map

map的三种遍历方式：

    Map<String,String> map = new HashMap<>();
    map.put("1","a");
    map.put("2","b");
    map.put("3","c");
    map.put("4","d");

第一种：遍历key

       //使用keyset（）方法获取到键的集合
        Set<String> set = map.keySet();
        //使用迭代器遍历
        Iterator iterator = set.iterator();
        while ((iterator.hasNext())){
            //从集合中取出键
            String key = (String) iterator.next();
            //通过key获取value
            System.out.println(key + map.get(key));
        }

第二种：遍历value

        //获取到所有的值
        Collection<String> value = map.values();
        //进行遍历
        Iterator iterator1 = value.iterator();
        while (iterator1.hasNext()){
            String value1 = (String) iterator1.next();
            System.out.println(value1);
        }

第三种：变量entry实例

        //获取到所有entry实例集合
        Set<Map.Entry<String,String>> set1 = map.entrySet();
        //通过遍历，取出每一个entri实例
        Iterator<Map.Entry<String, String>> entryIterator = set1.iterator();
        while (entryIterator.hasNext()){
            Map.Entry map3 = entryIterator.next();
            String key = (String) map3.getKey();
            String value2 = (String) map3.getValue();
            System.out.println(key+value2);
        }