此包中的類是由Java編譯器自動添加的。