HashMap代码实现

HashMap 是一种常用的数据结构，它可以用来存储键值对，通过键来快速查找对应的值。以下是一个简单的 HashMap 实现：

public class HashMap<K, V> {
    private static final int DEFAULT_CAPACITY = 16;
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;

    private Entry<K, V>[] table;
    private int size;
    private int threshold;
    private float loadFactor;

    public HashMap() {
        this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

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

    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity <= 0 || loadFactor <= 0 || Float.isNaN(loadFactor)) {
            throw new IllegalArgumentException();
        }
        this.loadFactor = loadFactor;
        this.table = new Entry[initialCapacity];
        this.threshold = (int) (initialCapacity * loadFactor);
    }

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

    public V get(K key) {
        if (key == null) {
            return getForNullKey();
        }
        int hash = hash(key.hashCode());
        int index = indexFor(hash, table.length);
        for (Entry<K, V> e = table[index]; e != null; e = e.next) {
            if (e.hash == hash && (e.key == key || key.equals(e.key))) {
                return e.value;
            }
        }
        return null;
    }

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

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

    private 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);
        }
    }

    private void resize(int newCapacity) {
        Entry<K, V>[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == Integer.MAX_VALUE) {
            threshold = Integer.MAX_VALUE;
            return;
        }
        Entry<K, V>[] newTable = new Entry[newCapacity];
        transfer(newTable);
        table = newTable;
        threshold = (int) (newCapacity * loadFactor);
    }

    private void transfer(Entry<K, V>[] newTable) {
        Entry<K, V>[] src = table;
        int newCapacity = newTable.length;
        for (int j = 0; j < src.length; j++) {
            Entry<K, V> e = src[j];
            if (e != null) {
                src[j] = null;
                do {
                    Entry<K, V> next = e.next;
                    int i = indexFor(e.hash, newCapacity);
                    e.next = newTable[i];
                    newTable[i] = e;
                    e = next;
                } while (e != null);
            }
        }
    }

    private int hash(int h) {
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

    private int indexFor(int hash, int length) {
        return hash & (length - 1);
    }

    static class Entry<K, V> {
        final int hash;
        final K key;
        V value;
        Entry<K, V> next;

        Entry(int hash, K key, V value, Entry<K, V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }
    }
}

这个实现使用了数组和链表来存储键值对，当添加新的键值对时，会根据键的哈希值来计算出它在数组中的位置，如果该位置已经有了其他键值对，就使用链表来解决冲突。当数组中的键值对数量超过了阈值时，会自动扩容数组。