在Java中生成给定大小的随机字符串

给定大小为 n，任务是生成此大小的随机字母数字字符串。

以下是生成给定大小的随机字母数字字符串的各种方法：

先决条件：在Java中生成随机数

方法一：使用 Math.random()

这里的函数getAlphaNumericString(n) 生成一个长度为字符串的随机数。这个数字是一个字符的索引，这个字符附加在临时局部变量 sb 中。最后 sb 被退回。

// Java program generate a random AlphaNumeric String
// using Math.random() method
  
public class RandomString {
  
    // function to generate a random string of length n
    static String getAlphaNumericString(int n)
    {
  
        // chose a Character random from this String
        String AlphaNumericString = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                                    + "0123456789"
                                    + "abcdefghijklmnopqrstuvxyz";
  
        // create StringBuffer size of AlphaNumericString
        StringBuilder sb = new StringBuilder(n);
  
        for (int i = 0; i < n; i++) {
  
            // generate a random number between
            // 0 to AlphaNumericString variable length
            int index
                = (int)(AlphaNumericString.length()
                        * Math.random());
  
            // add Character one by one in end of sb
            sb.append(AlphaNumericString
                          .charAt(index));
        }
  
        return sb.toString();
    }
  
    public static void main(String[] args)
    {
  
        // Get the size n
        int n = 20;
  
        // Get and display the alphanumeric string
        System.out.println(RandomString
                               .getAlphaNumericString(n));
    }
}

输出：

kU9vRVm9T1lFMbi3duO1

方法二：使用 CharSet

使用Java .nio.charset 包中的 Charset 随机生成 20 个字符长的字母数字字符串。

首先取 0 到 256 之间的 char 并遍历。
检查 char 是字母还是数字。
如果是，则在我们的字符串末尾添加
返回字符串

下面是上述方法的实现：

// Java program generate a random AlphaNumeric String
// using CharSet method
  
import java.util.*;
import java.nio.charset.*;
  
class RandomString {
  
    static String getAlphaNumericString(int n)
    {
  
        // length is bounded by 256 Character
        byte[] array = new byte[256];
        new Random().nextBytes(array);
  
        String randomString
            = new String(array, Charset.forName("UTF-8"));
  
        // Create a StringBuffer to store the result
        StringBuffer r = new StringBuffer();
  
        // Append first 20 alphanumeric characters
        // from the generated random String into the result
        for (int k = 0; k < randomString.length(); k++) {
  
            char ch = randomString.charAt(k);
  
            if (((ch >= 'a' && ch <= 'z')
                 || (ch >= 'A' && ch <= 'Z')
                 || (ch >= '0' && ch <= '9'))
                && (n > 0)) {
  
                r.append(ch);
                n--;
            }
        }
  
        // return the resultant string
        return r.toString();
    }
  
    public static void main(String[] args)
    {
        // size of random alphanumeric string
        int n = 20;
  
        // Get and display the alphanumeric string
        System.out.println(getAlphaNumericString(n));
    }
}

输出：

jj06CyZKfSBZQHM6KAUd

方法 3：使用正则表达式

首先取 0 到 256 之间的字符。
删除除 0-9、az 和 AZ 之外的所有字符。
随机选择一个字符
最后添加我们需要的变量

下面是上述方法的实现：

// Java program generate a random AlphaNumeric String
// using Regular Expressions method
  
import java.util.*;
import java.nio.charset.*;
  
class RandomString {
  
    static String getAlphaNumericString(int n)
    {
  
        // length is bounded by 256 Character
        byte[] array = new byte[256];
        new Random().nextBytes(array);
  
        String randomString
            = new String(array, Charset.forName("UTF-8"));
  
        // Create a StringBuffer to store the result
        StringBuffer r = new StringBuffer();
  
        // remove all spacial char
        String  AlphaNumericString
            = randomString
                  .replaceAll("[^A-Za-z0-9]", "");
  
        // Append first 20 alphanumeric characters
        // from the generated random String into the result
        for (int k = 0; k < AlphaNumericString.length(); k++) {
  
            if (Character.isLetter(AlphaNumericString.charAt(k))
                    && (n > 0)
                || Character.isDigit(AlphaNumericString.charAt(k))
                       && (n > 0)) {
  
                r.append(AlphaNumericString.charAt(k));
                n--;
            }
        }
  
        // return the resultant string
        return r.toString();
    }
  
    public static void main(String[] args)
    {
        // size of random alphanumeric string
        int n = 20;
  
        // Get and display the alphanumeric string
        System.out.println(getAlphaNumericString(n));
    }
}

输出：

4J8pirLzX6oIF0IIIaUU

方法四：生成大写字母/小写字母/数字的随机字符串

当字母数字字符串中需要某些特定字符时，例如仅大写字母或小写字母或数字，请使用此方法。下面的示例生成大小为 n 的随机小写字母字符串。

下面是上述方法的实现：

// Java program generate a random
// UpperCase or LowerCase or Number String
  
import java.util.*;
  
public class GFG {
  
    static String getAlphaNumericString(int n)
    {
  
        // lower limit for LowerCase Letters
        int lowerLimit = 97;
  
        // lower limit for LowerCase Letters
        int upperLimit = 122;
  
        Random random = new Random();
  
        // Create a StringBuffer to store the result
        StringBuffer r = new StringBuffer(n);
  
        for (int i = 0; i < n; i++) {
  
            // take a random value between 97 and 122
            int nextRandomChar = lowerLimit
                                 + (int)(random.nextFloat()
                                         * (upperLimit - lowerLimit + 1));
  
            // append a character at the end of bs
            r.append((char)nextRandomChar);
        }
  
        // return the resultant string
        return r.toString();
    }
  
    public static void main(String[] args)
    {
        // size of random alphanumeric string
        int n = 20;
  
        // Get and display the alphanumeric string
        System.out.println(getAlphaNumericString(n));
    }
}

输出：

qbhalyuzrenuwgvqidno