摘要

本博客主要讲述了Java的HashMap的底层实现

HashMap的底层原理

底层原理：数组+链表


过程总结：每一个Object的有一个哈希值，通过hashCode()函数获取哈希值，再通过自定义的hash()函数，得到一个值，也就是数组的下标。数组中的每个元素都是一个链表或为空。

哈希值转换为数组下标

//这就是hash函数，val就是key的哈希值，即val = key.hashCode()
//length 必须是2的整数幂
private int  hash(int val, int length){return val & (length - 1);}

节点

定义链表中的节点

public class Node2 {int hash;//hash对应数组下标Object key;Object value;Node2 next;
}

初始化

//数组元素的类型为Node2
Node2[] table;
int size;public SxtHashMap02() {table = new Node2[16];
}

put(Object key, Object value)

public void put(Object key, Object value){Node2 newNode = new Node2();newNode.hash = hash(key.hashCode(),table.length);newNode.key = key;newNode.value = value;newNode.next = null;Node2 last = null;//这个学习一下，记录最后一个节点int index = hash(key.hashCode(),table.length);if(table[index] == null){table[index] = newNode;size ++;}else{Node2 tmp = table[index];while(tmp != null){if(key.equals(tmp.key)){System.out.println("key重复了");tmp.value = value;return;}else {last = tmp;tmp = tmp.next;}}last.next = newNode;size ++;//size的增加与减少不要忘记}}

重写toString()

public String toString() {StringBuilder sb = new StringBuilder();sb.append("[");for(int i = 0; i < table.length; i ++){Node2 temp = table[i];while(temp != null){sb.append(temp.key + ":" + temp.value + ",");temp = temp.next;}}//这个套路学一下，将最后改为']'sb.setCharAt(sb.length() - 1,']');return sb.toString();
}	

这个toString()有什么用呢？在使用system.out.println()打印的时候，就会用到toString()。

get(Object key)

//根据Map的底层原理，就十分简单
public Object get(Object key){int hashCode = key.hashCode();int hash = hash(hashCode,table.length);Node2 temp = table[hash];while(temp != null){if(temp.key.equals(key)) return temp.value;temp = temp.next;}return null;
}

增加泛化

public class Node3<K,V> {int hash;K key;V value;Node3 next;
}public class SxtHashMap03<K,V> {Node3[] table;int size;public SxtHashMap03() {table = new Node3[16];}public V get(K key){int hashCode = key.hashCode();int hash = hash(hashCode,table.length);V value = null;Node3 temp = table[hash];while(temp != null){if(temp.key.equals(key)){value = (V)temp.value;}temp = temp.next;}return value;}public void put(K key, V value){Node3 newNode = new Node3();newNode.hash = hash(key.hashCode(),table.length);newNode.key = key;newNode.value = value;newNode.next = null;Node3 last = null;int index = hash(key.hashCode(),table.length);if(table[index] == null){table[index] = newNode;size ++;}else{Node3 tmp = table[index];while(tmp != null){if(key.equals(tmp.key)){System.out.println("key重复了");tmp.value = value;return;}else {last = tmp;tmp = tmp.next;}}last.next = newNode;size ++;}}
}

remove(K key)

 public void remove(K key){int index = hash(key.hashCode(), table.length);Node3 temp = table[index];if(temp == null) return;if(temp.key.equals(key)){table[index] = temp.next;size --;return;}Node3 last = null;while(temp != null){if(temp.key.equals(key)){last.next = temp.next;size --;return;}last = temp;temp = temp.next;}
}

参考: 手工实现HashMap