HashMap的自定义实现

Posted 2020-11-17 tenwood

一、背景：　　　　　　　　　　

HashMap到底是怎么实现的？

一对一对的存放，通过key找value；map的键不能重复；自己怎么实现呢？

 

代码：

Wife.java  辅助类

package com.cy.collection;

public class Wife {
    String name;
    
    public Wife(String name){
        this.name = name;
    }

    @Override
    public String toString() {
        return "Wife [name=" + name + "]";
    }
}

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Map.java:

package com.cy.collection;

/**
 * 自定义实现Map
 */
public class Map {
    private Entry[] arr = new Entry[1000];    //这里暂时不考虑扩容
    private int size;                        //这里都是private的，不暴露size属性
    
    /**
     * 1.键不能重复,如果含有这个键，就替换value
     * @param key
     * @param value
     */
    public void put(Object key, Object value){
        for(int i=0; i<size; i++){
            if(arr[i].key.equals(key)){
                arr[i].value = value;
                return;
            }
        }
        arr[size++] = new Entry(key, value);
    }
    
    //根据key获取
    public Object get(Object key){
        for(int i=0; i<size; i++){
            if(arr[i].key.equals(key)){
                return arr[i].value;
            }
        }
        return null;
    }
    
    //根据key删除
    public boolean remove(Object key){
        boolean success = false;
        for(int i=0;i<size;i++){
            if(arr[i].key.equals(key)){
                success = true;
                remove(i);
            }
        }
        return success;
    }
    private void remove(int i){
        int numMoved = size - i - 1;
        if(numMoved>0){
            System.arraycopy(arr, i+1, arr, i, numMoved);
        }
        arr[--size] = null;        //Let gc do its work
    }
    
    //containsKey
    public boolean containsKey(Object key){
        for(int i=0; i<size; i++){
            if(arr[i].key.equals(key)){
                return true;
            }
        }
        return false;
    }
    
    //containsValue 同containsKey
    
    //size
    public int size(){
        return size;
    }
}

/**
 * 用来存放键值对的条目
 */
class Entry{
    Object key;
    Object value;
    
    public Entry(Object key, Object value) {
        super();
        this.key = key;
        this.value = value;
    }
}

Test.java测试代码：

package com.cy.collection;

public class Test {

    public static void main(String[] args) {
        Map map = new Map();
        map.put("张三", new Wife("abc"));
        map.put("李四", new Wife("def"));
        map.put("王五", new Wife("ghi"));
        System.out.println(map.get("张三"));
        map.remove("李四");
        System.out.println(map.size());
        
        map.put("张三", new Wife("aaa"));
        System.out.println(map.get("张三"));
        System.out.println(map.containsKey("张三"));
    }

}

输出：
Wife [name=abc]
2
Wife [name=aaa]
true

View Code