HashSet随笔

HashSet说白了其实就是HashMap.Entry[capacity], HashMap.Entry实现的链表 相当于LinkedList.

参数loadFactor对其性能有很大影响.

capacity: the number of buckets.

initialCapacity: Default value is 16, 用于初始化数组的槽位, capacity永远 >= initialCapacity 并且capacity永远为2的n次幂 最大值为1<<30.

因为源码已经解释的很清楚了.

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);

        // Find a power of 2 >= initialCapacity
        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;

        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
        table = new Entry[capacity];

loadFactor: Default value is 0.75, 影响resize容器的频率与内存占用, 越小则需要更多内存占用, 越大则增加get()与put()/putAll()的系统开销.

threshold: 始终为当前capacity*loadFactor, 每次添加数据时会判断size()是否 >= threshold, 大于则resize(). resize()的同时会再次计算新的threshold值.