底层原理Hashmap源码解析实例

Map.java

package com.collection;

public interface Map<K, V> {
    public V put(K k, V v);

    public V get(K k);

    public int size();

    interface Entry<K, V> {
        public K getKey();

        public V getValue();
    }
}

HashMap.java

package com.collection;

public class HashMap<K, V> implements Map<K, V> {
    private Entry<K, V>[] table = null;
    private int size = 0;
    private static int defaultLength = 16;

    public HashMap() {
        table = new Entry[defaultLength];
    }

    @Override
    public V put(K k, V v) {
        int index = hash(k);
        Entry<K, V> entry = table[index];
        if (entry == null) {
            table[index] = new Entry<K, V>(k, v, null, index);
            size++;
        } else {
            table[index] = new Entry<K, V>(k, v, entry, index);
        }
        return table[index].getValue();
    }

    private int hash(K k) {
        int index = k.hashCode() & (defaultLength - 1);
        return Math.abs(index);
    }

    @Override
    public V get(K k) {
        if (size == 0) {
            return null;
        }
        int index = hash(k);
        Entry<K, V> entry = getEntry(k, index);

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

    private Entry<K, V> getEntry(K k, int index) {
        for (Entry<K, V> entry = table[index]; entry != null; entry = entry.next) {
            if (entry.hash == index && (k == entry.getKey() || k.equals(entry.getKey()))) {
                return entry;
            }
        }
        return null;
    }

    @Override
    public int size() {
        return 0;
    }

    class Entry<K, V> implements Map.Entry<K, V> {
        K k;
        V v;
        Entry<K, V> next;
        int hash;

        public Entry(K k, V v, Entry<K, V> next, int hash) {
            this.k = k;
            this.v = v;
            this.next = next;
            this.hash = hash;
        }

        @Override
        public K getKey() {
            return k;
        }

        @Override
        public V getValue() {
            return v;
        }
    }
}

TestHashMap.java

package com.collection;

public class TestHashMap {
    public static void main(String[] args) {
        com.collection.HashMap map = new com.collection.HashMap<>();


        for (int i = 0; i < 6666; i++) {
            map.put("Monkey" + i, "計算方法的事實");
            System.out.println(map.get("Monkey"));
        }

        System.out.println(map);

    }
}