如何在Java中从PGP公钥中获取用户id？

import org.bouncycastle.openpgp.PGPPublicKey;
import org.bouncycastle.openpgp.PGPPublicKeyRing;
import org.bouncycastle.openpgp.PGPUtil;
import org.bouncycastle.openpgp.operator.jcajce.JcaPGPObjectFactory;

import java.io.FileInputStream;
import java.io.InputStream;

public class PGPKeyExample {
    public static void main(String[] args) throws Exception {
        // 加载公钥文件
        InputStream keyInputStream = new FileInputStream("public_key.asc");

        // 创建PGPPublicKeyRing对象
        PGPPublicKeyRing keyRing = new PGPPublicKeyRing(PGPUtil.getDecoderStream(keyInputStream), new JcaPGPObjectFactory());

        // 获取第一个公钥
        PGPPublicKey publicKey = keyRing.getPublicKey();

        // 获取公钥指纹
        String fingerprint = new String(publicKey.getFingerprint());

        // 转换为十六进制字符串
        String hexFingerprint = toHex(fingerprint);

        // 提取用户ID
        String userId = hexFingerprint.substring(hexFingerprint.length() - 8);

        System.out.println("User ID: " + userId);
    }

    private static String toHex(String fingerprint) {
        StringBuilder hexString = new StringBuilder();
        for (int i = 0; i < fingerprint.length(); i++) {
            hexString.append(Integer.toHexString(fingerprint.charAt(i)));
        }
        return hexString.toString();
    }
}