Java Day 20

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

package com.company.Day020;

import com.oracle.webservices.internal.api.message.PropertySet;
import org.omg.Messaging.SYNC_WITH_TRANSPORT;

import java.io.IOException;
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Properties;
import java.util.Set;

/**
 * Created by junius on 2016/10/16.
 */
public class Demo001 {
    private static final String LINE_SEPARATOR = System.getProperty("line.separator");

    public static void main(String[] args) throws ParseException {
        //System.out.println(System.currentTimeMillis());
        //demo_1();
       // System.out.print("hello"+LINE_SEPARATOR+"world");
//        demo_3();
        String s1 = "2016-9-12";
        String s2 = "2016-9-16";
        int days = getDays(s1,s2);
        //System.out.print("days = "+days);
        int year =2012;
        showDate2(year);


    }

    public static void showDate2(int year) {
        Calendar c = Calendar.getInstance();
        c.set(year,2,1);
        c.add(Calendar.DAY_OF_MONTH,-1);
        showDate(c);
    }

    private static void showDate(Calendar c) {
        int year = c.get(Calendar.YEAR);
        int month = c.get(Calendar.MONTH)+1;
        int day = c.get(Calendar.DAY_OF_MONTH);
        int week = c.get(Calendar.DAY_OF_WEEK);
        System.out.print(year+"/"+month+"/"+day+" "+getWeek(week));
    }

    private static String getWeek(int i) {
        String[] week ={"","星期日","星期一","星期二","星期三","星期四","星期五","星期六"};
        return week[i];
    }

    private static int getDays(String s, String s1) throws ParseException {
        DateFormat dateFormat = DateFormat.getDateInstance();
        dateFormat = new SimpleDateFormat("yyyy-MM-dd");

        Date date_Str1 =dateFormat.parse(s);
        Date date_Str2 =dateFormat.parse(s1);

        long l1 = date_Str1.getTime();
        long l2 = date_Str2.getTime();

        long time = Math.abs(l1-l2);

        int day = getDay(time);
        return day;
    }

    private static int getDay(long time) {
        return (int)(time/1000/60/60/24);
    }

    private static void demo_3() throws ParseException {
        String str_date = "2016--10--16";
        DateFormat dateFormat = DateFormat.getDateInstance(DateFormat.LONG);
        dateFormat = new SimpleDateFormat("yyyy--MM--dd");
        Date date = dateFormat.parse(str_date);

        System.out.println(date);
    }

    private static void demo_2() {
        Date date = new Date();
        DateFormat dateFormat = DateFormat.getDateInstance(DateFormat.MEDIUM);
        //dateFormat = DateFormat.getDateTimeInstance();
        String str_date = dateFormat.format(date);
        System.out.print(str_date);
    }

    private static void demo_1() {

        Runtime r = Runtime.getRuntime();
        try {
            r.exec("notepad.exe");
        } catch (IOException e) {
            e.printStackTrace();
        }

        Properties pp = System.getProperties();
        Set<String> nameset = pp.stringPropertyNames();

        for(String name : nameset){
            String value = pp.getProperty(name);
            System.out.println(name+":"+value);
        }
    }


}

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
 

package com.company.Day020;

import java.io.FileWriter;
import java.io.IOException;

/**
 * Created by junius on 2016/10/16.
 */
public class File002{
    private static String LINE_SEPARATOR=System.getProperty("line.separator");

    public static void main(String[] args){
        FileWriter fw = null;
        try {
            fw = new FileWriter("demo.txt");
            fw.write("ww"+LINE_SEPARATOR+"w");
            fw.flush();
            fw.write("zz
z");
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            if(fw!=null)//如果不判断就会报空指针异常
            try {
                fw.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }


    }
}

 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[])
 

package com.company.Day020;

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.Reader;

/**
 * Created by junius on 2016/10/17.
 */
public class ReadDemo002 {
    public static void main(String[] args) throws IOException {
        Reader r = null;
        int i=0;

        r = new FileReader("demo.txt");

        while((i=r.read())!=-1){
            //System.out.print((char)i);
        }

        r.close();

        Reader r2 = new FileReader("demo.txt");

        char[] buf = new char[10];

        //int num = r2.read(buf);

        int len = 0;
        while((len=r2.read(buf))!=-1){
            System.out.print(new String(buf,0,len));
        }


    }
}