Frequently used methods: (JDK7)

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clear()
containsKey(Object key)
entrySet()
get(Object key)
getOrDefault(key) (JDK8)
isEmpty()
keySet()
put(K key, V value)
putAll(Map<? extends K,? extends V> m)
remove(Object key): remove entry

jdk7 doc

Entry

Map中采用Entry内部类表示一个映射，每个键值对就是一个Entry

  1. map.entrySet()得到整个entrySet
  2. map.entrySet().iterator()得到entrySet的iterator
  3. map.entrySet().iterator().next()得到每一个具体的Entry
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    Iterator<Map.Entry<Integer, Integer>> it = map.entrySet().iterator();
    	while(it.hasNext()) {
    	        Map.Entry<Integer,Integer> entry=it.next();
    	        int key=entry.getKey();
    	        int value=entry.getValue();
    	        System.out.println(key+" "+value);
    	    }

另外有entrySet()和keySet()的解释这里不做说明.

不同种类的遍历方式:

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public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("1", "val1");
        map.put("2", "val2");
        map.put("3", "val3");

        System.out.println("1. 通过Map.keySet遍历key和value");
        for (String key : map.keySet()) {
            System.out.println("key= " + key + ", value= " + map.get(key));
        }

        System.out.println("2. 通过Map.entrySet的iterator遍历key和value");
        Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, String> entry = iterator.next();
            System.out.println("key= " + entry.getKey() + ", value= " + entry.getValue());
        }

        System.out.println("3. 通过Map.entrySet遍历key和value");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println("key= " + entry.getKey() + ", value= " + entry.getValue());
        }

        System.out.println("4. 通过Map.values遍历所有的values，但是不能遍历key");
        for (String v : map.values()) {
            System.out.println("value= " + v);
        }
    }