Subex Java Developer Interview: Rounds, Questions and Preparation

What the job involves

As a Java Developer at Indian IT services companies like Subex, you will build and maintain backend services, APIs, and data processing modules for enterprise or telecom products. Freshers typically work in agile teams, writing Java code, fixing bugs, and integrating with databases and third-party systems. The role rewards strong fundamentals in Java, SQL, and problem solving over deep framework expertise.

Java and OOP fundamentals

1. 1

   Explain the difference between an interface and an abstract class in Java, with an example of when you would use each.

   Cover that interfaces support multiple inheritance of type and are for contracts, while abstract classes allow shared state and partial implementation. Give a short example like a Payment interface versus a BaseEntity abstract class.

2. 2

   How does a HashMap work internally in Java, and what happens during a collision?

   Mention hashing, buckets, equals and hashCode, and that Java 8+ converts long chains to balanced trees. Explain that collisions are resolved by chaining and that poor hashCode can degrade performance.

3. 3

   What is the difference between checked and unchecked exceptions, and when would you create a custom exception?

   Checked exceptions must be declared or handled and are for recoverable conditions. Unchecked exceptions indicate programming errors. A custom exception is useful when you need domain-specific error handling, like InvalidOrderException.

4. 4

   Write a Java method to reverse a string without using StringBuilder or reverse().

   Use a char array and swap from both ends, or build a new string in a loop. Explain time complexity O(n) and why you avoid repeated string concatenation.

SQL, Spring, and practical coding

1. 5

   Given an Employee table, write a query to find the second highest salary.

   Show a subquery with MAX and a condition, or use LIMIT with OFFSET. Mention that you should handle ties and explain which approach works in MySQL or Oracle.

2. 6

   What is dependency injection in Spring, and how does it help in a project with many services?

   Explain that DI lets Spring create and wire objects, reducing tight coupling. Mention constructor injection as preferred and how it makes unit testing easier with mocks.

3. 7

   How would you design a simple REST API to manage customer records, including endpoints and HTTP methods?

   List GET, POST, PUT, DELETE for /customers and /customers/{id}. Mention status codes like 200, 201, 404, and 400, and that request and response bodies should be JSON.

4. 8

   You join an existing Java project and see a NullPointerException in production logs. How do you debug it?

   Describe reading the stack trace, identifying the line and object, adding null checks or Optional, and writing a unit test. Mention using logs and a debugger rather than guessing.

Scenario and project questions

1. 9

   Tell me about a Java project you built in college. What was your role and what was the hardest bug you fixed?

   Use a clear structure: problem, your specific contribution, tools used, and how you solved the bug. Be honest if it was a team project and highlight what you personally coded.

2. 10

   If your code works on your machine but fails on the test server, what steps do you take?

   Check environment differences like Java version, database config, file paths, and environment variables. Suggest comparing logs, reproducing locally, and using a checklist before blaming the server.

3. 11

   You are asked to add a feature to a telecom billing module you have never seen before. How do you start?

   Show a methodical approach: read existing code and tests, ask a senior for a walkthrough, trace one request end to end, then make a small change and verify. Mention writing notes for yourself.

4. 12

   How would you handle a situation where you are stuck on a coding problem for more than an hour during a sprint?

   Say you would timebox your attempt, search documentation, then ask a teammate with a clear summary of what you tried. Emphasise that asking for help early is better than missing a deadline silently.

Behavioural and HR questions

1. 13

   Why do you want to work as a Java Developer in an IT services company rather than a product startup?

   Talk about exposure to multiple clients, structured training, and learning enterprise-grade systems. Avoid saying you are settling; show genuine interest in services delivery and client impact.

2. 14

   Are you comfortable with relocation, shifts, or working on a client site if required?

   Answer honestly but show flexibility. If you have constraints, state them politely and mention your willingness to adapt for project needs. Indian IT services often value this flexibility.

3. 15

   Describe a time you had a disagreement with a teammate during a college project. How did you resolve it?

   Use a real example and focus on listening, discussing the technical merits, and agreeing on a solution. Avoid blaming the teammate; show that you can separate people from the problem.

4. 16

   Where do you see yourself in three years as a Java Developer?

   Show a realistic path: becoming strong in backend development, learning a framework deeply, and taking ownership of modules. Mention wanting to contribute to design discussions and mentor juniors eventually.