1、HashMap简介

2、核心源码分析

public class HashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable
{
     //默认的初始容量（初始数组的长度）是16,且必须是2的整数次幂，
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    //数组的长度范围是[0,2的30次方]， 小于0会抛异常，大于MAXIMUM_CAPACITY会被它覆盖。
    static final int MAXIMUM_CAPACITY = 1 << 30;

    //默认的加载因子
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    //存放数据的Entry数组，实际容量必须是2的幂
     //每个Entry元素其实是一个链表
    transient Entry[] table;

    //HashMap中已经存放数据的个数,并非一定是数组的长度
    transient int size;

   
     //HashMap的阈值，如果size>threshold(threshold = 容量 * 加载因子)则HashMap需要rehash
    int threshold;

    //加载因子
    final float loadFactor;

    //HashMap被改变的次数
    transient volatile int modCount;

}

static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;
        final int hash;

        /**
         * 创建新的Entry，并让其next指针指向n
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }
          //获得key
        public final K getKey() {
            return key;
        }
          //获得value
        public final V getValue() {
            return value;
        }
          //设置value，并返回原来得value
        public final V setValue(V newValue) {
         V oldValue = value;
            value = newValue;
            return oldValue;
        }

          //比较两个Entry是否相等
          //首先，判断其key是否相等，然后再判断value是否相等，
          //只有key和value都相等，两个entry才相等
        public final boolean equals(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry e = (Map.Entry)o;
            Object k1 = getKey();
            Object k2 = e.getKey();
            if (k1 == k2 || (k1 != null && k1.equals(k2))) {
                Object v1 = getValue();
                Object v2 = e.getValue();
                if (v1 == v2 || (v1 != null && v1.equals(v2)))
                    return true;
            }
            return false;
        }
      
       //计算哈希值
public final int hashCode() {
            return (key==null   ? 0 : key.hashCode()) ^
                   (value==null ? 0 : value.hashCode());
        }

        public final String toString() {
            return getKey() + "=" + getValue();
        }

        //没做任何事情
        void recordAccess(HashMap<K,V> m) {
        }

        //没做任何事情
        void recordRemoval(HashMap<K,V> m) {
        }
    }

2.1 put方法

public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {//注意比较的顺序
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

private V putForNullKey(V value) {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
        addEntry(0, null, value, 0);
        return null;
    }

    //求hash值的方法，重新计算hash值 
    static int hash(int h) {   
        h ^= (h >>> 20) ^ (h >>> 12);   
        return h ^ (h >>> 7) ^ (h >>> 4);   
    }   
  
    // 返回h在数组中的索引值，这里用&代替取模，旨在提升效率  
    static int indexFor(int h, int length) {   
        return h & (length-1);   
    }

//添加新Entry元素
void addEntry(int hash, K key, V value, int bucketIndex) {
     Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
        if (size++ >= threshold) //如果size>阈值，则将数组长度扩大成原来的两倍
            resize(2 * table.length);
    }

//调整大小
void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        transfer(newTable);
        table = newTable;
        threshold = (int)(newCapacity * loadFactor);
    }

//转移Entry[]数组
void transfer(Entry[] newTable) {
        Entry[] src = table;
        int newCapacity = newTable.length;
        for (int j = 0; j < src.length; j++) {
            Entry<K,V> e = src[j];
            if (e != null) {
                src[j] = null;
                do {
                    Entry<K,V> next = e.next;
                    int i = indexFor(e.hash, newCapacity);
                    e.next = newTable[i];
                    newTable[i] = e;
                    e = next;
                } while (e != null);
            }
        }
    }

2.2 get()

public V get(Object key) {
        if (key == null)
            return getForNullKey();
        int hash = hash(key.hashCode());
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                return e.value;
        }
        return null;
    }

  private V getForNullKey() {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }