Java Day 20

Posted 2020-08-15 zhuzhuqwa

System类
 long currentTimeMillis();当前时间毫秒值
 
 getProperties()
  获取系统的属性信息，存储到properties集合
  使用自身的方法存取元素
  也可以自己定义一些属性信息

Runtime类
 Runtime 没有构造方法，不可以创建对象
 有非静态方法，则该类应该提供静态的返回该类对象的方法。
 单例设计模式

Math类
 final修饰
 ceil()
 floor()
 round()

Date类
 毫秒与日期转化
 
 日期对象转换成日期格式字符串
    DateFormat类中的format方法

 日期格式字符串转换成日期对象
    DateFormat类中的parse方法

  1 package com.company.Day020;
  2 
  3 import com.oracle.webservices.internal.api.message.PropertySet;
  4 import org.omg.Messaging.SYNC_WITH_TRANSPORT;
  5 
  6 import java.io.IOException;
  7 import java.text.DateFormat;
  8 import java.text.ParseException;
  9 import java.text.SimpleDateFormat;
 10 import java.util.Calendar;
 11 import java.util.Date;
 12 import java.util.Properties;
 13 import java.util.Set;
 14 
 15 /**
 16  * Created by junius on 2016/10/16.
 17  */
 18 public class Demo001 {
 19     private static final String LINE_SEPARATOR = System.getProperty("line.separator");
 20 
 21     public static void main(String[] args) throws ParseException {
 22         //System.out.println(System.currentTimeMillis());
 23         //demo_1();
 24        // System.out.print("hello"+LINE_SEPARATOR+"world");
 25 //        demo_3();
 26         String s1 = "2016-9-12";
 27         String s2 = "2016-9-16";
 28         int days = getDays(s1,s2);
 29         //System.out.print("days = "+days);
 30         int year =2012;
 31         showDate2(year);
 32 
 33 
 34     }
 35 
 36     public static void showDate2(int year) {
 37         Calendar c = Calendar.getInstance();
 38         c.set(year,2,1);
 39         c.add(Calendar.DAY_OF_MONTH,-1);
 40         showDate(c);
 41     }
 42 
 43     private static void showDate(Calendar c) {
 44         int year = c.get(Calendar.YEAR);
 45         int month = c.get(Calendar.MONTH)+1;
 46         int day = c.get(Calendar.DAY_OF_MONTH);
 47         int week = c.get(Calendar.DAY_OF_WEEK);
 48         System.out.print(year+"/"+month+"/"+day+" "+getWeek(week));
 49     }
 50 
 51     private static String getWeek(int i) {
 52         String[] week ={"","星期日","星期一","星期二","星期三","星期四","星期五","星期六"};
 53         return week[i];
 54     }
 55 
 56     private static int getDays(String s, String s1) throws ParseException {
 57         DateFormat dateFormat = DateFormat.getDateInstance();
 58         dateFormat = new SimpleDateFormat("yyyy-MM-dd");
 59 
 60         Date date_Str1 =dateFormat.parse(s);
 61         Date date_Str2 =dateFormat.parse(s1);
 62 
 63         long l1 = date_Str1.getTime();
 64         long l2 = date_Str2.getTime();
 65 
 66         long time = Math.abs(l1-l2);
 67 
 68         int day = getDay(time);
 69         return day;
 70     }
 71 
 72     private static int getDay(long time) {
 73         return (int)(time/1000/60/60/24);
 74     }
 75 
 76     private static void demo_3() throws ParseException {
 77         String str_date = "2016--10--16";
 78         DateFormat dateFormat = DateFormat.getDateInstance(DateFormat.LONG);
 79         dateFormat = new SimpleDateFormat("yyyy--MM--dd");
 80         Date date = dateFormat.parse(str_date);
 81 
 82         System.out.println(date);
 83     }
 84 
 85     private static void demo_2() {
 86         Date date = new Date();
 87         DateFormat dateFormat = DateFormat.getDateInstance(DateFormat.MEDIUM);
 88         //dateFormat = DateFormat.getDateTimeInstance();
 89         String str_date = dateFormat.format(date);
 90         System.out.print(str_date);
 91     }
 92 
 93     private static void demo_1() {
 94 
 95         Runtime r = Runtime.getRuntime();
 96         try {
 97             r.exec("notepad.exe");
 98         } catch (IOException e) {
 99             e.printStackTrace();
100         }
101 
102         Properties pp = System.getProperties();
103         Set<String> nameset = pp.stringPropertyNames();
104 
105         for(String name : nameset){
106             String value = pp.getProperty(name);
107             System.out.println(name+":"+value);
108         }
109     }
110 
111 
112 }

 

Calendar类

IO流
 字节流的抽象基类
 InputStream OutputStream
 字符流的抽象基类
 Reader Writer
 
 文字数据，优先考虑字符流
 如果文件不存在，就自动创建
 如果文件存在，就覆盖。

 换行和续写
  续写：使用构造函数
 

 1 package com.company.Day020;
 2 
 3 import java.io.FileWriter;
 4 import java.io.IOException;
 5 
 6 /**
 7  * Created by junius on 2016/10/16.
 8  */
 9 public class File002{
10     private static String LINE_SEPARATOR=System.getProperty("line.separator");
11 
12     public static void main(String[] args){
13         FileWriter fw = null;
14         try {
15             fw = new FileWriter("demo.txt");
16             fw.write("ww"+LINE_SEPARATOR+"w");
17             fw.flush();
18             fw.write("zz\r\nz");
19         } catch (IOException e) {
20             e.printStackTrace();
21         }finally {
22             if(fw!=null)//如果不判断就会报空指针异常
23             try {
24                 fw.close();
25             } catch (IOException e) {
26                 e.printStackTrace();
27             }
28         }
29 
30 
31     }
32 }

 

 IO异常处理
 
 FileReader -- 读取方式一
  1、确保文件一定存在

 读取方式二
  read(char[])
 

 1 package com.company.Day020;
 2 
 3 import java.io.FileNotFoundException;
 4 import java.io.FileReader;
 5 import java.io.IOException;
 6 import java.io.Reader;
 7 
 8 /**
 9  * Created by junius on 2016/10/17.
10  */
11 public class ReadDemo002 {
12     public static void main(String[] args) throws IOException {
13         Reader r = null;
14         int i=0;
15 
16         r = new FileReader("demo.txt");
17 
18         while((i=r.read())!=-1){
19             //System.out.print((char)i);
20         }
21 
22         r.close();
23 
24         Reader r2 = new FileReader("demo.txt");
25 
26         char[] buf = new char[10];
27 
28         //int num = r2.read(buf);
29 
30         int len = 0;
31         while((len=r2.read(buf))!=-1){
32             System.out.print(new String(buf,0,len));
33         }
34 
35 
36     }
37 }