Java中Comparable与Comparator的区别

1、Comparable

如果想让一个类的实例之间可以相互比较的，那么这个类就必须要实现Comparable接口，并且覆盖compareTo()方法

public class Main1{
    public static void main(String[] args){
        HDTV tv1 = new HDTV(32,"Changhong");
        HDTV tv2 = new HDTV(35,"Skyworth");
        System.out.println(tv1.compareTo(tv2));
    }
}

class HDTV implements Comparable<HDTV>{
    private int size;
    private String brand;
    
    public HDTV(int size,String brand){
        this.size = size;
        this.brand = brand;
    }
    
    public int getSize(){
        return this.size;
    }
    
    public void setSize(int size){
        this.size = size;
    }
    
    public String getBrand(){
        return this.brand;
    }
    
    public void setBrand(String brand){
        this.brand = brand;
    }
    
    @Override
    public int compareTo(HDTV tv){
        if(this.getSize()>tv.getSize())
            return 1;
        else if(this.getSize()<tv.getSize())
            return -1;
        else
            return 0;
    }
    
    
}

2、Comparator

在某些情况下，你可能不想让这个类具有比较性，在这样的案例中，你可能需要比较类中的某些字段；例如，2个人之间可以比较他们的身高和年龄；

这样的情况下，你就需要用到Comparator接口，并且实现compare()方法；使用Comparator的另外一个共同特性就是排序，Collections和Arrays工具类提供的接受比较器参数的排序方法

import java.util.*;
public class Main2{
    public static void main(String[] args){
        HPTV tv1 = new HPTV(36,"SamSung");
        HPTV tv2 = new HPTV(21,"Sony");
        HPTV tv3 = new HPTV(28,"Dell");
        
        ArrayList<HPTV> list = new ArrayList<HPTV>();
        list.add(tv1);
        list.add(tv2);
        list.add(tv3);
        
        Collections.sort(list, new BrandComparator());
        
        for(HPTV tv: list)
            System.out.println(tv.getSize()+"--"+tv.getBrand());
    }
    
}
class HPTV{
    private int size;
    private String brand;
    
    public HPTV(int size,String brand){
        this.size = size;
        this.brand = brand;
    }
    
    public int getSize(){
        return this.size;
    }
    
    public void setSize(int size){
        this.size = size;
    }
    
    public String getBrand(){
        return this.brand;
    }
    
    public void setBrand(String brand){
        this.brand = brand;
    }
    
}

class SizeComparator implements Comparator<HPTV>{
    @Override
    public int compare(HPTV tv1,HPTV tv2){
        int tv1Size = tv1.getSize();
        int tv2Size = tv2.getSize();
        if(tv1Size>tv2Size)
            return 1;
        else if(tv1Size<tv2Size)
            return  -1;
        else 
            return 0;
    }
}
class BrandComparator implements Comparator<HPTV>{
    @Override
    public int compare(HPTV tv1, HPTV tv2){
        String nm1 = tv1.getBrand();
        String nm2 = tv2.getBrand();
        return nm1.compareTo(nm2);
    }
}

有时，我们经常用到 Collections.reverseOrder()方法，这个方法对list进行反转

3、使用的时候怎么选择呢？

Comparable：如果一个类实现了Comaprable接口，那么这个类的实例之间可以互相比较；

Comparator：继承 Comparator 接口有两种情况

　　1）作为参数传递给排序方法：Collections.sort() or Arrays.sort()，精确的控制排序规则

　　2）精确的排序控制一些固定的数据结构，TreeSet和TreeMap

案例-TreeSet-Comparator

class Dog {
    int size;
 
    Dog(int s) {
        size = s;
    }
}
 
class SizeComparator implements Comparator<Dog> {
    @Override
    public int compare(Dog d1, Dog d2) {
        return d1.size - d2.size;
    }
}
 
public class ImpComparable {
    public static void main(String[] args) {
        TreeSet<Dog> d = new TreeSet<Dog>(new SizeComparator()); // pass comparator
        d.add(new Dog(1));
        d.add(new Dog(2));
        d.add(new Dog(1));
    }
}

案例-TreeSet-Comparable

class Dog implements Comparable<Dog>{
    int size;
 
    Dog(int s) {
        size = s;
    }
 
    @Override
    public int compareTo(Dog o) {
        return o.size - this.size;
    }
}
 
public class ImpComparable {
    public static void main(String[] args) {
        TreeSet<Dog> d = new TreeSet<Dog>();
        d.add(new Dog(1));
        d.add(new Dog(2));
        d.add(new Dog(1));
    }
}