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HashMap

时间:2016-07-01 18:07:42      阅读:210      评论:0      收藏:0      [点我收藏+]

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1、开始
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable
继承了抽象类AbstractMap,实现了Map接口,Cloneable接口(可克隆),Serializable接口(可序列化)

2、属性

    //默认初始化容量为16,容量必须是2的n次幂
    static final int DEFAULT_INITIAL_CAPACITY = 16;
    //最大容量为2的20次幂,再大就是Integer.MAX_VALUE
    static final int MAXIMUM_CAPACITY = 1 << 30;
    //默认加载因子为0.75
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    //Entry数组,其就是链表散列的数据结构,即数组+链表
    transient Entry<K,V>[] table;
    //已存储元素的数量
    transient int size;
    //扩容的临界值,只要存储元素的数量大于该临界值,就会自动扩容,其中threshold=capacity*load_factor
    int threshold;
    //加载因子
    final float loadFactor;
    //更改次数
    transient int modCount;

3、存储数据结构Entry

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

        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }

        public final K getKey() {
            return key;
        }

        public final V getValue() {
            return value;
        }

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        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;
        }

        public final int hashCode() {
            return (key==null   ? 0 : key.hashCode()) ^
                   (value==null ? 0 : value.hashCode());
        }

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

        //当向HashMap添加元素时调用该方法
        void recordAccess(HashMap<K,V> m) {
        }

        //当从HashMap中删除元素时调用该方法
        void recordRemoval(HashMap<K,V> m) {
        }
    }

4、构造器

    public HashMap(int initialCapacity, float loadFactor) {
        //校验容量大小
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
        //初始化容量大小的最大值
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        //校验加载因子
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " + loadFactor);

        //获取容量大小,使之是2的n次幂
        // Find a power of 2 >= initialCapacity
        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;

        //赋值
        //加载因子
        this.loadFactor = loadFactor;
        //扩容的临界值
        threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
        //存储元素的数组
        table = new Entry[capacity];
        //用于元素计算Hash值,定位元素在数组中的位置
        useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        //初始化时的一些其他操作
        init();
    }

    //指定初始容量
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    //使用默认容量和默认加载因子
    public HashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

    //使用现有元素,和默认加载因子
    public HashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
        putAllForCreate(m);
    }

5、添加
    
    public V put(K key, V value) {
        //当key为null时,存储在数组的第0个位置
        if (key == null)
            return putForNullKey(value);
        //计算HashCode值
        int hash = hash(key);
        //定位在数组中的位置,即确定该元素所在的链表
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            //对比hash值以及Key是否相等
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                //如果存在,则将旧值替换为新值
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                //返回旧值
                return oldValue;
            }
        }
        //更改次数
        modCount++;
        //在当前数组的i位置的链表中新增一节点
        addEntry(hash, key, value, i);
        return null;
    }

    private V putForNullKey(V value) {
        //key为null,其必在HashMap的第0个数组中
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null) {
                //当已存在,则替换
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
        //新增,hash=0,bucketIndex=0
        addEntry(0, null, value, 0);
        return null;
    }

    void addEntry(int hash, K key, V value, int bucketIndex) {
        //当前元素数量已经达到扩容临界点,则进行扩容
        if ((size >= threshold) && (null != table[bucketIndex])) {
            //扩容,为原来的2倍
            resize(2 * table.length);
            //重新计算当前key的hash值,
            hash = (null != key) ? hash(key) : 0;
            //以及在数组中的位置
            bucketIndex = indexFor(hash, table.length);
        }
        //为链表添加一新节点
        createEntry(hash, key, value, bucketIndex);
    }

    void createEntry(int hash, K key, V value, int bucketIndex) {
        //使用前插法,即新插入的元素必定在链表的头部
        Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        //元素数量加1
        size++;
    }

