Nisum Technologies India Java Developer Interview: Rounds, Questions and Preparation

What the job involves

A Java Developer at an Indian IT services or product-focused firm builds and maintains server-side applications, REST APIs, and database-backed modules for client projects. Freshers usually start by fixing bugs, writing unit tests, and adding small features under a senior developer's guidance. Over time, they take ownership of modules and interact with clients or QA teams in agile sprints.

Java and backend fundamentals

1. 1

   What is the difference between an ArrayList and a LinkedList in Java, and when would you use each?

   Mention that ArrayList gives fast random access but slower inserts in the middle, while LinkedList is better for frequent add or remove at the ends. Give a practical example like a queue of tasks versus a list you read often.

2. 2

   Explain how HashMap works internally in Java.

   Cover hashing, buckets, equals and hashCode contract, and what happens during a collision. Mention that since Java 8, long collision chains convert to a balanced tree for better performance.

3. 3

   What is the difference between checked and unchecked exceptions? Give one example of each.

   Checked exceptions must be declared or handled at compile time, like IOException. Unchecked exceptions are runtime errors, like NullPointerException. Explain when you would create a custom exception.

4. 4

   How does Spring Boot help you build a REST API faster than plain Java servlets?

   Talk about auto-configuration, embedded servers, starter dependencies, and annotations like @RestController and @GetMapping. Mention that it reduces boilerplate XML and setup time.

Databases, tools, and practical development

1. 5

   Write a SQL query to find the second highest salary from an Employee table.

   Show a subquery or a window function like DENSE_RANK. Explain how you would handle ties and what happens if the table has fewer than two rows.

2. 6

   What is the difference between inner join, left join, and full outer join?

   Use a simple example like customers and orders. Explain which rows are kept when there is no match on one side. Mention that full outer join may not be supported in all databases.

3. 7

   How do you use Git when two developers work on the same file and both push changes?

   Describe branching, pulling before pushing, and resolving merge conflicts manually. Show that you understand not to force push to a shared branch.

4. 8

   What is the purpose of JPA and Hibernate in a Spring Boot project?

   Explain that JPA is a specification and Hibernate is an implementation. Mention entity mapping, repositories, and how it reduces manual JDBC code. Also note lazy loading pitfalls.

Scenario and problem-solving questions

1. 9

   Your REST API returns a 500 error only in the production environment but works on your local machine. How do you debug it?

   Start by checking logs, environment variables, database connectivity, and configuration differences. Suggest reproducing with production-like data and adding temporary logging. Show a calm, step-by-step approach.

2. 10

   A client reports that a report page is very slow. How would you investigate and improve it?

   Mention checking SQL query plans, adding indexes, reducing N+1 queries, and caching repeated data. Also talk about measuring before and after changes.

3. 11

   You are asked to add a new field to an existing database table used by a live application. What steps do you take?

   Cover writing a migration script, making the column nullable or giving a default, testing in staging, and coordinating with the release. Mention backward compatibility.

4. 12

   How would you write a unit test for a service method that calculates an employee's bonus?

   Describe using JUnit and Mockito to mock dependencies like a repository. Test normal cases, boundary values, and exception cases. Keep the test independent and fast.

Behavioural and HR questions for freshers

1. 13

   Tell me about a project you built during college and your specific contribution.

   Use a clear structure: problem, your role, technologies used, and outcome. Be ready to explain any code or design choice you made. Avoid claiming work you did not do.

2. 14

   You are assigned to a legacy codebase with no documentation. How do you get started?

   Show willingness to read code, run the application, ask senior developers, and write small notes. Mention using debuggers and logs to understand flow. Highlight patience and curiosity.

3. 15

   Are you comfortable with relocation, flexible work hours, or working on client-facing projects?

   Answer honestly but positively. If you have constraints, state them clearly and professionally. Show that you understand IT services projects may need occasional extended hours during releases.

4. 16

   Where do you see yourself in two years as a Java developer?

   Focus on learning depth: mastering Spring, databases, and clean code. Mention taking more ownership and possibly mentoring juniors. Avoid saying you want to move into management immediately.