Java-生成固定长度的随机字符串、随机字符串开头的ID、写入文件、读取文件

package com.sgcc;

import java.io.*;
import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class Main {
        public static String generateMixedString(int length) {
        Random random = new Random();
        StringBuilder sb = new StringBuilder(length);
        for (int i = 0; i < length; i++) {
            // 生成随机数，当生成的数字为0时，表示生成字母,生成数字为1时，表示生成数字；
            int type = random.nextInt(2);
            if (type == 0) {
                // 生成随机字母（小写）
                char letter = (char) (random.nextInt(26) + 'a');
                sb.append(letter);
            } else {
                // 生成随机数字
                char digit = (char) (random.nextInt(10) + '0');
                sb.append(digit);
            }
        }

        return sb.toString().toUpperCase();
    }
    
    public static void main(String[] args) {
    //示例一： 生成5个长度，随机字符串
    Main  m = new Main();
    String result =  m.generateMixedString(5);
    System.out.println(result);
    System.out.println("------------------------------------");

    //示例二： 生成18为长度不重复字符串，前6位为随机字母和数字，后12位为数字序列
    int  limitNum = 10;
    DecimalFormat df = new DecimalFormat("000000000000");
    StringBuffer  stringBuffer = new StringBuffer();
    for (int i=0 ;i < limitNum;i++){
        stringBuffer.append(generateMixedString(6) + df.format(i)).append("\n");
    }

    //示例三：将数据写入到文件
    String filePath = "result.txt";
    try (BufferedWriter writer = new BufferedWriter(new FileWriter(filePath))) {
        writer.write(stringBuffer.toString());
    } catch (IOException e) {
        e.printStackTrace();
    }

    //示例四：读取文件
    final String CHARSET_NAME = "UTF-8";
    List<String> content = new ArrayList<>();
    try (BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(filePath), CHARSET_NAME))) {
        String line;
        while ((line = br.readLine()) != null) {
            content.add(line);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }

//  输出list中每一行的内容
//    content.forEach(System.out::println);
//  根据下表获取List每一行的内容
    for (int i=0;i<content.size();i++){
        System.out.println(content.get(i));
    }


    }
}