随机数 Stringbuffer

Random

package com.xiaohu.dao;

import java.util.Random;

public class DAO1 {
    public static void main(String[] args) {
        StringBuffer sa = new StringBuffer();
        Boolean flag = false;
        sa.append("hello");
        sa.append("world!");
        sa.append(flag);
        System.out.println(sa);
        
        
        Random rad = new Random();
        int num = rad.nextInt(50);
        
        
        Boolean b = rad.nextBoolean();
        System.out.println(num);
        System.out.println(b);
        
        Random rad1 = new Random(20);
        int r1 = rad1.nextInt();
        System.out.println(r1);
    }
}

综合案例

Canlendar
Calendar canlandar = Calendar.getInstance();

package com.xiaohu.dao;

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

public class DAO3 {
    public static void main(String[] args) {
        String oldName = "tt.jpg";
        String lastName = oldName.substring(oldName.lastIndexOf('.'));
        System.out.println(lastName);
        
        Date d = new Date();
        SimpleDateFormat f = new SimpleDateFormat("yyyyMMddhhmmss");
        String time =  f.format(d);
        
        Random rad = new Random();
        int num = rad.nextInt(Integer.MAX_VALUE);
        
        StringBuffer s = new StringBuffer();
        s.append(time);
        s.append(num);
        s.append(lastName);
        
        System.out.println(s);
    }
}

数学函数

package com.xiaohu.dao;

public class DAO4 {
    public static void main(String[] args) {
        double x1 = Math.abs(-23);//绝对值
        
        
        double x2 = Math.ceil(13.267);//向上取整
        double x3 = Math.floor(13.267);//向下取整
        double x4 = Math.max(11,22);//两数最大值
        
        double x5 = Math.round(11.5);
        double x6 = Math.round(11.2);//四舍五入
        
        double x7 = Math.random();//随机数
        
        double x8 = Math.sqrt(16);//开方
        
        System.out.println(x8);
    }
}