Edit
System-Date-Runtime-Calendar-Math-Random
一.System类
-
System:类中的方法和属性都是静态的;在java.lang包里面
out:标准输出,默认是控制台;
in:标准输入,默认是键盘;
-
获取系统属性信息:Properties getProperties();
Properties继承至Hashtable,所以可以用Map集合的方法对其进行操作,该集合存储的都是字符串,没有泛型定义
-
获取指定属性信息:String getProperty()
1.import java.util.*;
2.class SystemDemo
3.{
4. public static void main(String[] args)
5. {
6. //获取所有属性信息
7. Properties prop = System.getProperties();
8. for(Object obj : prop.keySet())
9. {
10. String value = (String)prop.get(obj);
11. System.out.println(obj+"::"+value);
12. }
13.
14. //获取指定属性信息
15. String str = System.getProperty("os.name");
16. System.out.println(str);
17.
18. //在系统中自定义特有信息
19. System.setProperty("mykey","myvalue");
20.
21. //在启动JVM时,动态加载属性信息
22. String v = System.getProperty("haha");
23. System.out.println("v="+v);//我们在cmd命令行输入:java -Dhaha=qqqqqq SystemDemo,输出结果为haha=qqqqqq
24. }
25.}
26.
二.Runtime类
Runtime类:在java.lang包里面,每个 Java 应用程序都有一个 Runtime 类实例，使应用程序能够与其运行的环境相连接。可以通过 getRuntime 方法获取当前运行时。
- 1.该类没有提供构造函数,说明该类不能new对象,且该类中方法不是静态的,说明该类肯定会提供方法获取本类对象,而且该方法- 是静态的,并返回值类型是本类对象,由这个特点可以看出该类使用了单例设计模式完成,保证该类对象的唯一性;
- 2.获取该类对象的方法是:static Runtime getRuntime()
- 3.Runtime类中的exec()方法:在单独的进程中执行指定的字符串命令。
1.class RuntimeDemo
2.{
3. public static void main(String[] args) throws Exception
4. {
5. Runtime r = Runtime.getRuntime();
6. //exec()要抛出IOException
7. Process p = r.exec("notepad.exe test.java");//记事本程序子进程,并打开test.java文件
8. Thread.sleep(3000);//让进程冻结3秒
9. p.destroy();//关闭子进程r,这里destroy()只能关闭在这里创建的进程
10. }
11.}
12.
三.Date类
1.import java.util.*;//Date类在java.util包中
2.import java.text.*;//因为DateFormat抽象类在java.test包中
3.class DateDemo
4.{
5. public static void main(String[] args)
6. {
7. Date d = new Date();
8. System.out.println(d);
9.
10. //将时间的模式封装到SimpleDateFormat对象中
11. DateFormat df = new SimpleDateFormat("yyyy年MM月dd日 E");
12.
13. //调用format()方法让模式格式化指定Date对象
14. String time = df.format(d);
15. System.out.println(time);
16.
17. }
18.}
四.Calendar类
1.import java.util.*;//Date类在java.util包中
2.class CalendarDemo
3.{
4. public static void main(String[] args)
5. {
6. //设置时间
7. SetCalendar();
8. //获取2016二月的天数
9. DaysOfMonth(2016);//2016年二月29天
10. //获取昨天的此刻时间
11. DaysOfMonth();
12.
13.
14.
15.
16. }
17. //设置时间
18. public static void SetCalendar()
19. {
20. Calendar c = Calendar.getInstance();
21. //设置时间,并且这里的2,实际指的是3月
22. c.set(2010,2,6);
23.
24. //年份加5
25. c.add(Calendar.YEAR,5);
26. //月份减5
27. c.add(Calendar.MONTH,-5);
28. CalendarPrint(c);
29. }
30.
31.
32. //获取时间
33. public static void CalendarPrint(Calendar c)
34. {
35. sop(c.get(Calendar.YEAR)+"年");
36. sop((c.get(Calendar.MONTH)+1)+"月");
37. sop(c.get(Calendar.DAY_OF_MONTH)+"日");
38.
39. //利用查表法来打印月份
40. String[] mons = {"一月","二月","三月","四月"
41. ,"五月","六月","七月","八月"
42. ,"九月","十月","十一月","十二月"};
43. sop(mons[c.get(Calendar.MONTH)]);
44. }
45.
46.
47. //练习:获取任意年的二月有多少天
48. public static void DaysOfMonth(int year)
49. {
50. Calendar c = Calendar.getInstance();
51. c.set(year,2,1);
52. c.add(Calendar.DAY_OF_MONTH,-1);
53. sop(c.get(Calendar.DAY_OF_MONTH));
54. }
55.
56.
57. //练习:获取昨天的此刻时间
58. public static void DaysOfMonth()
59. {
60. Calendar c = Calendar.getInstance();
61. c.add(Calendar.DAY_OF_MONTH,-1);
62. sop(c);
63. }
64. public static void sop(Object obj)
65. {
66. System.out.println(obj);
67. }
68.
69.}
五.Math和Random类
1.//Math类在lang包中,Random类在util包中
2.import java.util.*;
3.class MathRand
4.{
5. public static void main(String[] args)
6. {
7. double d = Math.ceil(23.51);//ceil()返回大于指定数据的最小整数
8. double d1 = Math.floor(32.12);//floor()返回小于指定数据的最大整数
9. double d2 = Math.round(12.35);//round()四舍五入
10. double d3 = Math.pow(2,3);//pow()为做幂次方运算2的3次方
11. double d4 = Math.random();//randow()返回一个大于等于0,小于1的随机数
12. System.out.println(d+" "+d1+" "+d2+" "+d3+" "+d4);
13.
14. int d5 = (int)(Math.random()*10+1);//返回1到10的随机数
15. System.out.println(d5);
16.
17. Random r = new Random();
18. //nextInt(int n)返回0到n的随机整数
19. int num = r.nextInt(10);
20. System.out.println(num);
21. }
22.}
23.
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本文深入讲解Java中的System、Runtime、Date、Calendar、Math和Random等核心类的功能与使用方法，包括如何获取系统属性、执行外部命令、日期时间操作及数学运算。
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