如何通过java检查用户输入的userid是否已经存在于firebase中？

dependencies {
    // Firebase Admin SDK
    implementation 'com.google.firebase:firebase-admin:8.0.0'
}

import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.ValueEventListener;

public class FirebaseUserChecker {
    public static void main(String[] args) {
        // 初始化Firebase Admin SDK
        FirebaseApp.initializeApp();

        // 获取Firebase数据库引用
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference usersRef = database.getReference("users");

        // 要检查的userid
        String userId = "your_user_id";

        // 添加事件监听器
        usersRef.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                if (dataSnapshot.hasChild(userId)) {
                    System.out.println("用户已存在于Firebase中");
                } else {
                    System.out.println("用户不存在于Firebase中");
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                System.out.println("检查用户存在性时发生错误：" + databaseError.getMessage());
            }
        });
    }
}