Create APIs with Firebase

1. Go to the Firebase console (https://console.firebase.google.com/) and create a new project.
2. Once you have created your project, navigate to the "Database" section in the left sidebar and click on the "Realtime Database" tab.
3. Click on the "Create database" button and choose "Start in test mode" to allow anyone to read and write to your database.
4. To connect to the Firebase Realtime Database from your Java application, you will need to install the Firebase SDK using Maven or Gradle, and include it in your code. Here's an example of how to do this in Java:

// Import the Firebase SDK
import com.google.firebase.*;
import com.google.firebase.database.*;

// Initialize Firebase
FirebaseOptions options = new FirebaseOptions.Builder()
  .setApiKey("your_api_key")
  .setAuthDomain("your_auth_domain")
  .setDatabaseUrl("your_database_url")
  .setProjectId("your_project_id")
  .setStorageBucket("your_storage_bucket")
  .setMessagingSenderId("your_messaging_sender_id")
  .build();
FirebaseApp.initializeApp(options);

1. Create the API endpoint(s) for your application. For example, here's a simple endpoint for creating a new record in the database:

app.post("/create", new Route() {
    @Override
    public Object handle(Request request, Response response) {
        // Get the new record from the request body
        Map<String, String> newRecord = new Gson().fromJson(request.body(), Map.class);

        // Get a reference to the database
        DatabaseReference ref = FirebaseDatabase.getInstance().getReference();

        // Push the new record to the database
        ref.push().setValue(newRecord);

        // Send a response
        response.status(200);
        return "Record created successfully.";
    }
});

1. To deploy your API, you can use a hosting service such as Firebase Hosting or any other service that supports Java.

2. Finally, you can test your API using a tool such as Postman or curl.

Please note that this is a basic example, it is recommended to also secure your API with Firebase Authentication and Firebase Cloud Firestore Security Rules.
You can find more detailed information and sample code in the Firebase Realtime Database documentation for Java: https://firebase.google.com/docs/database/admin/start

Also, you can use Firebase Admin SDK for Java to interact with Firebase Realtime Database, it has more functionalities like authentication, and security rules checking. https://firebase.google.com/docs/admin/setup.