13、实现自己的HashMap

一时失言乱红尘 2022-07-16 05:54 129阅读 0赞

在java编程语言中，最基本的结构就是两种，一个是数组，另外一个是模拟指针（引用），所有的数据结构都可以用这两个基本结构来构造的，HashMap也不例外。HashMap实际上是一个“链表散列”的数据结构，即数组和链表的结合体。

![20161008132916733][]

自己实现的HashMap代码如下:

package myhashmap;
    
    public class MyHashMap<K, V> {
    	// initialization capacity
    	private int capacity = 10;
    	// total entities
    	private int size = 0;
    	// hash表的数组
    	private Entity<K, V>[] entities = null;
    
    	@SuppressWarnings("unchecked")
    	public MyHashMap() {
    		entities = new Entity[capacity];
    	}
    
    	public void put(K key, V value) {
    		if (key == null) {
    			throw new RuntimeException("The key is null");
    		}
    		reHash();
    		Entity<K, V> newEntity = new Entity<K, V>(key, value);
    		put(newEntity, this.entities, this.capacity);
    	}
    
    	private void put(Entity<K, V> newEntity, Entity<K, V>[] entities,
    			int capacity) {
    		// 实体newEntity要存放在的Entity<K, V>[] entities中的下标
    		int index = newEntity.getKey().hashCode() % capacity;
    		Entity<K, V> entity = entities[index];
    		Entity<K, V> firstEntity = entities[index];
    		// 如果是第一次放进去元素，直接放入
    		if (entity == null) {
    			entities[index] = newEntity;
    			size++;
    		} else {
    			// 这里比较的是 the first entity
    			// Find the same key for ( the first entity ) ,
    			// if find then replace the old value to new value
    			if (newEntity.getKey().equals(entity.getKey())) {
    				newEntity.setNext(entity.getNext());
    				newEntity.setPre(entity.getPre());
    				if (entity.getNext() != null) {
    					entity.getNext().setPre(newEntity);
    				}
    				entities[index] = newEntity;
    			} else if (entity.getNext() != null) {
    				// 这里比较的是 all the next entity
    				// Find the same key for ( all the next entity ),
    				// if find then replace the old value to new value
    				while (entity.getNext() != null) {
    					entity = entity.getNext();
    					if (newEntity.getKey().equals(entity.getKey())) {
    						newEntity.setPre(entity.getPre());
    						newEntity.setNext(entity.getNext());
    						if (entity.getNext() != null) {
    							entity.getNext().setPre(newEntity);
    						}
    						entities[index] = newEntity;
    						return;
    					}
    				}
    				// Cannot find the same key, then insert the new entity at the
    				// header
    				newEntity.setNext(firstEntity);
    				newEntity.setPre(firstEntity.getPre());
    				firstEntity.setPre(newEntity);
    				entities[index] = newEntity;
    				size++;
    			} else {
    				// Cannot find the same key, then put the new entity in head
    				newEntity.setNext(firstEntity);
    				firstEntity.setPre(newEntity);
    				entities[index] = newEntity;
    				size++;
    			}
    		}
    	}
    
    	public V get(K key) {
    		if (key == null) {
    			throw new RuntimeException("The key is null");
    		}
    		int index = key.hashCode() % capacity;
    		Entity<K, V> entity = entities[index];
    		if (entity != null) {
    			if (entity.getKey().equals(key)) {
    				return entity.getValue();
    			} else {
    				entity = entity.getNext();
    				while (entity != null) {
    					if (entity.getKey().equals(key)) {
    						return entity.getValue();
    					}
    					entity = entity.getNext();
    				}
    			}
    
    		}
    		return null;
    	}
    
    	public void remove(K key) {
    		if (key == null) {
    			throw new RuntimeException("The key is null");
    		}
    		int index = key.hashCode() % capacity;
    		Entity<K, V> entity = entities[index];
    		if (entity != null) {
    			if (entity.getKey().equals(key)) {
    				if (entity.getNext() != null) {// remove the first entity
    					entity.getNext().setPre(entity.getPre());
    					entities[index] = entity.getNext();
    					entity = null;
    				} else {// empty this index
    					entities[index] = null;
    				}
    				size--;
    			} else {
    				entity = entity.getNext();
    				while (entity != null) {
    					if (entity.getKey().equals(key)) {
    						if (entity.getNext() != null) {
    							entity.getPre().setNext(entity.getNext());
    							entity.getNext().setPre(entity.getPre());
    							entity = null;
    						} else {
    							// release the found entity
    							entity.getPre().setNext(null);
    							entity = null;
    						}
    						size--;
    						return;
    					}
    					entity = entity.getNext();
    				}
    			}
    		}
    	}
    
