Introduction: Why Java Is Needed for Automation Testing
Java is one of the most widely used languages for API and automation testing. Modern testing is no longer limited to UI validation; companies expect testers to validate API, database, and UI together. Java acts as the common bridge between these layers.
Java is essential for API testing because it:
- Integrates seamlessly with REST API automation libraries
- Supports robust test frameworks
- Works well with TestNG, JUnit, Maven, Jenkins
- Enables end-to-end testing (API + DB + UI)
That’s why API testing with Java interview questions are now a core part of automation interviews.
Core Java Topics for Testing
1. Object-Oriented Programming (OOP)
Used for:
- Framework design
- Reusability
- Maintainability
Key concepts:
- Encapsulation
- Inheritance
- Polymorphism
- Abstraction
2. Collections Framework
Used to store:
- API request payloads
- Response data
- Database results
Common collections:
- List
- Set
- Map
3. Multithreading
Used for:
- Parallel API execution
- Faster regression runs
4. Exception Handling
Used to:
- Handle API failures
- Capture meaningful error messages
- Stabilize frameworks
5. Java 8 Streams
Used for:
- Filtering API responses
- Validating collections of data
API Testing with Java Interview Questions & Answers
Core Java + API Fundamentals (1–30)
1. What is API testing?
API testing validates business logic, data accuracy, and integration without UI.
2. Why is Java used for API testing?
- Platform independent
- Strong object-oriented design
- Rich libraries for HTTP, JSON, XML
- Easy integration with CI/CD
3. What types of APIs have you tested?
- REST
- SOAP
4. What is REST API?
REST is an architectural style using HTTP methods.
5. HTTP methods used in REST?
- GET
- POST
- PUT
- DELETE
- PATCH
6. What is HTTP status code?
Indicates API execution result.
| Code | Meaning |
| 200 | Success |
| 201 | Created |
| 400 | Bad Request |
| 401 | Unauthorized |
| 404 | Not Found |
| 500 | Server Error |
7. What is request payload?
Data sent to API.
8. What is response body?
Data returned by API.
9. What is endpoint?
API URL.
10. What is header?
Metadata sent with request.
11. What is authentication in API?
Verifying user identity.
12. Types of API authentication?
- Basic
- Bearer token
- OAuth
13. What is serialization?
Converting Java object to JSON.
14. What is deserialization?
Converting JSON to Java object.
15. What libraries are used for API testing in Java?
- REST-based API libraries
- Jackson / Gson for JSON
- TestNG / JUnit
16. Difference between PUT and PATCH?
| PUT | PATCH |
| Full update | Partial update |
17. What is idempotent API?
Same request gives same result.
18. What is stateless API?
Server does not store client state.
19. What is API schema validation?
Validating response structure.
20. What is JSON?
Lightweight data format.
21. JSON vs XML?
| JSON | XML |
| Lightweight | Heavy |
| Faster | Slower |
22. What is HTTP client in Java?
Used to send API requests.
23. What is API contract testing?
Validates API agreements.
24. What is rate limiting?
Restricts API usage.
25. What is API mocking?
Simulates API behavior.
26. Why API testing is faster than UI testing?
No UI dependency.
27. When should API testing be done?
Before UI testing.
28. What is negative API testing?
Validating invalid inputs.
29. What is API versioning?
Managing multiple API versions.
30. What is API regression testing?
Re-testing APIs after changes.
Java Coding Examples for API Testing
31. Simple Java API GET request (conceptual):
HttpURLConnection con = (HttpURLConnection) new URL(url).openConnection();
con.setRequestMethod(“GET”);
System.out.println(con.getResponseCode());
Expected Output:
200
32. Reading API response:
BufferedReader br = new BufferedReader(
new InputStreamReader(con.getInputStream()));
System.out.println(br.readLine());
33. Handling API exceptions:
try {
// API call
} catch(Exception e) {
System.out.println(e.getMessage());
}
34. Storing response in List:
List<String> users = new ArrayList<>();
users.add(“admin”);
35. Map usage for JSON key-value:
Map<String,String> payload = new HashMap<>();
payload.put(“name”,”John”);
Selenium + Java Coding Challenges (36–55)
Even API testers are expected to know basic Selenium integration.
36. Why combine API + Selenium?
Validate backend and frontend together.
