13.常用类

1.随机数类

package cn.cc.day15;

import java.util.Random;

/**
 * @author CC
 * 随机数
 */
public class TestRadom {

	public static void main(String[] args) {
		String[] name = {"胖虎","谷丰","胖子","cc"};
		int min = 0;
		int max = name.length-1;
		Random random = new Random();
		//指定范围的随机数
		int sum  = random.nextInt(max-min+1)+min;
		System.out.println(name[sum]);
	}

}

2.时间类

package cn.cc.day15;

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

import javax.xml.crypto.Data;

public class ClassDate {

	public static void main(String[] args) {
		//获取系统时间
		Date date = new Date();
		//格式化对象SimpleDateFormat
		SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
		//format(时间对象),返回的是准换后的字符串时间
		String strTime = simpleDateFormat.format(date);
		System.out.println(strTime);
	}

}

3.Math类

package cn.cc.day15;

public class MathTest {

	public static void main(String[] args) {
		int num = -10;
		//绝对值
		System.out.println(Math.abs(num));//10
		//立方根
		System.out.println(Math.cbrt(27));//3
		//平方根
		System.out.println(Math.sqrt(9));//3
		
		//向上取整
		System.out.println(Math.ceil(3.5));//4
		System.out.println(Math.ceil(3.1));//4
		System.out.println(Math.ceil(-1.1));//-1
		System.out.println(Math.ceil(-1.5));//-1
		
		//向下取整
		System.out.println(Math.floor(1.1));//1
		System.out.println(Math.floor(1.5));//1
		System.out.println(Math.floor(-2.2));//-3
		System.out.println(Math.floor(-2.6));//-3
		
		//四舍五入
		System.out.println(Math.round(2.6));//3
		System.out.println(Math.round(2.2));//2
		System.out.println(Math.round(-2.2));//-2
		System.out.println(Math.round(-2.6));//-3
		
		//找出两个数中的小数
		System.out.println(Math.min(2, 5));
		//找出两个数中的大数
		System.out.println(Math.max(2, 5));
		
	}

}

4.System类

System.currentTimeMillis()

获取从1970-1-1到当前的时间毫秒数

System.exit()

关闭程序

5.枚举类

package cn.cc.day15;

public enum Season {
	//先声明枚举值
	Spring("春季",1),Summer("夏季",2),Autumn("秋季",3),Winter("冬季",4);
	private Season(String seasonName,int index) {
		this.seasonName = seasonName;
		this.index = index;
	}
	//静态方法
	public static String getName(int index) {
		//遍历枚举
		for (Season s : Season.values()) {
			if (index == s.getIndex()) {
				return s.getSeasonName();
			}
		}
		return null;
	}
	//属性
	private String seasonName;
	private int index;
	public String getSeasonName() {
		return seasonName;
	}
	public void setSeasonName(String seasonName) {
		this.seasonName = seasonName;
	}
	public int getIndex() {
		return index;
	}
	public void setIndex(int index) {
		this.index = index;
	}
}

测试类:

package cn.cc.day15;

public class SeasonTest {

	public static void main(String[] args) {
	
		Season season = Season.Spring;
		System.out.println(season);//输出枚举值
		System.out.println(Season.getName(season.getIndex()));//根据标号输出对应枚举值的解释
	}

}

6.日历类

Calendar 对象=Calendar.getInstance();

• 获取年份:对象.get(Calendar.YEAR)
• 获取月份:对象.get(Calendar.MONTH)+1
• 获取天数:对象.get(Calendar.DATE)
• 获取小时:对象.get(Calendar.HOUR)
• 获取分钟:对象.get(Calendar.MINUTE)
• 获取秒数:对象.get(Calendar.SECOND)
• 获取星期几(周一是第一天):对象.get(Calendar.DAY_OF_WEEK

代码:

package cn.qf.day16.calendardemo;

import java.util.Calendar;

public class Test1 {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		//日历类，得到的单例对象
		//使用系统默认的时区 ,TimeZone
		//使用系统默认的区域，区域不同，一周开始的那个周几是不同
		//系统的时间是当前时间
		Calendar now=Calendar.getInstance();
		//从日历对象中获取时间信息
		//获取年份
		int year=now.get(Calendar.YEAR);
		System.out.println(year);
		//获取值的范围0-11   获取的月份后+1
		int month=now.get(Calendar.MONTH);
		System.out.println(month+1);
		//获取日:当前月的总天数
		//getMaximum   当前月中最大天数
		int day=now.getMaximum(Calendar.DAY_OF_MONTH);
		System.out.println(day);
		//获取小时
		int hour=now.get(Calendar.HOUR);
		System.out.println(hour);
		
		//分钟
		System.out.println(now.get(Calendar.MINUTE));
		//秒
		System.out.println(now.get(Calendar.SECOND));
		//星期几    周日是第一天
		System.out.println(now.get(Calendar.DAY_OF_WEEK));
		//几号
		System.out.println(now.get(Calendar.DATE));
	}

}

---

计算两个时间相差的天数:

package cn.qf.day16.calendardemo;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;

public class Test2 {

	public static void main(String[] args) {
		//计算两个时间相差的天数
		//得到日历对象
		Calendar cd=Calendar.getInstance();
		String strStart="2019-7-1";
		String strEnd="2019-7-22";
		DateFormat df=new SimpleDateFormat("yyyy-MM-dd");
		Calendar cd2=Calendar.getInstance();
		//设置cd日历对象的默认时间
		try {
			cd.setTime(df.parse(strStart));
			cd2.setTime(df.parse(strEnd));
			long diff=cd2.getTimeInMillis()-cd.getTimeInMillis();
			int days=(int)(diff/1000/60/60/24);
			System.out.println("已经过了："+days);
			//System.out.println(diff);
		} catch (ParseException e) {
			e.printStackTrace();
		}
	}
}

---

计算一段时间之后的时间

package cn.qf.day16.calendardemo;

import java.util.Calendar;

public class Test3 {

	public static void main(String[] args) {
		//一年之后的时间
		Calendar cd=Calendar.getInstance();
		//设置日历是一年之后的时间
		cd.add(Calendar.YEAR, 1);
		System.out.println("一年之后："+cd.get(Calendar.YEAR)+"年"+
		                   (cd.get(Calendar.MONTH)+1)+"月"+
				           cd.get(Calendar.DATE));
		
		//10天之后是几月几号
		Calendar cd2=Calendar.getInstance();
		cd2.add(Calendar.DATE, 10);
		System.out.println(cd2.get(Calendar.YEAR)+"年"+
                (cd2.get(Calendar.MONTH)+1)+"月"+
		           cd2.get(Calendar.DATE));
	}

}