# Connection 相关问题 **题目一** ~~~ You need to store elements in a collection that guarantees that no duplicates are stored and all elements can be access in nature order, which interface provies that capabiliy? A. java.util.Map B. java.util.Collection C. java.util.List D. java.util.Set ~~~ 答案 D **题目二** ~~~ List, Set, Map是否继承自Collection接口,它们有什么区别? ~~~ List，Set是，Map不是 Set 不允许有重复的元素.且没有顺路 Set取元素时,没法说取第几个,只能以Iterator接口取得所有的元素,再逐一遍历各个元素. List表示有先后顺序的集合并且允许重复 Map与List和Set不同，存储一对key/value，不能存储重复的key **题目三** ~~~ public static void main(){ Map<String,String> map = new HashMap<String,String>(); map.out(String.valueOf(System.currentTimeMillis())+"a",1); map.out(String.valueOf(System.currentTimeMillis())+"a",2); map.out(String.valueOf(System.currentTimeMillis())+"a",3); for(Map.Entry<String,String> entry : map.entrySet()){ System.out.printf(entry.getValue()); } } ~~~ 输出顺序是 123顺序无法确定. Map 中的键是 Set. Set 顺序是随机的.