实现自己的ArrayList和HashMap

超、凢脫俗 2022-09-18 02:58 156阅读 0赞

ArrayList的存储就是一个数组，

HashMap的存储是一个数组加一个链表，下面这张图来自: http://beyond99.blog.51cto.com/1469451/429789

![222554494.jpg][]

以下实现的MyArrayList及MyHashMap，在实际的工作中都是用不上的，最有可能用得到的地方就是面试找工作以及忽悠别人了。工作中虽然用不上，但是并不代表没有用，它可以帮助我们去理解他们的实现原理，等实现完后再去仔细查看JDK中的源码，就会发现别人实现当中那些可供学习的地方。

**MyArrayList**

public class MyArrayList<E> {
    	private int capacity = 10;
    	private int size = 0;
    	private E[] values = null;
    
    	@SuppressWarnings("unchecked")
    	public MyArrayList() {
    		values = (E[]) new Object[capacity];
    	}
    
    	@SuppressWarnings("unchecked")
    	public MyArrayList(int capacity) {
    		this.capacity = capacity;
    		values = (E[]) new Object[this.capacity];
    	}
    
    	public void put(E e) {
    		if (e == null) {
    			throw new RuntimeException("The value should not be null.");
    		}
    		if (size >= capacity) {
    			enlargeCapacity();
    		}
    		values[size] = e;
    		size++;
    	}
    
    	public E get(int index) {
    		if (index >= size) {
    			throw new RuntimeException("The index:" + index + " is out of band.");
    		}
    		return values[index];
    	}
    
    	public void remove(int index) {
    		if (index >= size) {
    			throw new RuntimeException("The index:" + index + " is out of band.");
    		}
    		for (int i = index; i < size - 1; i++) {
    			values[i] = values[i + 1];
    		}
    		values[size - 1] = null;
    		size--;
    	}
    
    	@SuppressWarnings("unchecked")
    	private void enlargeCapacity() {
    		capacity = capacity * 2;
    		E[] tmpValues = (E[]) new Object[capacity];
    		System.arraycopy(values, 0, tmpValues, 0, size);
    		values = tmpValues;
    	}
    
    	public String toString() {
    		StringBuilder sb = new StringBuilder();
    		sb.append("[");
    		for (int i = 0; i < size; i++) {
    			sb.append(values[i]).append(",");
    		}
    		if (size > 0) {
    			sb.deleteCharAt(sb.length() - 1);
    		}
    		sb.append("]");
    		return sb.toString();
    	}
    
    	/**
    	 * @param args
    	 */
    	public static void main(String[] args) {
    		MyArrayList<String> myList = new MyArrayList<String>();
    		myList.put("1");
    		myList.put("2");
    		myList.put("3");
    		myList.put("4");
    		myList.put("5");
    		myList.put("6");
    		myList.put("7");
    		myList.put("8");
    		myList.put("9");
    		myList.remove(7);
    		System.out.println(myList.toString());
    	}
    
    }

MyHashMap

public class MyHashMap<K, V> {
    	//initialization capacity
    	private int capacity = 10;
    	//total entities
    	private int size = 0;
    	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) {
    		int index = newEntity.getKey().hashCode() % capacity;
    		Entity<K, V> entity = entities[index];
    		Entity<K, V> firstEntity = entities[index];
    		if (entity == null) {
    			entities[index] = newEntity;
    			size++;
    		} else {
    			if (newEntity.getKey().equals(entity.getKey())) {//Find the same key for the first entity, if find then replace the old value to new value
    				newEntity.setNext(entity.getNext());
    				newEntity.setPre(entity.getPre());
    				if (entity.getNext() != null) {
    					entity.getNext().setPre(newEntity);
    				}
    				entities[index] = newEntity;
    			} else if (entity.getNext() != null) {
    				while (entity.getNext() != null) {//Find the same key for all the next entity, if find then replace the old value to new value
    					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;
    	}
    
    }

[222554494.jpg]: /images/20220828/c3f1ef675fa34a818e32c7bdfb89147f.png