比較器和可比

Question

我通過教程看如何比較和可比的作品，但我難倒了以下內容：ComparatorMain類調用

Collections.sort(listOfCountries,new Comparator<Country>() { 

        @Override 
        public int compare(Country o1, Country o2) { 
         return o1.getCountryName().compareTo(o2.getCountryName()); 
        } 
       }); 

但的compareTo（）的邏輯方法似乎是在做一個比較countryId，not countryName：

package org.arpit.javapostsforlearning; 
//If this.cuntryId < country.countryId:then compare method will return -1 
//If this.countryId > country.countryId:then compare method will return 1 
//If this.countryId==country.countryId:then compare method will return 0 
public class Country implements Comparable{ 
    int countryId; 
    String countryName; 

    public Country(int countryId, String countryName) { 
     super(); 
     this.countryId = countryId; 
     this.countryName = countryName; 
    } 

    @Override 
    public int compareTo(Object arg0) { 
     Country country=(Country) arg0; 
     return (this.countryId < country.countryId) ? -1: (this.countryId > country.countryId) ? 1:0 ; 
    } 

    public int getCountryId() { 
     return countryId; 
    } 

    public void setCountryId(int countryId) { 
     this.countryId = countryId; 
    } 

    public String getCountryName() { 
     return countryName; 
    } 

    public void setCountryName(String countryName) { 
     this.countryName = countryName; 
    } 

} 

那麼這怎麼可能起作用呢？

這裏是整個ComparatorMain類：

package org.arpit.javapostsforlearning; 

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

public class ComparatorMain { 

    /** 
    * @author Arpit Mandliya 
    */ 
    public static void main(String[] args) { 
     Country indiaCountry=new Country(1, 'India'); 
     Country chinaCountry=new Country(4, 'China'); 
     Country nepalCountry=new Country(3, 'Nepal'); 
     Country bhutanCountry=new Country(2, 'Bhutan'); 

      List<Country> listOfCountries = new ArrayList<Country>(); 
      listOfCountries.add(indiaCountry); 
      listOfCountries.add(chinaCountry); 
      listOfCountries.add(nepalCountry); 
      listOfCountries.add(bhutanCountry); 

      System.out.println('Before Sort by id : '); 
      for (int i = 0; i < listOfCountries.size(); i++) { 
       Country country=(Country) listOfCountries.get(i); 
       System.out.println('Country Id: '+country.getCountryId()+'||'+'Country name: '+country.getCountryName()); 
      } 
      Collections.sort(listOfCountries,new CountrySortByIdComparator()); 

      System.out.println('After Sort by id: '); 
      for (int i = 0; i < listOfCountries.size(); i++) { 
       Country country=(Country) listOfCountries.get(i); 
       System.out.println('Country Id: '+country.getCountryId()+'|| '+'Country name: '+country.getCountryName()); 
      } 

      //Sort by countryName 
      Collections.sort(listOfCountries,new Comparator<Country>() { 

       @Override 
       public int compare(Country o1, Country o2) { 
        return o1.getCountryName().compareTo(o2.getCountryName()); 
       } 
      }); 

      System.out.println('After Sort by name: '); 
      for (int i = 0; i < listOfCountries.size(); i++) { 
       Country country=(Country) listOfCountries.get(i); 
       System.out.println('Country Id: '+country.getCountryId()+'|| '+'Country name: '+country.getCountryName()); 
      } 
    } 

} 

教程： http://www.javacodegeeks.com/2013/03/difference-between-comparator-and-comparable-in-java.html

Answer 1

如果使用

Collections.sort(countries); 

那麼國家將利用自己的自然順序進行排序，即排序由其compareTo()方法定義。他們將因此按ID排序。如果集合包含Comparable對象的實例，則只能按此方式對集合進行排序。

如果使用

Collections.sort(countries, someComparator); 

那麼國家將使用由比較器（someComparator）中定義的順序排序傳入作爲參數。在你的情況下，由於比較器按名稱比較國家，它們將按名稱排序。你可以用這種方法排序任何類型的對象。

因此，簡而言之，傳遞一個比較器就是允許按照不同於該類本身定義的順序來排序的東西。