37. Launch browser:
WebDriver driver = new ChromeDriver();
driver.get(“https://example.com”);
38. Locator priority?
id → name → cssSelector → xpath
39. Dynamic XPath:
//input[contains(@id,’email’)]
40. Selenium waits?
- Implicit
- Explicit
- Fluent
41. Explicit wait example:
WebDriverWait wait = new WebDriverWait(driver, Duration.ofSeconds(10));
wait.until(ExpectedConditions.visibilityOfElementLocated(By.id(“login”)));
42. Thread.sleep vs Explicit wait?
| Thread.sleep | Explicit |
| Static | Dynamic |
43. Handling dropdown:
Select s = new Select(element);
s.selectByVisibleText(“India”);
44. Handle alerts:
driver.switchTo().alert().accept();
45. Screenshot capture:
File src = ((TakesScreenshot)driver).getScreenshotAs(OutputType.FILE);
46. Page Object Model?
Separates UI logic and test logic.
47. POM example:
@FindBy(id=”username”)
WebElement username;
48. Hybrid framework?
POM + Data-driven + Keyword-driven.
49. Maven usage?
Dependency management.
50. Jenkins role?
CI/CD execution.
51. Logging?
Log4j.
52. Reporting?
Extent Reports / Allure.
53. Parallel execution?
TestNG thread count.
54. Handling flaky tests?
Retries + waits.
55. Headless execution?
CI/CD pipelines.
JUnit & TestNG Interview Questions (56–75)
56. Why TestNG for API testing?
Parallel execution and reporting.
57. Common TestNG annotations?
| Annotation | Purpose |
| @Test | Test case |
| @BeforeMethod | Setup |
| @AfterMethod | Teardown |
58. DataProvider example:
@DataProvider
public Object[][] data() {
return new Object[][]{{“user1″,”pass1”}};
}
59. Hard vs Soft assertion?
| Hard | Soft |
| Stops execution | Continues |
60. TestNG XML usage?
Controls execution.
61. Groups?
Execute selected tests.
62. DependsOnMethods?
Controls dependency.
63. Retry analyzer?
Re-run failed tests.
64. Listener usage?
Captures execution events.
65. Parallel execution?
Multiple threads.
66. JUnit annotations?
@Test, @Before, @After
67. JUnit vs TestNG?
| JUnit | TestNG |
| Simple | Advanced |
68. Parameterization?
Multiple test data sets.
69. BDD framework?
Cucumber.
70. Feature file?
Written in Gherkin.
71. Step definition?
Maps steps to code.
72. Hooks?
@Before, @After
73. CI/CD integration?
Automated execution.
74. API assertions?
Status code, body, schema.
75. API response time validation?
Performance check.
Real-Time Interview Scenarios (76–90)
76. API + DB validation scenario:
- Create user via API
- Validate record in DB
77. JDBC connection:
Connection con = DriverManager.getConnection(url,user,pass);
78. Execute query:
ResultSet rs = stmt.executeQuery(“SELECT * FROM users”);
79. API + UI validation:
Create data via API, verify in UI.
80. End-to-end scenario:
API → DB → UI.
81. Handling authentication tokens?
Store token globally.
82. Handling environment URLs?
Config files.
83. Handling dynamic API data?
Randomization.
84. Schema validation?
Compare JSON structure.
85. Handling API failures?
Retry + logging.
86. Parallel API execution?
Thread safety.
87. CI/CD challenges?
Environment dependency.
88. Security testing?
Limited via automation.
89. Performance testing?
Handled by JMeter.
90. Framework scalability?
Modular design.
Common Mistakes in Java API Interviews
- Ignoring Java fundamentals
- Weak exception handling
- No framework explanation
- Memorizing answers
- No real-time scenarios
1-Page Revision Table / Notes
| Area | Key Focus |
| Java | OOP, Collections |
| API | Methods, Status Codes |
| Selenium | Locators, Waits |
| TestNG | Parallel execution |
| Framework | Hybrid, CI/CD |
FAQs (For Google Ranking)
Is Java mandatory for API testing roles?
Yes, Java is widely used.
Is Selenium required for API testing?
Basic knowledge is expected.
Is DB validation important?
Yes, backend validation is critical.
Are coding questions asked?
Yes, Java logic and API coding are common