HashMap

1、HashMap 线程不安全 key和value能存null值

        HashMap<Object, Object> map = new HashMap<>();
        map.put(1,1);
        map.put(1,1);
        map.put(null,2);
        map.put(null,"12");
        System.out.println(map.get(1));
````
### HashMap 能存null值 不会出现空指针的原因是：HashMap 的key如果是null 底层在去hashCode 的时候是默认赋值为0；

```java
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

2、ConcurrentHashMap 线程安全是因为加了锁 key和value不能存null值

ConcurrentHashMap<Object, Object> objectObjectConcurrentHashMap = new ConcurrentHashMap<>();
objectObjectConcurrentHashMap.put(null, null);

key 如果是null，key.hashCode() 会报空指针

• 部分原码

final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            synchronized (f) {}
            }
        }
}