5.28

import java.util.Random;

public class test1 {
    public static void main(String[] args) {
        Random r = new Random();
        for(int i = 0; i< 10; i++ ) {
            System.out.println("第"+(i+1)+"个随机数");
            int j = r.nextInt(101);
            System.out.println(j);
        }
    }
}

package demo;

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

public class test2 {
    public static void main(String[] args) {
        SimpleDateFormat sdf = new SimpleDateFormat("Gyyyy年MM月dd日 E HH:mm:ss");
        Date date = new Date();
        String format = sdf.format(date);
        System.out.println(format);
    }  
}

package demo;

import java.util.Scanner;

public class test3 {
    public static void main(String[] args) {        
        Scanner sc= new Scanner(System.in);
        System.out.println("输入一个邮箱地址");
        String string = sc.next();
        if(string.contains("@.") ==true) {
            int indexOf = string.indexOf("@.");
            String substring = string.substring(0, indexOf);
            System.out.println(substring);
        }else {
            System.out.println("邮箱不合法");
        }    
    }  
}