BigDecimal

先来看个案例,1.0 - 0.9The answer should have been0.1,但是如下图,输出的却是0.0999999998,这是因为double是近似值存储,There will be loss,在生活中,Some jobs require higher precision of numbers,所以doubleYou can't sign requirements,BigDecimalable to perform this task.

 BigDecimal概念和使用方法.例子

package com.zhang.BigDecimal;

import java.math.BigDecimal;

public class Demo01 {
    public static void main(String[] args) {
        /*double n1 = 1.0;
        double n2 = 0.9;
        System.out.println(n1-n2);*/
        //BigDecimal 大的浮点数精确计算
        BigDecimal n1 = new BigDecimal("1.0");//The value should be a string
        BigDecimal n2 = new BigDecimal("0.9");
        BigDecimal n3 = new BigDecimal("0.3");
        //减法
        BigDecimal r1 = n1.subtract(n2);
        System.out.println(r1);
        //加法
        BigDecimal r2 = n1.add(n2);
        System.out.println(r2);
        //乘法
        BigDecimal r3 = n1.multiply(n2);
        System.out.println(r3);
        //除法
        BigDecimal r4 = n2.divide(n3);
        System.out.println(r4);
    }
}

Date类 

DeteIt's enough to know something about the class

package com.zhang.Date;

import java.util.Date;

public class Dete {
    public static void main(String[] args) {
        //创建Date对象,
        //今天
        Date date1= new Date();
        System.out.println(date1.toString());
        System.out.println(date1.toLocaleString());
        //昨天
        Date date2 = new Date(date1.getTime()-(60*60*24*1000));
        System.out.println(date2.toLocaleString());
        //after before
        boolean b1 = date1.before(date2);
        boolean b2 = date2.before(date1);
        System.out.println(b1);
        System.out.println(b2);
        //比较compareTo（）
        int d3 = date1.compareTo(date2);
        System.out.println(d3);//结果为1或-1或0

    }
}

Calendar类

使用演示 

package com.zhang.calendar;

import java.util.Calendar;

public class Demo01 {
    public static void main(String[] args) {
        //创建对象
        Calendar calendar = Calendar.getInstance();
        System.out.println(calendar.getTime().toLocaleString());
        System.out.println(calendar.getTimeInMillis());//1970至今毫秒数
        //获取时间信息
        //年
        int year = calendar.get(Calendar.YEAR);//也可以表示为get（1）
        System.out.println("今年是"+year+"年");
        //月
        int month = calendar.get(Calendar.MONTH);//2
        System.out.println("当前为"+month+"月");//这里会有个问题,The resulting number will be less than the actual month1,This is the one it is from0month starts
        //日
        int day = calendar.get(Calendar.DAY_OF_MONTH);
        System.out.println("今天是"+day+"日");
        //时 HOUR12小时 HOUR_OF_DAY 24小时
        int hour = calendar.get(Calendar.HOUR);
        int hour2 = calendar.get(Calendar.HOUR_OF_DAY);
        System.out.println("当前为"+hour+"时");
        System.out.println("当前为"+hour2+"时");
        //分
        int minute = calendar.get(Calendar.MINUTE);
        System.out.println("现在是"+minute+"分");
        //秒
        int second = calendar.get(Calendar.SECOND);
        System.out.println("现在是"+second+"秒");
        //修改时间
        Calendar calendar2 = Calendar.getInstance();
        calendar2.set(Calendar.DAY_OF_MONTH,3);//日期改成3号
        System.out.println(calendar2.getTime().toLocaleString());
        //add 方法修改时间
        calendar.add(Calendar.HOUR,-1);//要修改的位置+,+数字（）Timing is plus,负数为减
        System.out.println(calendar.getTime().toLocaleString());
    }
}

SimpleDateFormat 

package com.zhang.sdf;

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

public class Demo01 {
    public static void main(String[] args) throws ParseException {
        //创建SimpleDateFormat 对象 y年 m月
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy年mm月dd日 hh时mm分ss秒");
        //创建Date
        Date date = new Date();
        //格式化date(把日期转成字符串）
        String str = sdf.format(date);
        System.out.println(str);
        //解析 （把字符串转成日期）
        Date date2 = sdf.parse("1990年07月05日 16时20分30秒");
        System.out.println(date2);
    }
}

System

package com.zhang.System;

public class Demo01 {
    public static void main(String[] args) {
        //arraycopy 数组复制
        //src;源数组
        //srcPos;Start copying the location
        //dest;目标数组
        //destPos;目标数组位置
        //length;复制的长度

        //创建数组
        int[] arr = {10,23,53,32,64,21,97};
        int[] dest = new int[7];

        System.arraycopy(arr,0,dest,0, arr.length);
        for (int i = 0;i<dest.length;i++){
            System.out.println(dest[i]);
        }
        //1970年到现在的毫秒数
        System.out.println(System.currentTimeMillis());
        //Code runtime timing
        //开始时间   Snippet to calculate   结束时间
        //运行时间=结束-开始
        long start = System.currentTimeMillis();
        for(int i = 0;i<=999999999;i++){
            for (int j = 0;j<999999;j++){
                int result = i+j;
            }
        }
        long end = System.currentTimeMillis();
        System.out.println("用时"+(end - start));

    }
}