【185天】黑马程序员27天视频学习笔记【Day14-下】

叨逼叨两句

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。

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

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");
    }
}

public static void exit(int status)

将状态码设置为0，是正常终止当前运行的JVM
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");
    }
}

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);
    }
}

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

public BigInteger add(BigInteger val):加
public BigInteger subtract(BigInteger val):减
public BigInteger multiply(BigInteger val):乘
public BigInteger divide(BigInteger val):除
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

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); 
    }
}

BigDecimal是不可变的、任意精度的有符号十进制数。

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

Date():空参构造表示当前时间。
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);
    
    }
}

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
    }
}

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

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
    }
}

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

        
    }
}

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

public static Calendar getInstance()
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;
        }
    }
    
}

public void add(int field,int amount)
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;
    }
}

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原文地址：https://www.cnblogs.com/tianshifu/p/7337775.html