去哪面试都会问的HashMap

//初始容量是16，且容量必须是2的倍数
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;

//最大容量是2的30次方
static final int MAXIMUM_CAPACITY = 1 << 30;

//负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;

static final Entry<?,?>[] EMPTY_TABLE = {};

//HashMap的主干是一个Entry数组,在需要的时候进行扩容，长度必须是2的被数
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

//放置的key-value对的个数
transient int size;

//进行扩容的阈值，值为 capacity * load factor，即容量 * 负载因子
int threshold;

//负载因子
final float loadFactor;

static class Entry<K,V> implements Map.Entry<K,V> {
    final K key;
    V value;
    Entry<K,V> next;//存储指向下一个Entry的引用，单链表结构
    int hash;//对key的hashcode值进行hash运算后得到的值，存储在Entry，避免重复计算

    Entry(int h, K k, V v, Entry<K,V> n) {
        value = v;
        next = n;
        key = k;
        hash = h;
    }
}

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;
    threshold = initialCapacity;
    init();
}

public V put(K key, V value) {
    //hashmap的数组为空
    if (table == EMPTY_TABLE) {
        inflateTable(threshold);
    }
    if (key == null)
        return putForNullKey(value);
    //获取hash值
    int hash = hash(key);
    //找到应该放到table的哪个位置
    int i = indexFor(hash, table.length);
    //遍历table[i]位置的链表，查找相同的key,若找到则使用新的value替换oldValue,并返回oldValue
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {
        Object k;
        //如果key已经存在，将value设置为新的，并返回旧的value值
        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }

    modCount++;
    //将元素放到table[i]，新的元素总在table[i]位置的第一个元素，原来的元素后移
    addEntry(hash, key, value, i);
    return null;
}

private void inflateTable(int toSize) {
    //返回大于或等于最接近2的幂数
    int capacity = roundUpToPowerOf2(toSize);

    threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
    table = new Entry[capacity];
    initHashSeedAsNeeded(capacity);
}

private V putForNullKey(V value) {
    for (Entry<K,V> e = table[0]; e != null; e = e.next) {
        if (e.key == null) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }
    modCount++;
    addEntry(0, null, value, 0);
    return null;
}

static int indexFor(int h, int length) {
    // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
    return h & (length-1);
}

void addEntry(int hash, K key, V value, int bucketIndex) {
    // 容量超过阈值，并且发生碰撞时进行扩容
    if ((size >= threshold) && (null != table[bucketIndex])) {
        // 数组扩容为原来的2倍，并将元素复制到新数组上
        resize(2 * table.length);
        // 重新计算hash值，如果不做特殊设置的话，和之前算出来的hash值一样
        hash = (null != key) ? hash(key) : 0;
        bucketIndex = indexFor(hash, table.length);
    }

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

void createEntry(int hash, K key, V value, int bucketIndex) {
    Entry<K,V> e = table[bucketIndex];
    table[bucketIndex] = new Entry<>(hash, key, value, e);
    size++;
}

void resize(int newCapacity) {
    Entry[] oldTable = table;
    int oldCapacity = oldTable.length;
    //容量已经达到最大
    if (oldCapacity == MAXIMUM_CAPACITY) {
        threshold = Integer.MAX_VALUE;
        return;
    }

    Entry[] newTable = new Entry[newCapacity];
    transfer(newTable, initHashSeedAsNeeded(newCapacity));
    table = newTable;
    threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}

void transfer(Entry[] newTable, boolean rehash) {
    int newCapacity = newTable.length;
    // 遍历数组
    for (Entry<K,V> e : table) {
        // 遍历链表
        while(null != e) {
            Entry<K,V> next = e.next;
            if (rehash) {
                e.hash = null == e.key ? 0 : hash(e.key);
            }
            int i = indexFor(e.hash, newCapacity);
            e.next = newTable[i];
            newTable[i] = e;
            e = next;
        }
    }
}

public V get(Object key) {
    if (key == null)
        return getForNullKey();
    Entry<K,V> entry = getEntry(key);

    return null == entry ? null : entry.getValue();
}

private V getForNullKey() {
    if (size == 0) {
        return null;
    }
    for (Entry<K,V> e = table[0]; e != null; e = e.next) {
        if (e.key == null)
            return e.value;
    }
    return null;
}

final Entry<K,V> getEntry(Object key) {
    if (size == 0) {
        return null;
    }

    int hash = (key == null) ? 0 : hash(key);
    for (Entry<K,V> e = table[indexFor(hash, table.length)];
         e != null;
         e = e.next) {
        Object k;
        if (e.hash == hash &&
            ((k = e.key) == key || (key != null && key.equals(k))))
            return e;
    }
    return null;
}