该方法返回map中所有key值的列表。
今天再代码中看到了Map集合中的HashMap的map.keySet()方法，首先看一下这个方法的定义
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
*
* @return a set view of the keys contained in this map
*/
Set<K> keySet();
大致的意思描述如下：
1）返回此映射中包含的键的 Set视图。
2）这个 set 受到映射支持，所以对映射的更改可在此 set 中反映出来，反之亦然。
3）如果对该 set 进行迭代的同时修改了映射（通过迭代器自己的 remove 操作除外），则迭代结果是不确定的。
4）set 支持元素移除，通过 Iterator.remove、 Set.remove、 removeAll、retainAll 和 clear 操作可从映射中移除（删除）相应的映射关系。
5）set不支持 add 或 addAll 两种添加操作。
6）返回值是：map包含的键的 set 视图
代码的使用：
Map<Integer, String> map = new HashMap<>();
//下面可以使用map.keySet()方法
map.keySet();
测试代码：
public class SourceCode {
public static void main(String[] args) {
Map<String,String> map = new HashMap<String, String>();
map.put("xiaocui1","gongchen");
map.put("xiaocui2","daima");
map.put("xiaocui3","xuexi");
map.put("xiaocui4","dagong");
System.out.println(map.keySet());
System.out.println("-----分割线-----");
for(String map1 : map.keySet()){
String string = map.keySet().toString();
System.out.println(string);
}
}
}
输出结果：
[xiaocui4, xiaocui1, xiaocui2, xiaocui3]
-----分割线-----
[xiaocui4, xiaocui1, xiaocui2, xiaocui3]
[xiaocui4, xiaocui1, xiaocui2, xiaocui3]
[xiaocui4, xiaocui1, xiaocui2, xiaocui3]
[xiaocui4, xiaocui1, xiaocui2, xiaocui3]
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