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Creating a RESTful API in Java for user creation involves setting up a web application using a framework like Spring Boot. Spring Boot makes it relatively easy to build RESTful services. Here's a step-by-step guide to creating a simple API for user creation:

1. Setup Project: Create a new Spring Boot project using your preferred IDE or use Spring Initializr (https://start.spring.io/). Add the necessary dependencies, including "Spring Web" and "Spring Data JPA."

2. Define Entity: Define a User entity class representing the user data. This class will also be mapped to a database table using JPA annotations.

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String username;
    private String email;

    // Constructors, getters, setters
}

3. Repository: Create a repository interface for the User entity to perform CRUD operations. Spring Data JPA will provide the implementation.

import org.springframework.data.repository.CrudRepository;

public interface UserRepository extends CrudRepository<User, Long> {
}

4. Service: Create a service class to handle business logic related to user operations. Here, you'll create a method to save a new user.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserService {
    private final UserRepository userRepository;

    @Autowired
    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public User createUser(User user) {
        return userRepository.save(user);
    }
}

5. Controller: Create a controller class to define your REST endpoints. In this case, you'll create an endpoint to handle user creation.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class UserController {
    private final UserService userService;

    @Autowired
    public UserController(UserService userService) {
        this.userService = userService;
    }

    @PostMapping("/users")
    public ResponseEntity<User> createUser(@RequestBody User user) {
        User createdUser = userService.createUser(user);
        return new ResponseEntity<>(createdUser, HttpStatus.CREATED);
    }
}

6. Application Entry Point: The main Spring Boot application class to start the application.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class RestApiApplication {
    public static void main(String[] args) {
        SpringApplication.run(RestApiApplication.class, args);
    }
}

7. Configuration: Configure your database connection in the application.properties or application.yml file.

spring.datasource.url=jdbc:mysql://localhost:3306/your_database_name
spring.datasource.username=your_username
spring.datasource.password=your_password
spring.jpa.hibernate.ddl-auto=update

8. Run the Application: Run your Spring Boot application, and it will start an embedded server and expose the /users endpoint for creating users.

To test the API, you can use tools like Postman or cURL to make POST requests to http://localhost:8080/users with JSON data containing user information.

Remember that this is a basic example. In a production environment, you would add more validation, error handling, security measures, and potentially use DTOs (Data Transfer Objects) to decouple your API contract from your internal entity structure.