您可以创建一个包含所有字母和数字的字符数组，然后可以从该字符数组中随机选择并创建自己的字符串密码。

char[] chars = new char[62]; // Sum of letters and numbers

int i = 0;

for(char c = 'a'; c <= 'z'; c++) { // For letters
    chars[i++] = c;
}

for(char c = '0'; c <= '9';c++) { // For numbers
    chars[i++] = c;
}

for(char c = 'A'; c <= 'Z';c++) { // For capital letters
    chars[i++] = c;
}

int numberOfCodes = 0;
String code = "";
while (numberOfCodes < 1) { // Enter how much you want to generate at one time
    int numChars = 8; // Enter how many digits you want in your password

    for(i = 0; i < numChars; i++) {
        char c = chars[(int)(Math.random() * chars.length)];
        code = code + c;
    }
    System.out.println("Code is:" + code);
}

我找到了生成随机十六进制编码字符串的解决方案。所提供的单元测试似乎符合我的主要用例。虽然，它比提供的一些其他答案稍微复杂一些。

/**
 * Generate a random hex encoded string token of the specified length
 *  
 * @param length
 * @return random hex string
 */
public static synchronized String generateUniqueToken(Integer length){ 
    byte random[] = new byte[length];
    Random randomGenerator = new Random();
    StringBuffer buffer = new StringBuffer();

    randomGenerator.nextBytes(random);

    for (int j = 0; j < random.length; j++) {
        byte b1 = (byte) ((random[j] & 0xf0) >> 4);
        byte b2 = (byte) (random[j] & 0x0f);
        if (b1 < 10)
            buffer.append((char) ('0' + b1));
        else
            buffer.append((char) ('A' + (b1 - 10)));
        if (b2 < 10)
            buffer.append((char) ('0' + b2));
        else
            buffer.append((char) ('A' + (b2 - 10)));
    }
    return (buffer.toString());
}

@Test
public void testGenerateUniqueToken(){
    Set set = new HashSet();
    String token = null;
    int size = 16;

    /* Seems like we should be able to generate 500K tokens 
     * without a duplicate 
     */
    for (int i=0; i<500000; i++){
        token = Utility.generateUniqueToken(size);

        if (token.length() != size * 2){
            fail("Incorrect length");
        } else if (set.contains(token)) {
            fail("Duplicate token generated");
        } else{
            set.add(token);
        }
    }
}

高效而简短。

/**
 * Utility class for generating random Strings.
 */
public interface RandomUtil {

    int    DEF_COUNT = 20;
    Random RANDOM    = new SecureRandom();

    /**
     * Generate a password.
     *
     * @return the generated password
     */
    static String generatePassword() {
        return generate(true, true);
    }

    /**
     * Generate an activation key.
     *
     * @return the generated activation key
     */
    static String generateActivationKey() {
        return generate(false, true);
    }

    /**
     * Generate a reset key.
     *
     * @return the generated reset key
     */
    static String generateResetKey() {
        return generate(false, true);
    }

    static String generate(boolean letters, boolean numbers) {
        int
            start = ' ',
            end   = 'z' + 1,
            count = DEF_COUNT,
            gap   = end - start;
        StringBuilder builder = new StringBuilder(count);

        while (count-- != 0) {
            int codePoint = RANDOM.nextInt(gap) + start;

            switch (getType(codePoint)) {
                case UNASSIGNED:
                case PRIVATE_USE:
                case SURROGATE:
                    count++;
                    continue;
            }

            int numberOfChars = charCount(codePoint);

            if (count == 0 && numberOfChars > 1) {
                count++;
                continue;
            }

            if (letters && isLetter(codePoint)
                || numbers && isDigit(codePoint)
                || !letters && !numbers) {

                builder.appendCodePoint(codePoint);
                if (numberOfChars == 2)
                    count--;
            }
            else
                count++;
        }
        return builder.toString();
    }
}

我使用的是一个非常简单的Java8解决方案。只需根据您的需求进行定制。

...
import java.security.SecureRandom;
...

//Generate a random String of length between 10 to 20.
//Length is also randomly generated here.
SecureRandom random = new SecureRandom();

String sampleSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_";

int stringLength = random.ints(1, 10, 21).mapToObj(x -> x).reduce((a, b) -> a).get();

String randomString = random.ints(stringLength, 0, sampleSet.length() - 1)
        .mapToObj(x -> sampleSet.charAt(x))
        .collect(Collector
            .of(StringBuilder::new, StringBuilder::append,
                StringBuilder::append, StringBuilder::toString));

我们可以使用它生成如下的字母数字随机字符串（返回的字符串将强制包含一些非数字字符以及一些数字字符）：

public String generateRandomString() {
            
    String sampleSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_";
    String sampleSetNumeric = "0123456789";
    
    String randomString = getRandomString(sampleSet, 10, 21);
    String randomStringNumeric = getRandomString(sampleSetNumeric, 10, 21);
    
    randomString = randomString + randomStringNumeric;
    
    //Convert String to List<Character>
    List<Character> list = randomString.chars()
            .mapToObj(x -> (char)x)
            .collect(Collectors.toList());
    
    Collections.shuffle(list);
    
    //This is needed to force a non-numeric character as the first String
    //Skip this for() if you don't need this logic

    for(;;) {
        if(Character.isDigit(list.get(0))) Collections.shuffle(list);
        else break;
    }
    
    //Convert List<Character> to String
    randomString = list.stream()
            .map(String::valueOf)
            .collect(Collectors.joining());
    
    return randomString;
    
}

//Generate a random number between the lower bound (inclusive) and upper bound (exclusive)
private int getRandomLength(int min, int max) {
    SecureRandom random = new SecureRandom();
    return random.ints(1, min, max).mapToObj(x -> x).reduce((a, b) -> a).get();
}

//Generate a random String from the given sample string, having a random length between the lower bound (inclusive) and upper bound (exclusive)
private String getRandomString(String sampleSet, int min, int max) {
    SecureRandom random = new SecureRandom();
    return random.ints(getRandomLength(min, max), 0, sampleSet.length() - 1)
    .mapToObj(x -> sampleSet.charAt(x))
    .collect(Collector
        .of(StringBuilder::new, StringBuilder::append,
            StringBuilder::append, StringBuilder::toString));
}

另一个解决方案。。。

public static String generatePassword(int passwordLength) {
    int asciiFirst = 33;
    int asciiLast = 126;
    Integer[] exceptions = { 34, 39, 96 };

    List<Integer> exceptionsList = Arrays.asList(exceptions);
    SecureRandom random = new SecureRandom();
    StringBuilder builder = new StringBuilder();
    for (int i=0; i<passwordLength; i++) {
        int charIndex;

        do {
            charIndex = random.nextInt(asciiLast - asciiFirst + 1) + asciiFirst;
        }
        while (exceptionsList.contains(charIndex));

        builder.append((char) charIndex);
    }
    return builder.toString();
}

您可以使用UUID类及其getLeastSignificantBits（）消息来获取64位随机数据，然后将其转换为基数36的数字（即由0-9、a-Z组成的字符串）：

Long.toString(Math.abs( UUID.randomUUID().getLeastSignificantBits(), 36));

这将产生一个长达13个字符的字符串。我们使用Math.abs（）来确保没有负号潜入。