HashMap API

API 1: put(K key, V value)

add new key-value pairs to the HashMap

API 2: clear()

API 3: containsKey(Object key)

check whether this map contains a mapping for the specified key

API 4: containsValue(Object value)

API 5: get(Object key)

API 6: getOrDefault(Object key, V defaultValue)

API 7: isEmpty()

API 8: putAll(Map<? extends K, ? extends V> m)

API 9: putIfAbsent(K key, V value)

API 10: remove(Object key)

API 11: remove(Object key, Object value)

API 12: size()

API 13: keySet()

API 14: entrySet()

• 遍历map的一种方法

for (Map.Entry<Integer, String> entry : map.entrySet())

API 14.1: getKey()

API 14:2 getValue()

API 15: values()

returns a Collection view of the values contained in this map

• 可以遍历values，但是不能找到对应的key

Iterator<Map.Entry<Integer, String>> entryIter = map.entrySet().iterator();

while (entryIter.hasNext()) {
    Map.Entry...?
}