    	public String toString() {
    		StringBuilder sb = new StringBuilder();
    		for (int i = 0; i < capacity; i++) {
    			sb.append("index=").append(i).append("[");
    			boolean hasEntity = false;
    			Entity<K, V> entity = entities[i];
    			if (entity != null) {
    				hasEntity = true;
    			}
    			while (entity != null) {
    				sb.append("[").append(entity.getKey()).append("=")
    						.append(entity.getValue()).append("]").append(",");
    				entity = entity.getNext();
    			}
    			if (hasEntity) {
    				sb.deleteCharAt(sb.length() - 1);
    			}
    			sb.append("]\n");
    		}
    		return sb.toString();
    	}
    
    	/**
    	 * Simple re-hash strategy, if the size is bigger than capacity, then do
    	 * re-hash action
    	 */
    	private void reHash() {
    		if (size >= capacity) {
    			int newCapacity = capacity * 2;
    			@SuppressWarnings("unchecked")
    			Entity<K, V>[] newEntities = new Entity[newCapacity];
    			for (int i = 0; i < capacity; i++) {
    				Entity<K, V> entity = entities[i];
    				while (entity != null) {
    					put(entity, newEntities, newCapacity);
    					entity = entity.getNext();
    				}
    			}
    			this.capacity = newCapacity;
    			this.entities = newEntities;
    		}
    	}
    
    	public static void main(String[] args) {
    		MyHashMap<String, String> map = new MyHashMap<String, String>();
    		map.put("one", "1");
    		map.put("two", "2");
    		map.put("three", "3");
    		map.put("four", "4");
    		map.put("five", "5");
    		map.put("six", "6");
    		map.put("seven", "7");
    		map.put("eight", "8");
    		map.put("nine", "9");
    		map.put("ten", "10");
    		System.out.println(map.get("one"));
    		System.out.println(map.get("two"));
    		System.out.println(map.get("three"));
    		System.out.println(map.get("four"));
    		System.out.println(map.get("five"));
    		System.out.println(map.get("six"));
    		System.out.println(map.get("seven"));
    		System.out.println(map.get("eight"));
    		System.out.println(map.get("nine"));
    		System.out.println(map.get("ten"));
    		System.out.println(map.toString());
    
    		map.remove("nine");
    		map.remove("three");
    		System.out.println(map.get("one"));
    		System.out.println(map.get("two"));
    		System.out.println(map.get("three"));
    		System.out.println(map.get("four"));
    		System.out.println(map.get("five"));
    		System.out.println(map.get("six"));
    		System.out.println(map.get("seven"));
    		System.out.println(map.get("eight"));
    		System.out.println(map.get("nine"));
    		System.out.println(map.get("ten"));
    		System.out.println(map.toString());
    	}
    }
    
    class Entity<K, V> {
    	private K key;
    	private V value;
    	private Entity<K, V> pre;
    	private Entity<K, V> next;
    
    	public Entity(K key, V value) {
    		this.key = key;
    		this.value = value;
    	}
    
    	public K getKey() {
    		return key;
    	}
    
    	public void setKey(K key) {
    		this.key = key;
    	}
    
    	public V getValue() {
    		return value;
    	}
    
    	public void setValue(V value) {
    		this.value = value;
    	}
    
    	public Entity<K, V> getPre() {
    		return pre;
    	}
    
    	public void setPre(Entity<K, V> pre) {
    		this.pre = pre;
    	}
    
    	public Entity<K, V> getNext() {
    		return next;
    	}
    
    	public void setNext(Entity<K, V> next) {
    		this.next = next;
    	}
    
    }

2、原博客地址

[http://blog.csdn.net/fenglibing/article/details/14166011][http_blog.csdn.net_fenglibing_article_details_14166011]

[20161008132916733]: /images/20220716/8fad28dc6a8b47a4bff3cba03bf95b14.png
[http_blog.csdn.net_fenglibing_article_details_14166011]: http://blog.csdn.net/fenglibing/article/details/14166011