手寫hashMap的簡單實現

手寫hashMap的簡單實現

public class MyHashMap<K, V> {
    private static final int DEFAULT_CAPITY = 16;
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;
    private Entry<K, V>[] table;
    private int capity;
    private float loadFactor;
    private int size;
    private int threshold;

    public MyHashMap() {
        this.capity = DEFAULT_CAPITY;
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        this.threshold = (int) (DEFAULT_CAPITY * DEFAULT_LOAD_FACTOR);
    }

    public MyHashMap(int capity) {
        this.capity = tableSizeFor(capity);
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        this.threshold = (int) (this.capity * this.loadFactor);
    }
    public void put (K key, V value) {
        if (null == table) {
            table = new Entry[capity];
        }
        int hashCode = hash(key);
        int index = indexFor(hashCode);
        for (Entry<K, V> entry = table[index]; entry != null; entry = entry.next) {
            if (entry.key.equals(key)) {
                entry.value = value;
                return;
            }
        }
        addEntry(hashCode, key, value, index);
    }
    public V get(K key) {
        int hashCode = hash(key);
        int index = indexFor(hashCode);
        for (Entry<K, V> entry = table[index]; entry != null; entry = entry.next) {
            if (entry.key.equals(key)) {
                return entry.value;
            }
        }
        return null;
    }
    private void addEntry(int hash, K key, V value, int index) {
        Entry<K, V> entry = new Entry<>(key, value, hash, table[index]);
        table[index] = entry;
        ++size;
        if (size > threshold) {
            resize();
        }
    }
    private void resize() {
        Entry<K, V>[] oldTable = table;
        int oldCap = oldTable.length;
        capity = capity << 1;
        threshold = threshold << 1;
        table = new Entry[capity];
        for (int i = 0; i < oldCap; ++i) {
            Entry<K, V> entry;
            if ((entry = oldTable[i]).next == null) {
                table[entry.hash & (capity - 1)] = entry;
            } else {
                Entry<K, V> loHead = null, loTail = null;
                Entry<K, V> hiHead = null, hiTail = null;
                Entry<K, V> next;
                do {
                    next = entry.next;
                    if ((entry.hash & oldCap) == 0) {
                        if (null == loHead) {
                            loHead = entry;
                        } else {
                            loTail.next = entry;
                        }
                        loTail = entry;
                    } else {
                        if (null == hiHead) {
                            hiHead = entry;
                        } else {
                            hiTail.next = entry;
                        }
                        hiTail = entry;
                    }
                } while ((entry = next) != null);
                if (null != loTail) {
                    table[i] = loHead;
                }
                if (null != hiTail) {
                    table[i + oldCap] = hiHead;
                }
            }
        }
    }
    
    private int indexFor(int hashCode) {
        return hashCode & (capity - 1);
    }
    
    private int hash(K key) {
        if (null == key) {
            return 0;
        }
        int h = key.hashCode();
        return h ^ (h >> 16);
    }

    private int tableSizeFor(int capity) {
        int n = capity - 1;
        n |= n >> 1;
        n |= n >> 2;
        n |= n >> 4;
        n |= n >> 8;
        n |= n >> 16;
        return n < 0 ? DEFAULT_CAPITY : (n + 1);
    }

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

        public Entry(K key, V value, int hash, Entry<K, V> next) {
            this.key = key;
            this.value = value;
            this.hash = hash;
            this.next = next;
        }
    }
    public static void main(String[] args) {
        MyHashMap<String, String> myhashMap = new MyHashMap<>(4);
        myhashMap.put("1", "1v");
        myhashMap.put("2", "2v");
        myhashMap.put("3", "3v");
        myhashMap.put("4", "4v");
        myhashMap.put("5", "5v");
        myhashMap.put("5", "6v");
        System.out.println(myhashMap.get("5"));
        System.out.println("capity : " + myhashMap.capity);
        System.out.println("size : " + myhashMap.size);
    }
}