HashMap初始容量和扩容

《阿里巴巴Java开发手册》中对于HashMap有推荐用法

那么HashMap是如何扩容的呢？通过查阅源码可以看出

static final float DEFAULT_LOAD_FACTOR = 0.75f;

public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

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);
    }

/**
     * 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HashMap#tableSizeFor可以看出HashMap的初始容量并非initialCapacity，而是min（2^n）>initialCapacity,即大于initialCapacity的2的n次幂的最小值。而扩容条件只要HashMap的容量大于threshold*loadFactor时就需要扩容，而非初始值initialCapacity，这样可能与我们的预期不符，本意可能是在initialCapacity范围内都不需要扩容，举例如下：

解决方案

其中Guava的Maps提供了解决方案

public static <K, V> HashMap<K, V> newHashMapWithExpectedSize(int expectedSize) {
    return new HashMap<>(capacity(expectedSize));
  }

static int capacity(int expectedSize) {
    if (expectedSize < 3) {
      checkNonnegative(expectedSize, "expectedSize");
      return expectedSize + 1;
    }
    if (expectedSize < Ints.MAX_POWER_OF_TWO) {
      // This is the calculation used in JDK8 to resize when a putAll
      // happens; it seems to be the most conservative calculation we
      // can make.  0.75 is the default load factor.
      return (int) ((float) expectedSize / 0.75F + 1.0F);
    }
    return Integer.MAX_VALUE; // any large value
  }

这样HashMap的容量在小于或等于expectedSize时都不需要扩容。