HashMap和HashTable的异同

1实现的接口继承的类不同

HashTable

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable

HashMap

public class HashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable

2基础大小不同

HashTable

public Hashtable() {
        this(11, 0.75f);
    }

HashMap

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
 static final float DEFAULT_LOAD_FACTOR = 0.75f;

HashTable的初始容量为11，HashMap为16，两者的初始因子都是0.75

3两者的Hash不同

HashTable

private int hash(Object k) {
        // hashSeed will be zero if alternative hashing is disabled.
        return hashSeed ^ k.hashCode();
    }

HashMap

final int hash(Object k) {
        int h = hashSeed;
        if (0 != h && k instanceof String) {
            return sun.misc.Hashing.stringHash32((String) k);
        }

        h ^= k.hashCode();

        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

4对NUll为Key的情况处理不同

HashTable

public synchronized V put(K key, V value) {
        // Make sure the value is not null
        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry tab[] = table;
        int hash = hash(key);
        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++;
        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;
    }

HashTable的Key不能为空，当用null当做Key的时候会抛出​​java.lang.NullPointerException​​​异常，因为其在计算Hash的时候调用了​​key.hashCode();​​​当Key为Null的时候就会抛出异常，而且在put中，并未单独对null进行判断。
HashMap

public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        if (key == null)
            return putForNullKey(value);
        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;
    }

HashMap通过​​putForNullKey​​专门处理Key为null的情况。

5扩容方式不同

HashTable

protected void rehash() {
        HashtableEntry e, old;
        int i, index;
        int oldCapacity = table.length;
        HashtableEntry oldTable[] = table;

        int newCapacity = oldCapacity * 2 + 1;
        HashtableEntry newTable[] = new HashtableEntry[newCapacity];

        threshold = (int)(newCapacity * loadFactor);
        table = newTable;

        for (i = oldCapacity ; i-- > 0 ;) {
            for (old = oldTable[i] ; old != null ; ) {
                e = old;
                old = old.next;
                index = (e.hash & 0x7FFFFFFF) % newCapacity;
                e.next = newTable[index];
                newTable[index] = e;
            }
        }
    }

其扩容方式为​​newCapacity = oldCapacity * 2 + 1​​​
HashMap

void addEntry(int hash, K key, V value, int bucketIndex) {
        if ((size >= threshold) && (null != table[bucketIndex])) {
            resize(2 * table.length);
            hash = (null != key) ? hash(key) : 0;
            bucketIndex = indexFor(hash, table.length);
        }

        createEntry(hash, key, value, bucketIndex);
    }

其扩容方式为2倍扩容。​​resize(2 * table.length);​​

6使用场景不同

HashTable是线程安全的，因为其有​​synchronized​​​修饰符
HashMap非线程安全