Exploring REST APIs in Java: A Comprehensive Guide to Interview Questions

Representational State Transfer (REST) is an architectural style for designing networked applications. RESTful APIs have become the standard for building web services that are scalable, stateless, and interoperable. In this article, we will delve into the concept of REST APIs in Java and explore some commonly asked interview questions related to implementing and consuming RESTful services. By the end, you will have a solid understanding of REST APIs in Java and be well-prepared to tackle REST API-related questions in your next interview.

  1. What is a REST API?

A REST API is an Application Programming Interface (API) that follows the principles of REST. It provides a standardized way of accessing and manipulating resources over the web using HTTP methods such as GET, POST, PUT, and DELETE. REST APIs use Uniform Resource Identifiers (URIs) to identify resources and support a stateless client-server communication model.

  1. What are the main components of a RESTful API?
  • Resources: Resources are the key entities exposed by a REST API. They represent the data or services that can be accessed and manipulated. Resources are identified by unique URIs.
  • HTTP Methods: REST APIs utilize various HTTP methods to perform operations on resources. The commonly used HTTP methods are GET (retrieve a resource), POST (create a new resource), PUT (update an existing resource), and DELETE (remove a resource).
  • Representations: Representations define the format in which resources are exchanged between clients and servers. Common representations include JSON (JavaScript Object Notation) and XML (eXtensible Markup Language).
  1. How do you implement a RESTful API in Java?

In Java, you can implement a RESTful API using various frameworks and libraries. Some popular options include:

  • JAX-RS (Java API for RESTful Web Services): JAX-RS is a Java programming language API that provides support for creating RESTful web services. It includes annotations and classes that simplify the development of REST APIs.
  • Spring Boot: Spring Boot is a popular framework for building Java applications, including RESTful services. It provides an opinionated approach to REST API development, offering features such as automatic configuration and dependency management.
  1. How do you consume a REST API in Java?

To consume a REST API in Java, you can use libraries such as:

  • Apache HttpClient: Apache HttpClient is a widely used Java library for making HTTP requests. It provides a high-level API for interacting with RESTful services.
  • Spring RestTemplate: RestTemplate is a part of the Spring framework and offers a convenient way to consume RESTful services. It provides methods for making HTTP requests and handling responses.
  • Java 11+ HttpClient: Starting from Java 11, the Java standard library includes the java.net.http.HttpClient class, which allows for making HTTP requests and handling responses.


Let’s consider a scenario where you want to build a simple RESTful API for managing a collection of books.

import javax.ws.rs.*;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import java.util.ArrayList;
import java.util.List;

public class BookResource {
    private List<Book> books;

    public BookResource() {
        books = new ArrayList<>();
        books.add(new Book(1, "Java in Action", "Joshua Bloch"));
        books.add(new Book(2, "Clean Code", "Robert C. Martin"));

    public List<Book> getAllBooks() {
        return books;

    public Response addBook(Book book) {
        return Response.status(Response.Status.CREATED).build();


    public Response updateBook(@PathParam("id") int id, Book book) {
        for (Book b : books) {
            if (b.getId() == id) {
                return Response.ok().build();
        return Response.status(Response.Status.NOT_FOUND).build();

    public Response deleteBook(@PathParam("id") int id) {
        for (Book b : books) {
            if (b.getId() == id) {
                return Response.ok().build();
        return Response.status(Response.Status.NOT_FOUND).build();

In the above example, we use JAX-RS annotations to define a resource class for managing books. The @Path annotation specifies the base path for the resource, and the @GET, @POST, @PUT, and @DELETE annotations define the HTTP methods for retrieving, creating, updating, and deleting books, respectively.


In this article, we have explored the concept of REST APIs in Java and covered some common interview questions related to implementing and consuming RESTful services. Understanding the key components of a RESTful API, the frameworks and libraries available for building and consuming REST APIs in Java, and the usage of annotations and HTTP methods is crucial for successfully working with RESTful services. By familiarizing yourself with these concepts and practicing their implementation, you will be well-prepared to tackle REST API-related questions in your Java interviews. Remember to explore real-world examples and experiment with different frameworks to strengthen your understanding of RESTful APIs in Java.

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