HashMap源码分析

一、源码分析

1.构造方法源码：

static final int DEFAULT_INITIAL_CAPACITY = 16;

static final int MAXIMUM_CAPACITY = 1 << 30;

static final float DEFAULT_LOAD_FACTOR = 0.75f;

transient Entry[] table;

transient int size;

int threshold;

final float loadFactor;

transient int modCount;

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);

        // 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];
        init();
}

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

shMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
        init();
}

由构造方法源码可知：

1、HashMap支持无参和有参数方式。

2、HashMap底层是一个Entry数组。

3、HashMap的默认容量为16，加载因子是0.75，因此默认的最大容量为：16*0.75=12

4、用户在新建HashMap的时候可以自定义容量和存储因子。

2.添加元素源码：

public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
}

void addEntry(int hash, K key, V value, int bucketIndex) {
        Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        if (size++ >= threshold)
            resize(2 * table.length);
}

由添加元素的源码可知：

1、添加的元素被放到了Entry中。

2、当添加的元素数量超过HashMap的默认容量的时候，HashMap会自动的扩容（比较耗时）。

二、HashMap的数据结构

来源：oschina

链接：https://my.oschina.net/u/2244961/blog/613060