01.HashMap源码学习整理思维导图

/** * 基于数组实现HashMap * * @author zx * @date 2021年05月05日 13:37 */
public class ArrayHashMap<K, V> {
   
    //先不考虑扩容 HashMap 初始容量为16. ；
    // 第一次；当存放的值 16*0.75 =12(容量 * 0.75) 进行扩容，扩容后得到的容量大小为: 16<<1 = 32;
    //第二次： 当存放的值32*0.75 = 25 进行扩容 ,扩容大小 32 << 1 = 64
    private final Entry[] entries = new Entry[1000];

    /** * 添加元素 * * @param k key值 * @param v value值 */
    public void add(K k, V v) {
   
        int index = hash(k);
        Entry oldEntry = entries[index];
        if (oldEntry == null) {
   
            entries[index] = new Entry<>(k, v);
        } else {
   
            if (index == 0) {
   
                entries[index] = new Entry<>(k, v);
            }
            oldEntry.next = new Entry<>(k, v);
        }
    }

    /** * 根据key值获取对应的value * * @param k key值 * @return 返回对应key的value值 */
    public V get(K k) {
   
        int index = hash(k);
        for (Entry oldEntry = entries[index]; oldEntry != null; oldEntry = oldEntry.next) {
   
            //hashcode相同，我们需要比较内容值
            if ((k == null && oldEntry.key == null) || oldEntry.key.equals(k)) {
   
                return (V) oldEntry.value;
            }
        }
        return null;
    }

    /** * 遍历Map */
    public void list() {
   
        if (entries == null) {
   
            System.out.println("集合中没有存放元素，不能为遍历");
        }
        for (Entry entry : entries) {
   
            if (entry != null) {
   
                Entry temp = entry.next;
                while (temp != null) {
   
                    System.out.println(temp.key + "---" + temp.value);
                    temp = temp.next;
                }
                System.out.println(entry.key + "---" + entry.value);
            }
        }
    }


    /** * 计算hash值 * * @param key key值 * @return hash值 */
    static int hash(Object key) {
   
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }


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

        public Entry(K key, V value) {
   
            this.key = key;
            this.value = value;
        }

        @Override
        public String toString() {
   
            return "Entry{" +
                    "key=" + key +
                    ", value=" + value +
                    '}';
        }
    }

    public static void main(String[] args) {
   
        ArrayHashMap<Object, Object> map = new ArrayHashMap<>();
        map.add(null, "null value");
        map.add(null, "null value2");
        map.add("a", "a is hashcode 97");
        map.add(97, "97 is hashcode 97");
        map.add(98, "98 is hashcode 98");
        map.add("b", "b is hashcode 98");
        System.out.println("------------------");
        map.list();
        System.out.println("------------------");
        System.out.println(map.get(null));
        System.out.println(map.get("a"));
        System.out.println(map.get(97));
    }
}