     void resize(int newCapacity) {
        //获取当前数组的引用,作为本地变量
        Entry[] oldTable = table;
        //老的数组的长度
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            //当前数组容量已经是最大时,则不需要进行扩容
            threshold = Integer.MAX_VALUE;
            return;
        }
        //创建新的数组
        Entry[] newTable = new Entry[newCapacity];
        boolean oldAltHashing = useAltHashing;
        useAltHashing |= sun.misc.VM.isBooted() &&
                (newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        //是否进行hash值重算
        boolean rehash = oldAltHashing ^ useAltHashing;
        //数组元素迁移到新数组中
        transfer(newTable, rehash);
        //最后将新数组引用赋给HashMap
        table = newTable;
        //重新计算扩容临界值
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }

   
    void transfer(Entry[] newTable, boolean rehash) {
        //新数组长度
        int newCapacity = newTable.length;
        for (Entry<K,V> e : table) {
            //遍历数组中的每一个元素,即每一个链表
            while(null != e) {
                //将每一个链表迁移到新数组中

                //记录当前节点的下一个节点,用于下次迁移
                Entry<K,V> next = e.next;
                if (rehash) {
                    //是否对当前节点的hash重计算
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                //重新定位该元素在新数组中的位置
                int i = indexFor(e.hash, newCapacity);
                //将新数组中该位置的链表元素都放在该元素后面,使用的是前插法
                e.next = newTable[i];
                //将链表挂在新数组中
                newTable[i] = e;
                //继续下一个节点的迁移
                e = next;
            }
        }
    }

6、删除

     public V remove(Object key) {
        //获取需要删除的key对应的元素
        Entry<K,V> e = removeEntryForKey(key);
        //返回该元素上的值
        return (e == null ? null : e.value);
    }

    final Entry<K,V> removeEntryForKey(Object key) {
        //计算该key的hash
        int hash = (key == null) ? 0 : hash(key);
        //定位该key的在数组中的位置
        int i = indexFor(hash, table.length);
        //找到链表的第一个节点
        Entry<K,V> prev = table[i];
        Entry<K,V> e = prev;
        //遍历链表
        while (e != null) {
            //当前节点的下一个节点,也是下一次遍历所需要的节点
            Entry<K,V> next = e.next;
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k)))) {
                //若当前节点是所要删除的节点,即hash相等和key也相等
                //更改次数
                modCount++;
                //元素数量减1
                size--;
                if (prev == e)
                    //当要删除的节点是链表的头结点时,则只需要将当前节点的下一个节点作为该链表的头结点,即可
                    table[i] = next;
                else
                    //当删除的节点不是链表的头结点时,则只需要当前节点的前一个节点的下一个是当前节点的下一个节点,即可
                    prev.next = next;
                //删除记录
                e.recordRemoval(this);
                //返回删除的节点
                return e;
            }
            prev = e;
            e = next;
        }
        //返回要删除的节点
        return e;
    }

7、访问和查找
    
    //根据key查找元素
    public V get(Object key) {
        if (key == null)
            //当key为null时,从数组的第0个位置查找
            return getForNullKey();
        //根据key查找
        Entry<K,V> entry = getEntry(key);

        return null == entry ? null : entry.getValue();
    }

    private V getForNullKey() {
        //从数组的第0个位置的链表头部开始查找
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            //当存在一个key为null的节点时
            if (e.key == null)
                //返回该节点中的值
                return e.value;
        }
        return null;
    }

    //是否包含键
    public boolean containsKey(Object key) {
        return getEntry(key) != null;
    }


    final Entry<K,V> getEntry(Object key) {
        //计算该key对应的hash
        int hash = (key == null) ? 0 : hash(key);
        //定位在数组中的位置,即找到某张链表,之后从该链表的头部开始遍历
        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))))
                //当hash相等,key也想等时,返回该节点
                return e;
        }
        return null;
    }

    //是否包含值
    public boolean containsValue(Object value) {
        if (value == null)
            //当值为null时
            return containsNullValue();
        Entry[] tab = table;
        //遍历当前数组,以及每一个链表
        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;
    }

 
    private boolean containsNullValue() {
        //值为null
        Entry[] tab = table;
        //遍历当前数组,以及每一个链表
        for (int i = 0; i < tab.length ; i++)
            for (Entry e = tab[i] ; e != null ; e = e.next)
                if (e.value == null)
                    return true;
        return false;
    }

