翔の學習淺談

在收集物件之後，對物件進行排序是常用的動作，你不用親自實作排序演算法，java.util.Collections提供有sort()方法，由於必須有索引才能進行排序，因此Collections的sort()方法接受List實作物件。

實際開發總是不斷有意外，如果你的物件無法實作Comparable呢？也許你拿不到原始碼！也許你不能修改原始碼！舉個例子來說，String本身有實作Comparable，所以你可以如下排序：

    public static void main(String[] args) {
        List words = Arrays.asList("B", "X", "A", "M", "F", "W", "O");
        Collections.sort(words);
        System.out.println(words);
    }

Out:

[A, B, F, M, O, W, X]

Comparator用在對象外面，相當於定義了一套排序演算法來排序。

實做Comparator

●假設員工預設是以員工編號排序，但下列情況

     --請以職稱排序:

import java.util.Comparator;

public class Employee implements Comparator<Employee> {
private int Id;
private String Name;

    @Override
    public int compare(Employee o1, Employee o2) {
        return o1.getName().compareTo(o2.getName());
     //return (o1.getName().compareTo(o2.getName()))*-1;     //順序會顛倒
    }

    public int getId() {
        return Id;
    }

    public String getName() {
        return Name;
    }

    public Employee() {
    }

    public Employee(int Id, String Name) {
        this.Id = Id;
        this.Name = Name;
    }

    @Override
    public String toString() {
        return "ID:" + this.getId() + " Name:" + this.getName();
    }   
}
 

public class Test2 {
    public static void main(String[] args) {
        List<Employee> words = Arrays.asList(new Employee(1,"B"),new Employee(2,"A"));
        Collections.sort(words, new Employee());
        System.out.println(words);
  
    }
}

Out:

[ID:2 Name:A, ID:1 Name:B]

參考資料: https://openhome.cc/Gossip/Java/ComparableComparator.html

以中文名字排序:

import java.text.Collator;
import java.util.Comparator;
import java.util.Locale;

public class Employee implements Comparator<Employee> {
private int Id;
private String Name;

    @Override
    public int compare(Employee o1, Employee o2) {
            Locale locale = new Locale(
            System.getProperty("user.language"),//ZH 中華音譯
            System.getProperty("user.country"));//TW 台灣
            Collator com = Collator.getInstance(locale);
            return com.compare(o1.getName(), o2.getName());
    }

    public int getId() {
        return Id;
    }

    public String getName() {
        return Name;
    }

    public Employee() {
    }

    public Employee(int Id, String Name) {
        this.Id = Id;
        this.Name = Name;
    }

    @Override
    public String toString() {
        return "ID:" + this.getId() + " Name:" + this.getName();
    }   
}
 

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class Test2 {
    public static void main(String[] args) {
        List<Employee> words = Arrays.asList(new Employee(1,"林"),new Employee(2,"王"));
        Collections.sort(words, new Employee());
        System.out.println(words);
    }
}
 

Out:
[ID:2 Name:王, ID:1 Name:林]