9、
import java.util.*;
//这里省略部分代码
HashMap props = new HashMap();
props.put("hello", "javaer");
props.put("welcome", "to java contest");
props.put("wish", "you have a good grade");
Set s = props.keySet();
//在这里插入代码
System.out.println(s);
插入哪一个选项里的代码能够使程序对 props 里的键进行排序并输出
10、
这里有一张员工信息表(只展示了部分数据,数据格式类似)
+-------+---------------+-------+
| id | language | name |
+-------+---------------+-------+
| 1 | C++ | MA |
| 2 | C++ | VA |
| 3 | Go | VC |
| 4 | Python | MB |
| 5 | Php | VA |
+-------+---------------+-------+
查询哪些员工会使用 3 种及以上的编程语言
A.select name,count(*) as count from info having
count > 2 order by count desc;
B.select name,count(*) as count from info group by
name and count > 2 order by count desc;
C.select name,count(*) as count from info group by
name where count >= 2 order by count desc;
D.select name,count(*) as count from info group by
name having count > 2 order by count desc;
11、以下是 java concurrent 包下的 4 个类,选出差别最大的一个?
12、以下程序运行的结果为:
public class Example extends Thread{
@Override