Java API Testing Interview Questions

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Introduction: Why Java Is Needed for Automation & API Testing

In today’s testing landscape, API testing is as important as UI testing. Most modern applications follow a microservices architecture, where APIs handle the core business logic. As a result, companies expect automation testers to know Java-based API testing in addition to Selenium UI automation.

Java is widely used for API testing because:

  • Java provides strong OOP concepts for reusable test frameworks
  • Popular API testing libraries like Rest Assured are Java-based
  • Easy integration with Selenium, TestNG, JUnit, Maven, Jenkins
  • Supports end-to-end testing (UI + API + Database)
  • Highly demanded skill in Automation Tester and SDET interviews

That’s why java api testing interview questions are frequently asked in automation, backend testing, and full-stack QA interviews.

Core Java Topics for API & Automation Testing

Before API concepts, interviewers validate your Core Java fundamentals.

1. Object-Oriented Programming (OOP)

  • Class & Object
  • Inheritance
  • Polymorphism
  • Encapsulation
  • Abstraction

2. Java Collections

  • List → ArrayList, LinkedList
  • Set → HashSet
  • Map → HashMap, LinkedHashMap

3. Exception Handling

  • try, catch, finally
  • Checked vs Unchecked exceptions

4. Multithreading (Basic Level)

  • Thread class
  • Runnable interface

5. Java 8 Features

  • Streams
  • Lambda expressions
  • forEach()

Java API Testing Interview Questions (Core Java + API Basics)

Q1. What is API testing?

API testing validates backend services by sending requests and verifying responses without using the UI.

Q2. Why is Java used for API testing?

Java supports REST libraries, OOP-based frameworks, and seamless automation integration.

Q3. What is REST API?

REST API is an architectural style using HTTP methods to communicate between systems.

Q4. Common HTTP methods in API testing?

  • GET – Retrieve data
  • POST – Create data
  • PUT – Update data
  • DELETE – Delete data

Q5. What is HTTP status code?

Status codes indicate the result of an API request (e.g., 200, 201, 400, 401, 404, 500).

Q6. What is JVM?

JVM executes Java bytecode and makes Java platform-independent.

Q7. Explain OOP in simple terms.

OOP organizes code into objects to improve reuse and maintainability.

Q8. Inheritance example in Java.

class API {

    void connect() {

        System.out.println(“API Connected”);

    }

}

class RestAPI extends API {

    void getRequest() {

        System.out.println(“GET Request”);

    }

}

public class Test {

    public static void main(String[] args) {

        RestAPI api = new RestAPI();

        api.connect();

        api.getRequest();

    }

}

Output

API Connected

GET Request

Q9. What is polymorphism?

Same method name performing different actions.

Q10. What is encapsulation?

Hiding data using private variables and exposing via public methods.

Q11. ArrayList vs LinkedList?

  • ArrayList → Faster access
  • LinkedList → Faster insertion/deletion

Q12. HashMap example.

HashMap<String,String> headers = new HashMap<>();

headers.put(“Content-Type”,”application/json”);

System.out.println(headers.get(“Content-Type”));

Output

application/json

Q13. Checked vs Unchecked exception?

  • Checked → IOException
  • Unchecked → NullPointerException

Q14. Exception handling example.

try {

    int x = 10 / 0;

} catch (ArithmeticException e) {

    System.out.println(“Exception handled”);

}

Output

Exception handled

Q15. Java 8 Stream example.

List<Integer> codes = Arrays.asList(200,404,500);

codes.stream().filter(c -> c >= 400).forEach(System.out::println);

Output

404

500

Java API Testing Interview Questions – REST Assured

Q16. What is Rest Assured?

Rest Assured is a Java library used to test REST APIs.

Q17. How do you perform a GET request in Java API testing?

Response response = RestAssured.get(“/users”);

System.out.println(response.getStatusCode());

Output

200

Q18. How do you validate response body?

response.then().body(“name”, equalTo(“John”));

Q19. How do you pass headers in API request?

given()

.header(“Content-Type”,”application/json”)

.when()

.get(“/users”);

Q20. How do you send POST request?

given()

.body(“{\”name\”:\”John\”}”)

.when()

.post(“/users”)

.then()

.statusCode(201);

Q21. What is JSON?

JSON is a lightweight data-interchange format used in APIs.

Q22. How do you parse JSON response?

JsonPath jp = response.jsonPath();

System.out.println(jp.getString(“name”));

Q23. Difference between PUT and PATCH?

  • PUT → Full update
  • PATCH → Partial update

Q24. What is authentication in API testing?

Validating API access using tokens, OAuth, or basic authentication.

Q25. Token-based authentication example.

given()

.header(“Authorization”,”Bearer token”)

.when()

.get(“/profile”);

Selenium + Java + API Interview Questions

Q26. Why combine API and Selenium testing?

API testing validates backend logic faster; Selenium validates UI behavior.

Q27. Selenium code to open browser.

WebDriver driver = new ChromeDriver();

driver.get(“https://example.com”);

Q28. When should API testing be done before UI testing?

API testing should be done first to validate backend stability.

Q29. How do you validate API data with UI?

Compare API response values with UI displayed values.

Real-Time API Testing Interview Scenarios

Scenario 1: API + UI Validation

Steps

  1. Call login API
  2. Capture token
  3. Login to UI
  4. Validate user details with API response

Scenario 2: API + Database Validation

Steps

  1. Call create-user API
  2. Validate response
  3. Verify record in database

Scenario 3: End-to-End Flow

  1. Create order via API
  2. Validate order in UI
  3. Verify order in database

JUnit Interview Questions for API Testing

Q30. What is JUnit?

JUnit is a unit testing framework for Java.

Q31. Common JUnit annotations?

  • @Test
  • @Before
  • @After

TestNG Interview Questions for API Testing

Q32. What is TestNG?

TestNG is an advanced testing framework inspired by JUnit.

Q33. TestNG DataProvider example.

@DataProvider

public Object[][] apiData(){

    return new Object[][] {{“user1”},{“user2”}};

}

Q34. Why TestNG is preferred for API automation?

Supports parallel execution, grouping, and reports.

Framework Design Questions (API + Java)

Q35. What is Hybrid Framework?

Combination of POM + Data-Driven + Keyword-Driven frameworks.

Q36. How do you design API automation framework?

  • Base request class
  • API endpoints class
  • Utilities (JSON, config)
  • Test classes
  • Reports

Q37. CI/CD tools used for API automation?

  • Jenkins
  • GitHub Actions
  • Azure DevOps

Common Mistakes in Java API Testing Interviews

  • Weak Core Java concepts
  • Not understanding HTTP methods
  • Ignoring response validation
  • Hard-coded test data
  • No real-time API project experience

1-Page Revision Table / Notes

AreaKey Focus
Core JavaOOP, Collections, Streams
APIREST, HTTP Methods
Rest AssuredGET, POST, JSON
SeleniumUI validation
FrameworkHybrid, Data-Driven
CI/CDJenkins

FAQs – Java API Testing Interview Questions

Q1. Is Java mandatory for API testing interviews?
 Yes, Java is widely used for API automation.

Q2. Is Selenium required for API testing roles?
 Not mandatory, but integration knowledge is a plus.

Q3. What library is most used for Java API testing?
 Rest Assured is the most popular.

Q4. Do API testers need database knowledge?
 Yes, DB validation is often required.

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