Javaは、固定長のランダム文字列(大文字と小文字と数字)を生成します.

package org.jimmy.autosearch2019.test;

import java.util.ArrayList;
import java.util.Random;

/**
 * @author ラピスラズリ(Dawn)
 * @date 2019 5 30    3:18:48
 * @detail             
 */
public class TestRandom2019053001 {

    public static ArrayList strList = new ArrayList();
    public static Random random = new Random();
    public static final int RANDOM_LENGTH1 = 256;
    public static final int RANDOM_LENGTH2 = 512;
    public static final int RANDOM_LENGTH3 = 1024;
    
    static {
        init();
    }
    
    public static void main(String[] args) {
        String randomStr = generateRandomStr(RANDOM_LENGTH1);
        System.out.println(RANDOM_LENGTH1 + "    :" + randomStr);
        randomStr = generateRandomStr(RANDOM_LENGTH2);
        System.out.println(RANDOM_LENGTH2 + "    :" + randomStr);
        randomStr = generateRandomStr(RANDOM_LENGTH3);
        System.out.println(RANDOM_LENGTH3 + "    :" + randomStr);
    }
    
    public static String generateRandomStr(int length) {
        StringBuffer sb = new StringBuffer();
        for(int i = 0; i < length; i++) {
            int size = strList.size();
            String randomStr = strList.get(random.nextInt(size));
            sb.append(randomStr);
        }
        return sb.toString();
    }
    
    public static void init() {
        int begin = 97;
        //      ,     
        for(int i = begin; i < begin + 26; i++) {
            strList.add((char)i + "");
        }
        //      ,     
        begin = 65;
        for(int i = begin; i < begin + 26; i++) {
            strList.add((char)i + "");
        }
        // 0-9       
        for(int i = 0; i < 10; i++) {
            strList.add(i + "");
        }
    }
    
}

実行後の効果:
256ビット乱数:qZP 66 hBKD 2 qZUNNw 0 seH 2 auRjw 7 wvtVVVVAUalV 6 ZZZZZZXzL 8 H 6 MXSKcA 3 a 5 ae116 DhMLD 0 jXlbNQp 1 YlRpDIpijbn 9 EtqPlWsIbvlrE 1 fZtnY 8 K 88 N 6 uhOYsCZc 44 A 064 arq 0 bK 4 tkCvVcXLPTyGeHJkBfujOrQfpkkyRGGGVVVVVVVVVVVVkkkkkkkkkpkkkydRGVVVVVVVVVVVVVVtyDUjBpNf 393 WRygZKSDUy 704 B 2 hzXWw 7 lTRImD 5 Y 1 evtXmglo 4 xmJn 7 XogvlsnenKlNtnZhfIz 4 512ビット乱数:z 70 aSppoq 72 xBjElVsedc 3 jVrscJKczb 42 kwgn 1 DNK 3 GoLKhmsiG 0 qme 7 A PZB5kEXBer82Wz8DvGXQhLo2sZq1aCEKcviC0NUYBKxe0r6dTCrvrdMiIJfUv9O0idBEveM8ivLwbFmO1CO8nhNQftdBVFc1eaMwnHnM0VoXzHuqIpkmiII3tcwFi6pSWNzAhCXXtCuXjvEfh2KQS7hYqn2UdMGt347zRSKJqnJA7jjwsbg4tqrLxMl6YdqJMvWDRVWgcoL99aHCAbi4ruNJdRPef386FVF2toKXFOPekKrSQAU4oNwJj7hNdmum95ZP1XejYQ7VKSOwKXWbFKJrlGZVI2nex0zYQLLBKlDxbiiaTV24wjGDA6TWBysxjkWcnWu31CVF6cKOWZcBGvkw1seJ2UBYiYi 8 RMOqns 01 yN 8 vMDCcmxsnlcgPr p 1 ETrGIbveuyoBmnGUblPeNaVwAabJXiMEjylsuerVgIIXebVVVPFZZZJYsikoEMXbSWTCu 1 v 1024ビット乱数:xQWiJx 91 GDPeIJwWWH 3 chn 0 0 dayRF 8 NlKhWf 9 gdMkkkkkkkVImLmgY 4 PzpVcPzPzPzPzMnT 0 htcc 6 vPxCD 8 YVVVYf8 seTikKc 7 XXXjjjXXXJJJJJJJJJJJJJJJJJrsMMA 9 W 6 GBlRtynSfdPR 1 sTZT 67 TM 2 JDilkYKBuZSSOvOsXWMrMnsFEcMnAJHj 4 N 0 nAYfqFgKin 0 q 4 vdfsYOJv 5 zZ 2 OBnviqpQ 7 Hezgew 6 LEMo 5 JAGVcL 8 qZCnr 50 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