8、迭代器

    //用于HashMap的迭代器抽象类
    private abstract class HashIterator<E> implements Iterator<E> {
        Entry<K,V> next;        // next entry to return
        int expectedModCount;   // For fast-fail
        int index;              // current slot
        Entry<K,V> current;     // current entry

        HashIterator() {
            expectedModCount = modCount;
            if (size > 0) { // advance to first entry
                Entry[] t = table;
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
        }

        public final boolean hasNext() {
            return next != null;
        }

        final Entry<K,V> nextEntry() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Entry<K,V> e = next;
            if (e == null)
                throw new NoSuchElementException();

            if ((next = e.next) == null) {
                Entry[] t = table;
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
            current = e;
            return e;
        }

        public void remove() {
            if (current == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Object k = current.key;
            current = null;
            HashMap.this.removeEntryForKey(k);
            expectedModCount = modCount;
        }
    }

    //值迭代器类
    private final class ValueIterator extends HashIterator<V> {
        public V next() {
            return nextEntry().value;
        }
    }
    //键迭代器类
    private final class KeyIterator extends HashIterator<K> {
        public K next() {
            return nextEntry().getKey();
        }
    }
    //Entry迭代器类
    private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
        public Map.Entry<K,V> next() {
            return nextEntry();
        }
    }

    //获取键迭代器对象
    // Subclass overrides these to alter behavior of views‘ iterator() method
    Iterator<K> newKeyIterator()   {
        return new KeyIterator();
    }
    //获取值迭代器对象
    Iterator<V> newValueIterator()   {
        return new ValueIterator();
    }
    //获取Entry迭代器类对象
    Iterator<Map.Entry<K,V>> newEntryIterator()   {
        return new EntryIterator();
    }

    private transient Set<Map.Entry<K,V>> entrySet = null;
    transient volatile Set<K>        keySet = null;
    transient volatile Collection<V> values = null;

    //获取HashMap中的所有键对Set
    public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null ? ks : (keySet = new KeySet()));
    }

    private final class KeySet extends AbstractSet<K> {
        //获取迭代器
        public Iterator<K> iterator() {
            return newKeyIterator();
        }
        //Set大小
        public int size() {
            return size;
        }
        //是否包含
        public boolean contains(Object o) {
            return containsKey(o);
        }
        //删除元素
        public boolean remove(Object o) {
            return HashMap.this.removeEntryForKey(o) != null;
        }
        //清空
        public void clear() {
            HashMap.this.clear();
        }
    }

    //获取HashMap中的所有值对Collection
    public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null ? vs : (values = new Values()));
    }

    private final class Values extends AbstractCollection<V> {
        //获取迭代器
        public Iterator<V> iterator() {
            return newValueIterator();
        }
        //Collection大小
        public int size() {
            return size;
        }
         //是否包含
        public boolean contains(Object o) {
            return containsValue(o);
        }
        //清空
        public void clear() {
            HashMap.this.clear();
        }
    }

    //获取HashMap中的所有键值对Set
    public Set<Map.Entry<K,V>> entrySet() {
        return entrySet0();
    }

    private Set<Map.Entry<K,V>> entrySet0() {
        Set<Map.Entry<K,V>> es = entrySet;
        return es != null ? es : (entrySet = new EntrySet());
    }

    private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        //获取迭代器
        public Iterator<Map.Entry<K,V>> iterator() {
            return newEntryIterator();
        }
         //是否包含
        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<K,V> e = (Map.Entry<K,V>) o;
            Entry<K,V> candidate = getEntry(e.getKey());
            return candidate != null && candidate.equals(e);
        }
        //删除元素
        public boolean remove(Object o) {
            return removeMapping(o) != null;
        }
        //Set大小
        public int size() {
            return size;
        }
        //清空
        public void clear() {
            HashMap.this.clear();
        }
    }

9、清空所有元素
    public void clear() {
        modCount++;
        Entry[] tab = table;
        for (int i = 0; i < tab.length; i++)
            tab[i] = null;
        size = 0;
    }

参考资料:
http://www.cnblogs.com/tstd/p/5055286.html

http://tengj.top/2016/04/15/javajh3hashmap/

 

HashMap

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原文地址:http://www.cnblogs.com/xiaoxian1369/p/5633763.html

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