java的比較器有兩類，分別是Comparable介面和Comparator介面。

在為物件陣列進行排序時，比較器的作用非常明顯，首先來講解Comparable介面。

讓需要進行排序的物件實現Comparable介面，重寫其中的compareTo(T o)方法，在其中定義排序規則，那麼就可以直接呼叫java.util.Arrays.sort()來排序物件陣列，例項如下：

class Student implements Comparable<Student>{
    private String name;
    private int age;
    private float score;
    
    public Student(String name, int age, float score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }
    
    public String toString()
    {
        return name+"\t\t"+age+"\t\t"+score;
    }

    @Override
    public int compareTo(Student o) {
        // TODO Auto-generated method stub
        if(this.score>o.score)//score是private的，為什麼能夠直接呼叫,這是因為在Student類內部
            return -1;//由高到底排序
        else if(this.score<o.score)
            return 1;
        else{
            if(this.age>o.age)
                return 1;//由底到高排序
            else if(this.age<o.age)
                return -1;
            else
                return 0;
        }
    }
}

public class ComparableDemo01 {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Student stu[]={new Student("zhangsan",20,90.0f),
                new Student("lisi",22,90.0f),
                new Student("wangwu",20,99.0f),
                new Student("sunliu",22,100.0f)};
        java.util.Arrays.sort(stu);
        for(Student s:stu)
        {
            System.out.println(s);
        }
    }
}
 

執行的結果是

sunliu 22 100.0
wangwu 20 99.0
zhangsan 20 90.0
lisi 22 90.0

但是在設計類的時候，可能沒有考慮到讓類實現Comparable介面，那麼就需要用到另外的一個比較器介面Comparator。

從上面的例項我們可以發現，compareTo(T o)只有一個引數，而Comparator介面中必須要實現的compare(T o1,T o2)就有兩個引數。

程式碼例項

package edu.sjtu.ist.comutil;

import java.util.Comparator;

class Student {
    private String name;
    private int age;
    private float score;
    
    public Student(String name, int age, float score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public float getScore() {
        return score;
    }
    public void setScore(float score) {
        this.score = score;
    }

    public String toString()
    {
        return name+"\t\t"+age+"\t\t"+score;
    }

}

class StudentComparator implements Comparator<Student>{

    @Override
    public int compare(Student o1, Student o2) {
        // TODO Auto-generated method stub
        if(o1.getScore()>o2.getScore())
            return -1;
        else if(o1.getScore()<o2.getScore())
            return 1;
        else{
            if(o1.getAge()>o2.getAge())
                return 1;
            else if(o1.getAge()<o2.getAge())
                return -1;
            else 
                return 0;
        }
    }
    
}


public class ComparableDemo02 {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Student stu[]={new Student("zhangsan",20,90.0f),
                new Student("lisi",22,90.0f),
                new Student("wangwu",20,99.0f),
                new Student("sunliu",22,100.0f)};
        java.util.Arrays.sort(stu,new StudentComparator());
        for(Student s:stu)
        {
            System.out.println(s);
        }
    }

}
 

程式執行的結果和上面是一樣的

Array.sort是對陣列進行排序，假如我們不想使用陣列，想使用Collection介面下的集合，如想使用List，那麼需要稍微做些修改：

package comparatorTest;

/**
 * 定義一個學生類
 * 包括學號，姓名，數學成績，語文成績
 * @author zhangnan
 *
 */

public class Student{
	private String Name;
	private int ID;
	private int scoreMath;
	private int scoreChi;
	public Student (String name,int ID,int score1,int score2){
		this.Name=name;
		this.ID=ID;
		this.scoreMath=score1;
		this.scoreChi=score2;
	}
	public String getName(){
		return this.Name;
	}
	public void setName(String pname){
		this.Name=pname;
	}
	public int getID(){
		return this.ID;
	}
	public void setID(int pID){
		this.ID=pID;
	}
	public int getMathScore(){
		return scoreMath;
	}
	public void setMathScore(int score1){
		this.scoreMath=score1;
	}
	public float getChiScore(){
		return scoreChi;
	}
	public void setChiScore(int score2){
		this.scoreChi=score2;
	}
	/**
	 * 返回學生資訊
	 */
	public String   toString(){
		return 	Integer.toString(ID)+"\t\t"+Name+"\t\t"+Integer.toString(scoreMath)+"\t\t"+Integer.toString(scoreChi);
	}
	
	
}
--------------------------------------------------------------------------------
package comparatorTest;

import java.util.Comparator;

public class ComparatorSort implements Comparator {

	public int compare(Student s1, Student s2) {
		// TODO Auto-generated method stub
		if (s1.getID() > s2.getID()) {
			return 1;
		} else if (s1.getID() < s2.getID()) {
			return -1;
		} else {
			if (s1.getMathScore() > s2.getMathScore())
				return -1;
			else if (s1.getMathScore() < s2.getMathScore())
				return 1;
			else
				return 0;
		}

	}

}
--------------------------------------------------------------------------------
package comparatorTest;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Random;

public class test {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Random random=new Random();
		ArrayList st=new ArrayList();
		Student s1= new Student("zhangnan1",random.nextInt(10),random.nextInt(100),random.nextInt(100));
		Student s2= new Student("zhangnan2",random.nextInt(10),random.nextInt(100),random.nextInt(100));
		Student s3= new Student("zhangnan3",random.nextInt(10),random.nextInt(100),random.nextInt(100));
		Student s4= new Student("zhangnan4",random.nextInt(10),random.nextInt(100),random.nextInt(100));
		st.add(s1);
		st.add(s2);
		st.add(s3);
		st.add(s4);
		System.out.println("全部的學生：");
		Collections.sort(st,new ComparatorSort());
		for(Student s:st){
			
			System.out.println(s);
		}

	}

}

 

全部的學生：

0zhangnan3 6778

0zhangnan2 390

2zhangnan1 5796

3zhangnan4 6253