五、JAVA常用类库之Math、Random、Date

1.Math类：

1.2基本类库：

1.3代码示例： 

public class MathRandomDemo {

	public static void main(String[] args) {
		
		//pi
		System.out.println(Math.PI);
		//返回double的绝对值
		System.out.println(Math.abs(-1));
		//Math.random()伪随机数流大于等于0小于1
		System.out.println(Math.random());
		//返回最接近参数并返回某一整数的Double值
		System.out.println(Math.round(Math.random()*1000)/100.0);
		//平方根
		System.out.println(Math.sqrt(2));
		
		//返回最小的数
		System.out.println(Math.min(10, 6));
		//返回最大的数
		System.out.println(Math.max(3, -12));
	}

}

2.Random类：

此类的实例用于生成伪随机数流

2.1代码示例： 

import java.util.Random;

public class MathRandomDemo {

	public static void main(String[] args) {
		
	   Random r = new Random();
	   //0到long的最大值
	   System.out.println(r.nextLong());
	   //0到5的最大值
	   System.out.println(r.nextInt(5));
	}

}

 

3.Date：

import java.util.Date;

public class DateDemo {
	public static void main(String[] args) {
		
		Date date = new Date();
		System.out.println(date);
	}

}

4.Calendar：

import java.util.Calendar;
import java.util.GregorianCalendar;

public class DateDemo {
	public static void main(String[] args) {
		
		
		Calendar c1 = Calendar.getInstance();
		Calendar c2 = new GregorianCalendar();
		
		int year = c1.get(Calendar.YEAR);
		int month = c1.get(Calendar.MONTH);
		int data = c1.get(Calendar.DAY_OF_MONTH);
		//24小时制
		int hour = c1.get(Calendar.HOUR_OF_DAY);
		int minute = c1.get(Calendar.MINUTE);
		int second = c1.get(Calendar.SECOND);
		int millisecond = c1.get(Calendar.MILLISECOND);
		StringBuilder sb = new StringBuilder();
		sb.append(year).append("年").append(month).append("月").append(data).append("日").append(" ")
		  .append(hour).append(":").append(minute).append(":").append(second)
		  .append(" ").append(millisecond);
		System.out.println(sb.toString());
		
	}

}

上述输出有点繁琐：为了简单化，引入DateFormat

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class DateDemo {
	public static void main(String[] args) {
		
		Date date = new Date();
		System.out.println(date);
		
		
		Calendar c1 = Calendar.getInstance();
		Calendar c2 = new GregorianCalendar();
		
		int year = c1.get(Calendar.YEAR);
		int month = c1.get(Calendar.MONTH);
		int data = c1.get(Calendar.DAY_OF_MONTH);
		//24小时制
		int hour = c1.get(Calendar.HOUR_OF_DAY);
		int minute = c1.get(Calendar.MINUTE);
		int second = c1.get(Calendar.SECOND);
		int millisecond = c1.get(Calendar.MILLISECOND);
		StringBuilder sb = new StringBuilder();
		sb.append(year).append("年").append(month).append("月").append(data).append("日").append(" ")
		  .append(hour).append(":").append(minute).append(":").append(second)
		  .append(" ").append(millisecond);
		System.out.println(sb.toString());
		
		DateFormat af = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss SSS");
		String newDate = af.format(new Date());
		System.out.println(newDate);
		
	}

}