如何用Java编程方法生成一个指定长度的随机字符串？

在Java中，可以使用`java.util.Random`和`StringBuilder`类来生成指定长度的随机字符串。以下是一个简单的示例，该示例会生成包含大写字母、小写字母和数字的随机字符串：

import java.security.SecureRandom;
import java.util.Random;

public class RandomStringGenerator {
    private static final String CHAR_LOWER = "abcdefghijklmnopqrstuvwxyz";
    private static final String CHAR_UPPER = CHAR_LOWER.toUpperCase();
    private static final String NUMBER = "0123456789";
    private static final String ALL_CHARS = CHAR_LOWER + CHAR_UPPER + NUMBER;

    public static String generateRandomString(int length) {
        if (length <= 0) {
            throw new IllegalArgumentException("Length should be positive");
        }
        
        SecureRandom random = new SecureRandom(); // 使用SecureRandom以获得更好的随机性
        StringBuilder sb = new StringBuilder(length);
        for (int i = 0; i < length; i++) {
            int index = random.nextInt(ALL_CHARS.length());
            sb.append(ALL_CHARS.charAt(index));
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        int stringLength = 10;
        System.out.println(generateRandomString(stringLength));
    }
}