重要参数

 

/**     * 默认的初始容量，必须是2的幂。     */    static final int DEFAULT_INITIAL_CAPACITY = 16;    /**     * 最大容量（必须是2的幂且小于2的30次方，传入容量过大将被这个值替换）     */    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * 默认装载因子，这个后面会做解释     */    static final float DEFAULT_LOAD_FACTOR = 0.75f;    /**     * 存储数据的Entry数组，长度是2的幂。看到数组的内容了，接着看数组中存的内容就明白为什么博文开头先复习数据结构了     */    transient Entry[] table;    /**     * map中保存的键值对的数量     */    transient int size;    /**     * 需要调整大小的极限值（容量*装载因子）     */    int threshold;    /**     *装载因子     */    final float loadFactor;    /**     * map结构被改变的次数     */    transient volatile int modCount;

 

 

 

/**     * The load factor used when none specified in constructor.     */    static final float DEFAULT_LOAD_FACTOR = 0.75f;

 

初始长度：16

/**     * The default initial capacity - MUST be a power of two.     */    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

 

 

 

构造器

HashMap有4个构造器

　　public HashMap()；

　　public HashMap(int initialCapacity) ；

　　 public HashMap(int initialCapacity, float loadFactor) ；

　　 public HashMap(Map<? extends K, ? extends V> m) 

详细说来：

空构造器：初始长度和负载因子使用默认值；

/**     * Constructs an empty HashMap with the specified initial     * capacity and the default load factor (0.75).     *     * @param  initialCapacity the initial capacity.     * @throws IllegalArgumentException if the initial capacity is negative.     */    public HashMap(int initialCapacity) {        this(initialCapacity, DEFAULT_LOAD_FACTOR);//(16,0.75f)    }

自定义长度，自然数，默认因子=0.75

/**     * Constructs an empty HashMap with the specified initial     * capacity and the default load factor (0.75).     *     * @param  initialCapacity the initial capacity.     * @throws IllegalArgumentException if the initial capacity is negative.     */    public HashMap(int initialCapacity) {        this(initialCapacity, DEFAULT_LOAD_FACTOR);//(initialCapacity,0.75f)

}

全自定义参数

/**     * Constructs an empty HashMap with the specified initial     * capacity and load factor.     *     * @param  initialCapacity the initial capacity     * @param  loadFactor      the load factor     * @throws IllegalArgumentException if the initial capacity is negative     *         or the load factor is nonpositive     */    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);        this.loadFactor = loadFactor;        this.threshold = tableSizeFor(initialCapacity);    }

直接放一个map，默认因子0.75

/**     * Constructs a new HashMap with the same mappings as the     * specified Map.  The HashMap is created with     * default load factor (0.75) and an initial capacity sufficient to     * hold the mappings in the specified Map.     *     * @param   m the map whose mappings are to be placed in this map     * @throws  NullPointerException if the specified map is null     */    public HashMap(Map
        m) {        this.loadFactor = DEFAULT_LOAD_FACTOR;        putMapEntries(m, false);    }

 

获取threshold，容器当期极限值

>>>    :     无符号右移，忽略符号位，空位都以0补齐

this.threshold = tableSizeFor(initialCapacity);....static final int tableSizeFor(int cap) {        int n = cap - 1;        n |= n >>> 1;        n |= n >>> 2;        n |= n >>> 4;        n |= n >>> 8;        n |= n >>> 16;        return (n < 0) ? 1 : (n >= (1 << 30)) ? (1 << 30) : n + 1;    }