Java中HashMap、TreeMap与Comparable、Comparator的关联使用

﹏ヽ暗。殇╰゛Y 2022-05-26 02:40 168阅读 0赞

我们知道，在Map接口中，HashMap类存储元素是按照元素的key的hashcode存放的，HashMap里面存入的键值对在取出的时候是随机的,它根据键的HashCode值存储数据,根据键可以直接获取它的值，具有很快的访问速度。在Map 中插入、删除和定位元素，HashMap 是最好的选择。但是如果想要使元素按照自定义的顺序排序，就要用到TreeMap了。

TreeMap不是按照hashcode存放的，而是按照实现的comparable接口的compareTo这个方法来存储的，只要compareTo的返回结果为0就表示两个对象相等，那么就存不进去两个对象，后put的会把前面的覆盖掉。

1、通过实现comparable接口排序

先创建TestPerson类

import java.util.Objects;
    
    public class TestPerson implements Comparable{
    	private String name;
    	private Integer age;
    	TestPerson(String name,int age){
    		this.name = name;
    		this.age = age;
    	}
    	public String getName(){
    		return this.name;
    	}
    	public int getAge(){
    		return this.age;
    	}
    	@Override 
    	public boolean equals(Object o){
    		if(o instanceof TestPerson){
    			TestPerson p = (TestPerson)o;
    			return this.name.equals(p.name) && this.age==p.age;
    		}
    		else
    			return false;
    	}
        @Override  
        public int hashCode() {  
            return Objects.hash(name, age);  
        }
    	@Override
    	public int compareTo(Object o) {
    		// TODO Auto-generated method stub
    		TestPerson tp = (TestPerson)o;
    		return this.age > tp.getAge() ? 1 : this.age == tp.getAge() ? 0 : -1;
    	}
    	@Override
    	public String toString() {
    		// TODO Auto-generated method stub
    		return name + "(" + age.toString() + ")";
    	}
    }

测试用例：

import java.util.Map;
    import java.util.TreeMap;
     
    public class TreeMapTest {
    	public static void main(String[] args) {
    		Map<TestPerson,String> m = new TreeMap<TestPerson,String>();
    		m.put(new TestPerson("Jack Ma",51),"Alibaba");
    		m.put(new TestPerson("Jackey Cheng",60), "YaolaiYingCheng");
    		m.put(new TestPerson("Jay Chou",35),"YeHuimei");
    		m.put(new TestPerson("Ma Huateng",53),"Tencent");
    		System.out.println(m);
    	}	
    }

输出：

\{Jay Chou(35)=YeHuimei, Jack Ma(51)=Alibaba, Ma Huateng(53)=Tencent, Jackey Cheng(60)=YaolaiYingCheng\}

2、通过实现comparator接口实现排序

前面的例子是按年龄排序，本例按姓名排序

import java.util.Comparator;
    import java.util.Objects;
    
    public class TestPerson implements Comparator<TestPerson>{
    	private String name;
    	private Integer age;
    	public TestPerson(String name,int age){
    		this.name = name;
    		this.age = age;
    	}
    	public TestPerson(){
    		
    	}
    	public String getName(){
    		return this.name;
    	}
    	public int getAge(){
    		return this.age;
    	}
    	@Override 
    	public boolean equals(Object o){
    		if(o instanceof TestPerson){
    			TestPerson p = (TestPerson)o;
    			return this.name.equals(p.name) && this.age==p.age;
    		}
    		else
    			return false;
    	}
        @Override  
        public int hashCode() {  
            return Objects.hash(name, age);  
        }
    	@Override
    	public String toString() {
    		// TODO Auto-generated method stub
    		return name + "(" + age.toString() + ")";
    	}
    	@Override
    	public int compare(TestPerson p1, TestPerson p2) {
    		// TODO Auto-generated method stub
    		//return p1.getAge()>p2.getAge()? 1 : p1.getAge() == p2.getAge()? 0:-1;
    		return p1.getName().compareTo(p2.getName());
    	}
    }

测试用例

import java.util.Map;
    import java.util.TreeMap;
     
    public class TreeMapTest {
    	public static void main(String[] args) {
    		Map<TestPerson,String> m = new TreeMap<TestPerson,String>(new TestPerson());
    		m.put(new TestPerson("Jack Ma",51),"Alibaba");
    		m.put(new TestPerson("Jackey Cheng",60), "YaolaiYingCheng");
    		m.put(new TestPerson("Jay Chou",35),"YeHuimei");
    		m.put(new TestPerson("Ma Huateng",53),"Tencent");
    		System.out.println(m);
    	}	
    }

输出

\{Jack Ma(51)=Alibaba, Jackey Cheng(60)=YaolaiYingCheng, Jay Chou(35)=YeHuimei, Ma Huateng(53)=Tencent\}