【185天】黑马程序员27天视频学习笔记【Day14-下】

叨逼叨两句

1. 不容易，白天被叫去帮忙，不得已晚上来挑灯夜战，熬到2点，总算完成任务了。
2. 我打算下周开始换一个更新时间，每次把deadline设置为晚上12点，都会接近或者超过这个时间，之后改成中午12点为警戒线，晚上6点为deadline，这样可以倒逼自己的身体去早点完成任务，不至于晚上熬夜影响第二天的工作学习。

14-（13-21）：常用类

System

1. public static void gc():提醒垃圾回收器执行finalize（）方法回收垃圾。【finalize（）方法不能由程序员手动调用，只能由垃圾回收器来调用】

   
       package com.test.regex;
   
   public class Demo14 {
       public static void main(String[] args) {
           for(int i = 0; i < 1000; i++){
               new Demo();
               System.gc();
           }
       }
   }
   
   class Demo {
       public void finalize(){
           System.out.println("垃圾被清扫了");
       }
   }
   

2. public static void exit(int status)

   1. 将状态码设置为0，是正常终止当前运行的JVM
   2. 将状态码设置为非0，是异常终止当前运行的JVM

       package com.test.regex;
   
   public class Demo15 {
       public static void main(String[] args) {
           System.exit(0);
           System.out.println("111111");
       }
   }
   

3. public static long currentTimeMillis()

   package com.test.regex;
   
   public class Demo15 {
       public static void main(String[] args) {
           long start = System.currentTimeMillis();
           for (int i = 0; i < 1000 ; i++) {
               System.out.println(i);
           }
           long end = System.currentTimeMillis();
           System.out.println(end - start);
       }
   }

4. public static void arraycopy(Object src,int srcPos,Object dest,int destPos,int length)

   package com.test.regex;
   
   public class Demo15 {
       public static void main(String[] args) {
           int[] src = {11,22,33};
           int[] dest = new int[8];
           for (int i = 0; i < dest.length; i++) {
               System.out.println(dest[i]);
           }
   
           System.arraycopy(src, 0, dest, 0, src.length);
           System.out.println("--------------");
   
           for (int i = 0; i < dest.length; i++) {
               System.out.println(dest[i]);
           }
   
       }
   }
   
   

BigInteger

不可变的任意精度的整数

1. public BigInteger add(BigInteger val):加
2. public BigInteger subtract(BigInteger val):减
3. public BigInteger multiply(BigInteger val):乘
4. public BigInteger divide(BigInteger val):除
5. public BigInteger[] divideAndRemainder(BigInteger val)

package com.test.regex;

import java.math.BigInteger;

public class Demo15 {
    public static void main(String[] args) {
        BigInteger bi1 = new BigInteger("100");
        BigInteger bi2 = new BigInteger("2");

        System.out.println(bi1.add(bi2));        //+
        System.out.println(bi1.subtract(bi2)); //-
        System.out.println(bi1.multiply(bi2)); //*
        System.out.println(bi1.divide(bi2));       ///(除)

        BigInteger[] arr = bi1.divideAndRemainder(bi2);

        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }

    }
}

BigDecimal

1. float和double类型运算时非常容易丢失精度

   package com.test.regex;
   
   public class Demo15 {
       public static void main(String[] args) {
           System.out.println(2.0-1.1);
       }
   }
   

2. BigDecimal是不可变的、任意精度的有符号十进制数。

3. 加减乘除和BigInteger相同

   package com.test.regex;
   
   import java.math.BigDecimal;
   
   public class Demo15 {
       public static void main(String[] args) {
   
       BigDecimal bd1 = new BigDecimal(2.0);
       BigDecimal bd2 = new BigDecimal(1.1);
       System.out.println(bd1.subtract(bd2)); //开发不推荐，依然不精确
   
       BigDecimal bd3 = new BigDecimal("2.0");
       BigDecimal bd4 = new BigDecimal("1.1");
       System.out.println(bd3.subtract(bd4)); //开发推荐
   
       BigDecimal bd5 = BigDecimal.valueOf(2.0);
       BigDecimal bd6 = BigDecimal.valueOf(1.1);
       System.out.println(bd5.subtract(bd6)); //开发推荐
   
       }
   }
   

Date

1. Date():空参构造表示当前时间。
2. Date(long date):有参构造表示特定瞬间，精确到毫秒。

package com.test.regex;

import java.util.Date;

public class Demo15 {
    public static void main(String[] args) {

        Date d1 = new Date();
        System.out.println(d1);

        Date d2 = new Date(0);
        System.out.println(d2);

    }
}

1. public long getTime():返回毫秒数

   package com.test.regex;
   
   import java.util.Date;
   
   public class Demo15 {
       public static void main(String[] args) {
           Date d1 = new Date();
           System.out.println(d1.getTime()); //用Date对象来获取当前时间的毫秒值
           System.out.println(System.currentTimeMillis());//用System对象来获取当前时间的毫秒值
       }
   }

