java学习笔记（5）-HashSet类

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1.HashSet介绍

HashSet基于Set接口的实现，内部存储数据是用的HashMap，其操作都是基于HashMap的

2.几个重要的参数

private transient HashMap<E,Object> map;//内部存储数据private static final Object PRESENT = new Object();//构造一个空虚拟的对象

    public HashSet() {        map = new HashMap<>();//默认使用HashMap的初始容量和加载因子16和0.75    }        public HashSet(int initialCapacity, float loadFactor) {        map = new HashMap<>(initialCapacity, loadFactor);    }    public HashSet(int initialCapacity) {        map = new HashMap<>(initialCapacity);    }    HashSet(int initialCapacity, float loadFactor, boolean dummy) {        map = new LinkedHashMap<>(initialCapacity, loadFactor);    }

3.主要看一下HashSet的add方法

    public boolean add(E e) {        return map.put(e, PRESENT)==null;    }

明显使用的是HashMap的put方法，定位到HashMap中

HashMap.java    public V put(K key, V value) {        return putVal(hash(key), key, value, false, true);    }    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,                   boolean evict) {        Node<K,V>[] tab; Node<K,V> p; int n, i;        if ((tab = table) == null || (n = tab.length) == 0)            n = (tab = resize()).length;        if ((p = tab[i = (n - 1) & hash]) == null)//根据key的hash找到在table中的位置index，取出该节点p            tab[i] = newNode(hash, key, value, null);//table中没有则直接添加新节点，直接执行最后的return null        else {//如果hash相等，也即是有冲突发生            Node<K,V> e; K k;            if (p.hash == hash &&                ((k = p.key) == key || (key != null && key.equals(k))))//hash相等，并且key也相等，则取出p赋值给e，不走下面的else                e = p;            else if (p instanceof TreeNode)                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);            else {                for (int binCount = 0; ; ++binCount) {                    if ((e = p.next) == null) {                        p.next = newNode(hash, key, value, null);                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st                            treeifyBin(tab, hash);                        break;                    }                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k))))                        break;                    p = e;                }            }            if (e != null) { // existing mapping for key                V oldValue = e.value;                if (!onlyIfAbsent || oldValue == null)                    e.value = value;//这里将虚拟对象重新赋值成虚拟对象PRESENT                afterNodeAccess(e);                return oldValue;//返回PRESENT，HashSet中map.put(e, PRESENT)==null返回false            }        }        ++modCount;        if (++size > threshold)            resize();        afterNodeInsertion(evict);        return null;//HashSet中没有相同的的元素时执行到这里    }

注意：在HashSet中是没有get方法的，要取出HashSet中的元素可以用迭代器遍历

4.contains(Object o)方法

    public boolean contains(Object o) {        return map.containsKey(o);    }

HashMap.java    public V get(Object key) {        Node<K,V> e;        return (e = getNode(hash(key), key)) == null ? null : e.value;    }    final Node<K,V> getNode(int hash, Object key) {        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;        if ((tab = table) != null && (n = tab.length) > 0 &&            (first = tab[(n - 1) & hash]) != null) {            if (first.hash == hash && // always check first node                ((k = first.key) == key || (key != null && key.equals(k))))                return first;//找到了Object key            if ((e = first.next) != null) {                if (first instanceof TreeNode)                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);//有冲突，并且节点是红黑树，直接在树中查找                do {//否则，还是有冲突，并且节点是链表，则循环查找                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k))))                        return e;                } while ((e = e.next) != null);            }        }        return null;    }

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