自己实现HashMap集合

如图，HashMap的结果就是这个样子。横向是链表，纵向是数组。所以hashmap是由数组和链表组成的。

下面来实现HashMap

import java.util.Collection;
import java.util.Map;
import java.util.Set;

public class HashMapTemplate<K,V> implements Map<K,V>{
    //初始大小
     private int capacity = 16;  
     private int size = 0;  
     Entity<K,V>[] entities=null;
     
    @Override
    public void clear() {
        // TODO Auto-generated method stub
        entities=null;
    }

    @Override
    public boolean containsKey(Object key) {
        //处理key为null的情况
        if (key==null) {
            Entity entity=entities[0];
            if (entity==null) {
                return false;
            }else {
                if (entity.getKey()==key) {
                    return true;
                }else{
                    while(entity.getNextEntity()!=null){
                        if (entity.getNextEntity().getKey().equals(key)) {
                            return true;
                        }
                        entity=entity.getNextEntity();
                    }
                    return false;
                }
            }
        }else{
            int index=key.hashCode()%capacity;
            Entity entity=entities[index];
            if (entity.getKey().equals(key)) {
                return true;
            }else {
                while(entity.getNextEntity()!=null){
                    if (entity.getNextEntity().getKey().equals(key)) {
                        return true;
                    }
                    entity=entity.getNextEntity();
                }
                return false;
                
            }
        }
    }

    @Override
    public boolean containsValue(Object value) {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public Set<java.util.Map.Entry<K, V>> entrySet() {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public V get(Object key) {
        if (key==null) {
            Entity entity=entities[0];
            if (entity.getKey()==key) {
                return (V) entity.getValue();
            }else {
                while(entity.getNextEntity()!=null){
                    if (entity.getNextEntity().getKey().equals(key)) {
                        return (V) entity.getValue();
                    }
                    entity=entity.getNextEntity();
                }
                return null;
                
            }
        }else{
            int index=key.hashCode()%capacity;
            Entity entity=entities[index];
            if (entity.getKey().equals(key)) {
                return (V) entity.getValue();
            }else {
                while(entity.getNextEntity()!=null){
                    if (entity.getNextEntity().getKey().equals(key)) {
                        return (V) entity.getValue();
                    }
                    entity=entity.getNextEntity();
                }
                return null;
                
            }
        }
    }

    @Override
    public boolean isEmpty() {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public Set<K> keySet() {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public V put(K key, V value) {
        reCapacity();  
        Entity<K, V> newEntity = new Entity<K, V>(key, value);  
        put(newEntity, this.entities, this.capacity); 
        return null;
    }
    private void put(Entity<K, V> newEntity, Entity<K, V>[] entities, int capacity) {
        if (newEntity.getKey()==null) {
            Entity entity=entities[0];
            if (entity==null) {
                entities[0]=newEntity;
                size++;
            }
        }else{
            int index=newEntity.getKey().hashCode()%capacity;
            Entity entity=entities[index];
            Entity firstEntity =entities[index];
            if (entity==null) {
                entities[index]=newEntity;
                size++;
            }else {
                if (newEntity.getKey().equals(entity.getKey())) {
                    newEntity.setNextEntity(entity.getNextEntity());
                    newEntity.setPreEntity(entity.getPreEntity());
                    if (entity.getNextEntity()!=null) {
                        entity.getNextEntity().setPreEntity(newEntity);
                    }
                    entities[index]=newEntity;
                }else if (entity.getNextEntity()!=null) {
                    while (entity.getNextEntity()!=null) {
                          entity = entity.getNextEntity();  
                            if (newEntity.getKey().equals(entity.getKey())) {  
                                newEntity.setPreEntity(entity.getPreEntity());  
                                newEntity.setNextEntity(entity.getNextEntity());  
                                if (entity.getNextEntity() != null) {  
                                    entity.getNextEntity().setPreEntity(newEntity);  
                                }  
                                entities[index] = newEntity;  
                            }  
                    }
                    newEntity.setNextEntity(firstEntity);  
                    newEntity.setPreEntity(firstEntity.getPreEntity());  
                    firstEntity.setPreEntity(newEntity);  
                    entities[index] = newEntity;  
                    size++;  
                }else {
                     newEntity.setNextEntity(firstEntity);  
                        firstEntity.setPreEntity(newEntity);  
                        entities[index] = newEntity;  
                        size++; 
                }
            }
        }
     }
    @Override
    public void putAll(Map<? extends K, ? extends V> m) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public V remove(Object key) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int size() {
        // TODO Auto-generated method stub
        return this.size;
    }

    @Override
    public Collection<V> values() {
        // TODO Auto-generated method stub
        return null;
    }
    public HashMapTemplate() {  
            //初始化容量
            entities = new Entity[capacity];  
    }
    /**
     * 扩容
     */
     private void reCapacity() {  
            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.getNextEntity();  
                    }  
                }  
                this.capacity = newCapacity;  
                this.entities = newEntities;  
            } 
        }

    @Override
    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.getNextEntity();  
                }  
                if (hasEntity) {  
                    sb.deleteCharAt(sb.length() - 1);  
                }  
                sb.append("]
");  
            }  
            return sb.toString();  
    }  
}
@SuppressWarnings("unchecked")
class Entity<K,V>{
    private K key;
    private V value;
    private Entity<K,V> PreEntity;//前节点
    private Entity<K,V> nextEntity;//后节点
    
    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 getPreEntity() {
        return PreEntity;
    }
    
    public void setPreEntity(Entity preEntity) {
        PreEntity = preEntity;
    }
    public Entity getNextEntity() {
        return nextEntity;
    }
    public void setNextEntity(Entity nextEntity) {
        this.nextEntity = nextEntity;
    }
    public Entity(K k,V v){
        this.key=k;
        this.value=v;
    }
    
}

上面有一个点需要注意一下：

1.HashMap允许key为null，所以需要处理一下

2.HashMap存放键值对的时候，需要检查容量是否充足，但是因为我们这里是引用传递，所以需要单独写一个put方法通过局部变量的临近原则解决对象的问题。

3.完成hashmap部分功能。并没有完全写完。