What Is an API in Java?

From web development to mobile apps, and especially in Java, APIs are essential to building scalable, modular, and maintainable applications. But what exactly is an API in Java? How does it work? Why is it important?

In Java, APIs are very important and used almost everywhere. They include Java’s built-in libraries as well as custom REST APIs that you can create using tools like Spring Boot or Jakarta REST. In this article, we will explore what an API is, what it means specifically in Java, why it is significant, and how you can get started with developing your own API using Java.

1. Understanding the Concept of an API

API (Application Programming Interface) is a set of defined rules and protocols that allow one software application to communicate with another. It acts as a contract between different software components.

APIs can be:

• Library APIs: Like Java’s Collections API, which allows us to use predefined classes and methods for data structures.
• Web APIs: Like RESTful services that allow remote systems to communicate over HTTP.
• Hardware APIs: That let software interact with device functionalities.

In essence, an API exposes a set of functionalities and hides the internal logic, promoting abstraction, reusability, and modularity.

2. What Is an API in the Context of Java?

In Java, an API can refer to:

• Java API (Standard Library): A collection of prebuilt packages, classes, and interfaces provided by the Java Development Kit (JDK), such as java.util, java.io, java.net, etc.
• Third-party APIs: Libraries developed by others, like Apache Commons, Google Guava, or Jackson.
• Custom APIs: Code you write and expose to other systems, such as RESTful APIs created using Java frameworks.

2.1 Java Standard API Example

Here is an example using the Java Collections API:

public class JavaApiExample {

    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("Thomas");
        names.add("Charles");
        names.add("Jennifer");

        Collections.sort(names);

        System.out.println("Sorted Names: " + names);
    }
}

In this code:

• ArrayList and Collections are part of the Java API.
• You use these classes without worrying about the internal implementation of sorting or list management.

3. The Significance of APIs in Java

APIs play a critical role in Java development, as they define clear boundaries and contracts between different parts of a program or between separate systems. Whether you are working with the standard Java libraries, integrating third-party services, or building your own reusable modules, APIs help structure your code in a consistent and maintainable way.

Here are several reasons why APIs are important in Java:

• Encapsulation and Abstraction: APIs allow developers to interact with complex systems or components without needing to understand the internal workings. For example, when you use the List interface in Java, you don’t need to know how ArrayList or LinkedList works under the hood. This abstraction simplifies development and reduces the likelihood of errors.
• Code Reusability: Once an API is defined, it can be reused across different projects, modules, or even by other developers. This promotes modular design and helps teams avoid rewriting the same logic multiple times.
• Interoperability: APIs make it easier for different software components or systems to work together. In Java, this is especially useful when your application needs to communicate with web services, databases, or external libraries. RESTful APIs, for instance, allow Java applications to exchange data with other systems over HTTP.
• Scalability and Maintainability: By clearly defining what each part of the application does through its API, it becomes easier to scale the application or make updates without breaking other parts of the code. This makes long-term maintenance more manageable and efficient.
• Improved Productivity: Java’s vast ecosystem of built-in and third-party APIs helps developers build powerful applications faster. For instance, using the Spring Framework’s APIs for dependency injection, web development, or data access lets you focus more on business logic rather than boilerplate code.

4. Getting Started with API Development in Java (Creating a REST API)

There are several ways to create a RESTful Web API in Java, including using frameworks like Jakarta EE, Micronaut, or Quarkus. However, in this article, we will focus on using Spring Boot, which is a widely used framework for building REST APIs in Java. Let’s look at how to create a simple RESTful Web API using Spring Boot.

Create a Spring Boot Project

You can use https://start.spring.io or set it up manually. Use the following dependencies:

• Spring Web
• Spring Boot DevTools

Maven POM snippet

	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-devtools</artifactId>
			<scope>runtime</scope>
			<optional>true</optional>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

Define a Model

Create a Java class to represent the data structure your API will work with.

public class Book {

    private String id;
    private String title;
    private String author;

    public Book(String id, String title, String author) {
        this.id = id;
        this.title = title;
        this.author = author;
    }

    public Book() {
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }
        
}

Create a Controller (Your API)

Set up the REST endpoints that handle HTTP requests and responses.

@RestController
@RequestMapping("/api/books")
public class BookController {

    private final Map<String, Book> bookRepository = new HashMap<>();

    @PostMapping
    public Book createBook(@RequestBody Book book) {
        bookRepository.put(book.getId(), book);
        return book;
    }

    @GetMapping("/{id}")
    public Book getBook(@PathVariable String id) {
        return bookRepository.get(id);
    }

    @GetMapping
    public Collection<Book> getAllBooks() {
        return bookRepository.values();
    }

    @DeleteMapping("/{id}")
    public void deleteBook(@PathVariable String id) {
        bookRepository.remove(id);
    }
}

This exposes a RESTful API with the following endpoints:

• POST /api/books – Add a new book
• GET /api/books/{id} – Retrieve a book by ID
• GET /api/books – List all books
• DELETE /api/books/{id} – Delete a book

Run the Application

Here is the main class to bootstrap Spring Boot:

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

@SpringBootApplication
public class SimpleRestapiApplication {

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

}

Test the API

To run the application, use the following command from the root directory of your project:

./mvnw spring-boot:run

You can test the API using tools like Postman, curl, or a browser:

# Create a new book
curl -X POST -H "Content-Type: application/json" \
     -d '{"id":"1","title":"Effective Java","author":"Joshua Bloch"}' \
     http://localhost:8080/api/books

# Get the book
curl http://localhost:8080/api/books/1

# Get all books
curl http://localhost:8080/api/books

5. Conclusion

In Java, an API is a tool that allows us to build modular, reusable, and scalable applications. Whether we are consuming Java’s built-in APIs or developing our own RESTful APIs using frameworks like Spring Boot, Jakarta REST, Miconaut or Quarkus, understanding the concept of APIs is critical.

In this article, we examined what an API is both conceptually and in the Java context, reviewed the benefits of using APIs, and built a simple example of a RESTful API in Java. In the journey of becoming a more skilled Java developer, APIs become essential to nearly every project, whether it is building a backend service, a desktop application, or integrating different systems.

6. Download the Source Code

This article explained what an API is in Java.

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