Matisse: MimeType中的HashSet需要优化一下

[liuzhanta]

PNG("image/png", new HashSet(1) {
{
add("png");
}
}),
HashMap的默认大小是4，这样会浪费很多内存

[nekocode]

Pull request welcome~

[gejiaheng]

@liuzhanta
As @nekocode said, would you like to optimize this? It's fine if you don't, I'll do it myself then.

[gejiaheng]

Extract from the source code of HashMap:

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

public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY) {
            initialCapacity = MAXIMUM_CAPACITY;
        } else if (initialCapacity < DEFAULT_INITIAL_CAPACITY) {
            initialCapacity = DEFAULT_INITIAL_CAPACITY;
        }

        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        // Android-Note: We always use the default load factor of 0.75f.

        // This might appear wrong but it's just awkward design. We always call
        // inflateTable() when table == EMPTY_TABLE. That method will take "threshold"
        // to mean "capacity" and then replace it with the real threshold (i.e, multiplied with
        // the load factor).
        threshold = initialCapacity;
        init();
    }

So it won't take effect if you pass 1 as the initial capacity.

[liuzhanta]

如果超过默认大小，HashMap会自动扩容，当前大小*2,所以还是会造成一定浪费，可能我的问题太刁钻了。

/**
 * Adds a new entry with the specified key, value and hash code to
 * the specified bucket.  It is the responsibility of this
 * method to resize the table if appropriate.
 *
 * Subclass overrides this to alter the behavior of put method.
 */
void addEntry(int hash, K key, V value, int bucketIndex) {
    if ((size >= threshold) && (null != table[bucketIndex])) {
        resize(2 * table.length);
        hash = (null != key) ? sun.misc.Hashing.singleWordWangJenkinsHash(key) : 0;
        bucketIndex = indexFor(hash, table.length);
    }

    createEntry(hash, key, value, bucketIndex);
}

[gejiaheng]

@liuzhanta
It's not a problem at all that you brought up this issue. As a matter of fact, it's better to do as you said above on some conditions. But Matisse has a very special case, so it won't make any difference if you pass the initial capacity or not.
You're always welcome to contribute or file an issue.