import java.util.*;
import javax.swing.*;

public class alphanumeric {
    public static void main(String args[]) {
        String nval, lenval;
        int n, len;

        nval = JOptionPane.showInputDialog("Enter number of codes you require: ");
        n = Integer.parseInt(nval);

        lenval = JOptionPane.showInputDialog("Enter code length you require: ");
        len = Integer.parseInt(lenval);

        find(n, len);
    }

    public static void find(int n, int length) {
        String str1 = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        StringBuilder sb = new StringBuilder(length);
        Random r = new Random();

        System.out.println("\n\t Unique codes are \n\n");
        for(int i=0; i<n; i++) {
            for(int j=0; j<length; j++) {
                sb.append(str1.charAt(r.nextInt(str1.length())));
            }
            System.out.println("  " + sb.toString());
            sb.delete(0, length);
        }
    }
}

这里是Java语言：

import static java.lang.Math.round;
import static java.lang.Math.random;
import static java.lang.Math.pow;
import static java.lang.Math.abs;
import static java.lang.Math.min;
import static org.apache.commons.lang.StringUtils.leftPad

public class RandomAlphaNum {
  public static String gen(int length) {
    StringBuffer sb = new StringBuffer();
    for (int i = length; i > 0; i -= 12) {
      int n = min(12, abs(i));
      sb.append(leftPad(Long.toString(round(random() * pow(36, n)), 36), n, '0'));
    }
    return sb.toString();
  }
}

下面是一个运行示例：

scala> RandomAlphaNum.gen(42)
res3: java.lang.String = uja6snx21bswf9t89s00bxssu8g6qlu16ffzqaxxoy

您可以为此使用Apache Commons库RandomStringUtils：

RandomStringUtils.randomAlphanumeric(20).toUpperCase();

如果密码必须包含数字和字母特殊字符，则可以使用以下代码：

private static final String NUMBERS = "0123456789";
private static final String UPPER_ALPHABETS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
private static final String LOWER_ALPHABETS = "abcdefghijklmnopqrstuvwxyz";
private static final String SPECIALCHARACTERS = "@#$%&*";
private static final int MINLENGTHOFPASSWORD = 8;

public static String getRandomPassword() {
    StringBuilder password = new StringBuilder();
    int j = 0;
    for (int i = 0; i < MINLENGTHOFPASSWORD; i++) {
        password.append(getRandomPasswordCharacters(j));
        j++;
        if (j == 3) {
            j = 0;
        }
    }
    return password.toString();
}

private static String getRandomPasswordCharacters(int pos) {
    Random randomNum = new Random();
    StringBuilder randomChar = new StringBuilder();
    switch (pos) {
        case 0:
            randomChar.append(NUMBERS.charAt(randomNum.nextInt(NUMBERS.length() - 1)));
            break;
        case 1:
            randomChar.append(UPPER_ALPHABETS.charAt(randomNum.nextInt(UPPER_ALPHABETS.length() - 1)));
            break;
        case 2:
            randomChar.append(SPECIALCHARACTERS.charAt(randomNum.nextInt(SPECIALCHARACTERS.length() - 1)));
            break;
        case 3:
            randomChar.append(LOWER_ALPHABETS.charAt(randomNum.nextInt(LOWER_ALPHABETS.length() - 1)));
            break;
    }
    return randomChar.toString();
}

我认为这是这里最小的解决方案，或者几乎是最小的方案之一：

 public String generateRandomString(int length) {
    String randomString = "";

    final char[] chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz01234567890".toCharArray();
    final Random random = new Random();
    for (int i = 0; i < length; i++) {
        randomString = randomString + chars[random.nextInt(chars.length)];
    }

    return randomString;
}

代码工作正常。如果您正在使用此方法，我建议您使用超过10个字符。在5个字符/30362次迭代时发生冲突。这花了9秒。

最佳随机字符串生成器方法

public class RandomStringGenerator{

    private static int randomStringLength = 25 ;
    private static boolean allowSpecialCharacters = true ;
    private static String specialCharacters = "!@$%*-_+:";
    private static boolean allowDuplicates = false ;

    private static boolean isAlphanum = false;
    private static boolean isNumeric = false;
    private static boolean isAlpha = false;
    private static final String alphabet = "abcdefghijklmnopqrstuvwxyz";
    private static boolean mixCase = false;
    private static final String capAlpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    private static final String num = "0123456789";

    public static String getRandomString() {
        String returnVal = "";
        int specialCharactersCount = 0;
        int maxspecialCharacters = randomStringLength/4;

        try {
            StringBuffer values = buildList();
            for (int inx = 0; inx < randomStringLength; inx++) {
                int selChar = (int) (Math.random() * (values.length() - 1));
                if (allowSpecialCharacters)
                {
                    if (specialCharacters.indexOf("" + values.charAt(selChar)) > -1)
                    {
                        specialCharactersCount ++;
                        if (specialCharactersCount > maxspecialCharacters)
                        {
                            while (specialCharacters.indexOf("" + values.charAt(selChar)) != -1)
                            {
                                selChar = (int) (Math.random() * (values.length() - 1));
                            }
                        }
                    }
                }
                returnVal += values.charAt(selChar);
                if (!allowDuplicates) {
                    values.deleteCharAt(selChar);
                }
            }
        } catch (Exception e) {
            returnVal = "Error While Processing Values";
        }
        return returnVal;
    }

    private static StringBuffer buildList() {
        StringBuffer list = new StringBuffer(0);
        if (isNumeric || isAlphanum) {
            list.append(num);
        }
        if (isAlpha || isAlphanum) {
            list.append(alphabet);
            if (mixCase) {
                list.append(capAlpha);
            }
        }
        if (allowSpecialCharacters)
        {
            list.append(specialCharacters);
        }
        int currLen = list.length();
        String returnVal = "";
        for (int inx = 0; inx < currLen; inx++) {
            int selChar = (int) (Math.random() * (list.length() - 1));
            returnVal += list.charAt(selChar);
            list.deleteCharAt(selChar);
        }
        list = new StringBuffer(returnVal);
        return list;
    }   

}