1. public void setTime(long time)：

package com.test.regex;

import java.util.Date;

public class Demo15 {
    public static void main(String[] args) {
        Date d1 = new Date();
        d1.setTime(1000);
        System.out.println(d1);
    }
}

SimpleDateFormat

1. DateFormat是SimpleDateFormat的父类

package com.test.regex;

import java.text.DateFormat;

public class Demo15 {
    public static void main(String[] args) {
        //DateFormat df = new DateFormat(); //DateFormat是抽象类，无法创建实例
        DateFormat df1 = DateFormat.getDateInstance(); //相当于父类引用指向子类对象，右边的方法返回一个子类对象
    }
}

1. 将日期对象转换为时间字符串

package com.test.regex;

import java.text.SimpleDateFormat;
import java.util.Date;

public class Demo15 {
    public static void main(String[] args) {
        Date d = new Date(); //获取当前时间对象
        SimpleDateFormat sdf = new SimpleDateFormat(); //创建日期格式化类对象
        System.out.println(sdf.format(d));  //17-8-10 上午1:02

        SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        System.out.println(sdf2.format(d));//2017/08/10 01:04:53

    }
}

1. 将时间字符串转换为日期对象

package com.test.regex;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class Demo15 {
    public static void main(String[] args) throws ParseException {
        String str = "2000年08月08日08:08:08";
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日HH:mm:ss");
        Date d = sdf.parse(str);
        System.out.println(d);
    }
}

练习

package com.test.regex;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class Demo15 {
    public static void main(String[] args) throws ParseException {
        String birthday = "1990年5月21日";
        String today = "2090年6月6日";
        SimpleDateFormat sdf = www.rcsx.org new SimpleDateFormat("yyyy年MM月dd日");
        Date d1 = sdf.parse(birthday);
        Date d2 = sdf.parse(today);
        long time = d2.getTime() - d1.getTime();
        System.out.println(time/1000/60/60/24/365);

    }
}

Calendar

1. public static Calendar getInstance()
2. public int get(int field)

package com.test.regex;

import java.util.Calendar;

public class Demo15 {
    public static void main(String[] args)  {
        Calendar c = Calendar.getInstance();
        System.out.println(c.get(Calendar.YEAR));  // 返回年
        System.out.println(c.get(Calendar.MONTH)); //返回月，0代表1月
        System.out.println(c.get(Calendar.DAY_OF_MONTH)); // 月中的第几天
        System.out.println(c.get(Calendar.DAY_OF_WEEK)); // 一周的第几天，周日是1
        System.out.println(c.get(Calendar.YEAR)+"年"+(c.get(Calendar.MONTH)+1)+"月"+c.get(Calendar.DAY_OF_MONTH)+
                "日"+getWeek(c.get(Calendar.DAY_OF_WEEK)));
        System.out.println(c.get(Calendar.YEAR)+"年"+getNum(c.get(Calendar.MONTH)+1)+"月"+getNum(c.get(Calendar.DAY_OF_MONTH)
                )+"日"+getWeek(c.get(Calendar.DAY_OF_WEEK)));

    }

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

    public static String getNum(int num){
        if(num > 9){
             return "" + num;
        } else {
            return "0" + num;
        }
    }

}

1. public void add(int field,int amount)
2. public final void set(int year,int month,int date)

package com.test.regex;

import java.util.Calendar;

public class Demo15 {
    public static void main(String[] args)  {
        Calendar c = Calendar.getInstance();
        //c.add(Calendar.YEAR, -1); //对指定字段进行操作，加或者减
        //c.set(Calendar.YEAR,2000); //对指定字段进行设置
        c.set(2000,8,8); //年、月、日

        System.out.println(c.get(Calendar.YEAR)+"年"+getNum(c.get(Calendar.MONTH)+1)+"月"+getNum(c.get(Calendar.DAY_OF_MONTH)
                )+"日"+getWeek(c.get(Calendar.DAY_OF_WEEK)));
    }

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

    public static String getNum(int num){
        if(num > 9){
             return "" + num;
        } else {
            return "0" + num;
        }
    }

}

练习:键盘录入任意一个年份，判断是闰年还是平年

 

package com.test.regex;

import java.util.Calendar;
import java.util.Scanner;

public class Demo16 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入年份，判断该年份是闰年还是平年：");
        //int year = sc.nextInt();
        String line = sc.nextLine();
        int year = Integer.parseInt(line);
        boolean b = getYear(year);
         System.out.println(b);
    }

    public static boolean getYear(int year) {
        Calendar c = Calendar.getInstance();
        c.set(year, 2,1);
        c.add(Calendar.DAY_OF_MONTH, -1);
        return c.get(Calendar.DAY_OF_MONTH) == 29;
    }
}