JAVA中遍历Map和Set方法，取出map中所有的key

Java遍历Set集合

1、迭代器遍历：
Set<String> set = new HashSet<String>();
Iterator<String> it = set.iterator();
while (it.hasNext()) {
  String str = it.next();
  System.out.println(str);
}
2、for循环遍历：
for (String str : set) {
      System.out.println(str);
}
3、优点还体现在泛型 假如 set中存放的是Object
Set<Object> set = new HashSet<Object>();
for循环遍历：
for (Object obj: set) {
      if(obj instanceof Integer){
                int aa= (Integer)obj;
             }else if(obj instanceof String){
               String aa = (String)obj
             }
              ........
}   

Java中遍历Map

Map<String, String> map = new HashMap(); 

        map.put("郑州", "河南");
        map.put("长沙", "湖南"); 

        //第一種
        Set<String> set = map.keySet(); //取出所有的key值
        for (String key:set) {
            System.out.println("第一种："+map.get(key));
        }

        //第二种
        Set<Map.Entry<String, String>> entryseSet=map.entrySet();
        for (Map.Entry<String, String> entry:entryseSet) {
            System.out.println("第二种："+entry.getKey()+":"+entry.getValue());
        }

        //第三种
        Iterator it = map.keySet().iterator();
        while(it.hasNext()){
           //int key = (Integer) it.next();
           String value = map.get(it.next());
           System.out.println("第三种："+value);
        }