生成验证码，可以有大写字母，小写字母和数字，共五位，数字占一位，字母占四位的java程序

在Java中，生成包含随机大写字母、小写字母和数字的六位验证码（其中数字仅占一位）可以使用`Random`类和`StringBuilder`来实现。以下是一个简单的示例：

import java.security.SecureRandom;
import java.text.DecimalFormat;

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

    public static void main(String[] args) {
        StringBuilder captcha = generateCaptcha(5);
        System.out.println("Generated captcha: " + captcha.toString());
    }

    public static StringBuilder generateCaptcha(int length) {
        SecureRandom random = new SecureRandom();
        StringBuilder captchaBuilder = new StringBuilder(length);

        // 确保数字只有一位
        int digitPos = random.nextInt(length);
        captchaBuilder.append(NUMBER.charAt(random.nextInt(NUMBER.length()))); // 添加数字

        // 剩余三位填入大写字母和小写字母
        for (int i = 0; i < length - 1; i++) {
            if (i == digitPos) continue;
            captchaBuilder.append(CHAR_LOWER.charAt(random.nextInt(CHAR_LOWER.length())));
            captchaBuilder.append(CHAR_UPPER.charAt(random.nextInt(CHAR_UPPER.length())));
        }

        return captchaBuilder;
    }
}