HashMap指定初始容量的构造函数-基于源码分析

客官°小女子只卖身不卖艺 2024-04-19 06:49 102阅读 0赞

今日主题是源码分析之HashMap指定初始容量的构造函数（以下都是基于jdk1.8）。

HashMap可以指定初始容量大小的构造函数有两个：

/** * The maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. */
        static final int MAXIMUM_CAPACITY = 1 << 30;
    
         /** * The load factor used when none specified in constructor. */
        static final float DEFAULT_LOAD_FACTOR = 0.75f;
    
    	/** * The next size value at which to resize (capacity * load factor). * * @serial */
        // (The javadoc description is true upon serialization.
        // Additionally, if the table array has not been allocated, this
        // field holds the initial array capacity, or zero signifying
        // DEFAULT_INITIAL_CAPACITY.)
        int threshold;
    
        /** * The load factor for the hash table. * * @serial */
        final float loadFactor;
        
        /** * Constructs an empty <tt>HashMap</tt> 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);
        }
        
         /** * Constructs an empty <tt>HashMap</tt> 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);
        }

看第一个构造函数即可，因为第二个也是通过第一个来实现的，只是加载因子是默认值0.75 。

接下来看tableSizeFor方法：

/** * Returns a power of two size for the given target capacity. */
        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 >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
        }

通过注释可以看到，我们传进来的初始容量值，经过一系列的无符号右移运算和位的或运算，最终给我们返回最接近我们指定的初始容量值的2的幂次方。

这里不需要考虑指定初始容量值为负数的情况，因为在构造函数中已经进行了判断。如果为负数，则程序将抛出异常。

为什么要先进行一步减1的操作呢？这是因为当我们指定初始容量正好是2的幂次方时，比如8，经过上述一系列的运算，最终会返回16，然而最接近于指定值的2的幂次方就是它本身。为了解决这一问题，程序首先执行了减1操作。

tableSizeFor方法的返回值赋值给了变量threshold，如果现在你还看不出来这个变量的作用也没关系，在接下来的博客中你将继续看到它的身影~