三、HashMap源码分析

一、HashMap源码分析

HashMap源码分析:
HashMap 底层是数组加链表的形式

    //hashMap默认的容量大小为16 必须是2的幂次方
    static final int DEFAULT_INITIAL_CAPACITY = 16;
    //hashMap 2的幂次方容量 <= 1<<30
    static final int MAXIMUM_CAPACITY = 1 << 30;
    //默认负载系数 用于判断hashMap什么时候进扩容
    static final float DEFAULT_LOAD_FACTOR = 0.75f
    //hashMap底层是Entry数组
    transient Entry<K,V>[] table
    //容量大小
    transient int size;
    // threshold=容量*负载系数
    int threshold;
    // 负载系数
    final float loadFactor;
    // the HashMap fail-fast 集合在遍历时，不能进行修改，增加和删除
    transient int modCount;

HashMap 构造函数

    //无参构造函数 容量为默认的16 负载系数为0.75
    public HashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }
    //指定容量大小的  负载系数为0.75
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
    //指定容量和负载系数
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        int capacity = 1;
        //遍历 找到2的幂 >= 指定容量 确认hashMap的容量
        while (capacity < initialCapacity)
            capacity* <<= 1;
        this.loadFactor = loadFactor;
        //记录 capacity*loadFactor 的大小
        threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
        //创建长度为capacity 的数组
        table = new Entry[capacity];
        useAltHashing = sun.misc.VM.isBooted() &&
                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        init();
    }

hashMap 存入键值对

    //put
    public V put(K key, V value) {
        //如果存入的key为null
        if (key == null)
            return putForNullKey(value);
        //确认hash值
        int hash = hash(key);
        //确认桶的位置
        int i = indexFor(hash, table.length);
        //遍历桶的位置上是否有相同的可以，有就替换，返回旧值
        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))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }
    //存入key为null的值
    private V putForNullKey(V value) {
        //取第一个桶，如果也存在key为null值，则替换，返回旧值
        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++;
        //则hash值为0 ，放在第一个桶里 返回null
        addEntry(0, null, value, 0);
        return null;
    }
    //将添加元素放入到数组中
    void addEntry(int hash, K key, V value, int bucketIndex) {
        //如果当前集合的大小大于等于 threshold(当前容量*负载系数) 并且插入的数组位置不为空，则进行扩容
        if ((size >= threshold) && (null != table[bucketIndex])) {
            //扩容为原来的2倍
            resize(2 * table.length);
            //扩容后需要重新计算hash值
            hash = (null != key) ? hash(key) : 0;
            //根据hash值和数组的长度确认放入桶的位置
            bucketIndex = indexFor(hash, table.length);
        }

        createEntry(hash, key, value, bucketIndex);
    }
    //扩容
    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];
        boolean oldAltHashing = useAltHashing;
        useAltHashing |= sun.misc.VM.isBooted() &&
                (newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        boolean rehash = oldAltHashing ^ useAltHashing;
        //将旧数组的元素转移到新数组中
        transfer(newTable, rehash);
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }
    //扩容后将旧数组转移到新数组
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        for (Entry<K,V> e : table) {
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) {
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                int i = indexFor(e.hash, newCapacity);
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }
    // 确认元素存入桶的位置
    static int indexFor(int h, int length) {
        return h & (length-1);
    }
    //添加元素进桶
    void createEntry(int hash, K key, V value, int bucketIndex) {
        Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        size++;
    }

hashMap根据key获取值

     public V get(Object key) {
        if (key == null)
            return getForNullKey();
        Entry<K,V> entry = getEntry(key);
        return null == entry ? null : entry.getValue();
    }
    //如果key为null则去第一个桶的，如果有在返回，没有就返回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;
    }
    //根据key获取Entry
    final Entry<K,V> getEntry(Object key) {
        //首先通过key获取hash值
        int hash = (key == null) ? 0 : hash(key);
        //然后找到元素存放的桶，遍历桶上的元素，没有返回null
        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 != null && key.equals(k))))
                return e;
        }
        return null;
    }

hashMap清空

 public void clear() {
        modCount++;
        Entry[] tab = table;
        for (int i = 0; i < tab.length; i++)
            tab[i] = null;
        size = 0;
    }

判断hashMap中是否包含value

    public boolean containsValue(Object value) {
        if (value == null)
            return containsNullValue();
        Entry[] tab = table;
        //遍历所有数组中的Entry
        for (int i = 0; i < tab.length ; i++)
            for (Entry e = tab[i] ; e != null ; e = e.next)
                if (value.equals(e.value))
                    return true;
        return false;
    }

hashMap 内部类 Entry

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

        /**
         * Creates new entry.
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }
    }

总结：
1.hashMap底层是数组和加链表的形式，元素都是存放在一个一个桶里，以key的hash值和数组的大小确认
2.hashMap默认的容量大小为16，负载系数为0.75 也就是当容量大小为12是，就会进行扩容，容量为原来的2倍，这方面平时需要注意，尽量减少扩容
3.hashMap中的Entry分别存放了 key，value，本身，还有hash值