2019.3.19

                                        今日一练

问题?

        hashMap  hashTable

        hashMap中的key与value可以为null吗?

        hashTable中的key与value可以为null吗?

      

1.1hashMap中的key  与value都可以为null

 jdk1.8的源码中可以看到

public V put(K key, V value) {
    if (table == EMPTY_TABLE) {
        inflateTable(threshold);
    }
    if (key == null)    //key可以为null 
        return putForNullKey(value);   //当key为null返回说明此时到这里代码已经不走了.返回此方法
    int hash = hash(key);
    int i = indexFor(hash, table.length);
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {
        Object k;
        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }

    modCount++;
    addEntry(hash, key, value, i);
    return null;
}

当key为null返回 putForNullKey(value);这个方法    可以看出value也可以为null

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

1.2  hashTable中的key与value均不可以为null

public synchronized V put(K key, V value) {
    // Make sure the value is not null
    if (value == null) {
        throw new NullPointerException();     //当value为null则出现空指针异常.
    }

    // Makes sure the key is not already in the hashtable.
    Entry tab[] = table;
    int hash = hash(key);   //此时如果key为null   个人认为直接就是编译出现异常,
    int index = (hash & 0x7FFFFFFF) % tab.length;
    for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            V old = e.value;
            e.value = value;
            return old;
        }
    }

    modCount++;//记录计算次数
    if (count >= threshold) {
        // Rehash the table if the threshold is exceeded
        rehash();

        tab = table;
        hash = hash(key);
        index = (hash & 0x7FFFFFFF) % tab.length;
    }

    // Creates the new entry.
    Entry<K,V> e = tab[index];
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
    return null;
}