自己手撸一个hashMap

自定义Map接口

package com.example.myhashmap.service;

public interface MyMap {

    public V put(K k,V v);
    public V get(K k);

    interface MyEntry{
        public K getKey();
        public V getValue();
        public void setValue(V value);
        MyEntry next();
    }

}

自定义一个接口 接口中定义了自定义的Map具备的两个方法 put(),get() 并定义了一个内部接口Entry

实现接口

实现MyMap接口

实现接口方法中主要定义了几个常量，分别是默认初始化长度 阈值比例

package com.example.myhashmap.service.impl;

import com.example.myhashmap.service.MyMap;
import com.example.myhashmap.service.entry.Entry;

import java.util.ArrayList;
import java.util.List;

public class MyHashMap implements MyMap {
    //默认初始化长度
    private static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
    //阈值比例
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;


    //默认大小
    private int defaultInitSize;
    //阈值比例
    private float defaultLoadFactor;
    //map中entry的数量
    private int entryUserSize;
    //数组
    private Entry[] table;

    public MyHashMap() {
        this.defaultInitSize = DEFAULT_INITIAL_CAPACITY;
        this.defaultLoadFactor = DEFAULT_LOAD_FACTOR;
        table = new Entry[this.defaultInitSize];
    }

    public MyHashMap(int defaultInitSize, float defaultLoadFactor) throws IllegalAccessException {
        if (defaultInitSize <= 0) {
            throw new IllegalAccessException("defaultInitSize init error" + defaultInitSize);
        }
        if (defaultLoadFactor <= 0) {
            throw new IllegalAccessException("defaultLoadFactor init error" + defaultLoadFactor);
        }
        this.defaultInitSize = defaultInitSize;
        this.defaultLoadFactor = defaultLoadFactor;

        table = new Entry[this.defaultInitSize];
    }

    @Override
    public V put(K k, V v) {
        V oldValue = null;
        //是否需要扩容 扩容完成是否需要重新散列
        if (entryUserSize >= defaultInitSize * defaultLoadFactor) {
            resize(2 * defaultInitSize);
        }
        //得到hash值 计算出数组中的位置
        int index = hash(k) & (defaultInitSize - 1);
        if (table[index] == null) {
            table[index] = new Entry<>(k, v, null);
        } else {
            //遍历链表
            Entry entry = table[index];
            Entry e = entry;
            while (e != null) {
                if (k == e.getKey() || k.equals(e.getKey())) {
                    oldValue = e.getValue();
                    e.setValue(v);
                    return oldValue;
                }
                e = e.next();
            }
            table[index] = new Entry<>(k, v, entry);
        }
        ++entryUserSize;
        return null;
    }

    @Override
    public V get(K k) {
        int index = hash(k) & (defaultInitSize - 1);
        if (table[index] == null) {
            return null;
        } else {
            Entry entry = table[index];
            do {
                if (k == entry.getKey() || k.equals(entry.getKey())) {
                    return entry.getValue();
                }
                entry = entry.next();
            } while (entry != null);

        }
        return null;
    }

    //取hash值
    private int hash(K k) {
        int hashCode = k.hashCode();
        hashCode ^= (hashCode >>> 20) ^ (hashCode >>> 12);
        return hashCode ^ (hashCode >>> 7) ^ (hashCode >>> 4);
    }

    //扩容
    private void resize(int i) {
        Entry[] newTable = new Entry[i];
        defaultInitSize = i;
        entryUserSize = 0;
        rehash(newTable);
    }


    private void rehash(Entry[] newTable) {
        //得到原来老的集合 遍历单链表
        List> entryList = new ArrayList<>();
        for (Entry entry : this.table) {
            if (entry != null) {
                do {
                    entryList.add(entry);
                    entry = entry.next();
                } while (entry != null);
            }
        }

        //覆盖旧的引用
        if (newTable.length > 0) {
            this.table = newTable;
        }

        //重新put entry 到 hashMap
        for (Entry entry : entryList) {
            put(entry.getKey(), entry.getValue());
        }

    }

}

实现内部Entry接口

定义了map的数据结构 其中next为发生hash冲突时使用单链表进行存储

package com.example.myhashmap.service.entry;

import com.example.myhashmap.service.MyMap;

public class Entry implements MyMap.MyEntry {

    private K key;
    private V value;
    private Entry next;

    public Entry(K key,V value,Entry next){
        this.key = key;
        this.value =value;
        this.next = next;
    }

    @Override
    public Entry next(){
        return this.next;
    }

    @Override
    public K getKey() {
        return key;
    }

    @Override
    public V getValue() {
        return value;
    }

    @Override
    public void setValue(V value) {
        this.value = value;
    }
}

实现方法中的get方法与put方法的解析

put方法

 public V put(K k, V v) {
        V oldValue = null;
        //是否需要扩容 扩容完成是否需要重新散列
        if (entryUserSize >= defaultInitSize * defaultLoadFactor) {
            resize(2 * defaultInitSize);
        }
        //得到hash值 计算出数组中的位置
        int index = hash(k) & (defaultInitSize - 1);
        if (table[index] == null) {
            table[index] = new Entry<>(k, v, null);
        } else {
            //遍历链表
            Entry entry = table[index];
            Entry e = entry;
            while (e != null) {
                if (k == e.getKey() || k.equals(e.getKey())) {
                    oldValue = e.getValue();
                    e.setValue(v);
                    return oldValue;
                }
                e = e.next();
            }
            table[index] = new Entry<>(k, v, entry);
        }
        ++entryUserSize;
        return null;
    }

hash方法

    //取hash值
    private int hash(K k) {
        int hashCode = k.hashCode();
        hashCode ^= (hashCode >>> 20) ^ (hashCode >>> 12);
        return hashCode ^ (hashCode >>> 7) ^ (hashCode >>> 4);
    }

resize扩容方法

 //扩容
    private void resize(int i) {
        Entry[] newTable = new Entry[i];
        defaultInitSize = i;
        entryUserSize = 0;
        rehash(newTable);
    }


    private void rehash(Entry[] newTable) {
        //得到原来老的集合 遍历单链表
        List> entryList = new ArrayList<>();
        for (Entry entry : this.table) {
            if (entry != null) {
                do {
                    entryList.add(entry);
                    entry = entry.next();
                } while (entry != null);
            }
        }

        //覆盖旧的引用
        if (newTable.length > 0) {
            this.table = newTable;
        }

        //重新put entry 到 hashMap
        for (Entry entry : entryList) {
            put(entry.getKey(), entry.getValue());
        }

    }

get方法

    @Override
    public V get(K k) {
        //获取hash值 与put 中的计算方法一致
        int index = hash(k) & (defaultInitSize - 1);
        //直接获取数组中的指定位置的值
        if (table[index] == null) {
            return null;
        } else {
            //如果指定位置为链表的话 继续遍历
            Entry entry = table[index];
            do {
                if (k == entry.getKey() || k.equals(entry.getKey())) {
                    return entry.getValue();
                }
                entry = entry.next();
            } while (entry != null);

        }
        return null;
    }

测试方法

package com.example.myhashmap.service.test;


import com.example.myhashmap.service.impl.MyHashMap;

public class Test {

    public static void main(String[] args) {
        MyHashMap myMap = new MyHashMap<>();

        for (int i = 0; i < 500; i++) {
            myMap.put("key"+i,"value"+i);
        }

        for (int i = 0; i <500 ; i++) {
            System.out.println(myMap.get("key"+i));
        }
    }

}