Java Developer Interview Questions UK: 8 Real Questions and What Interviewers Want

Applying for a Java Developer job in the UK? Here is what you are likely to be asked. These questions come from our Java Developer question bank of 25 questions, written for UK employers at mid-level level.

Eight Java Developer interview questions, and what the interviewer is thinking

1. What is the difference between an abstract class and an interface in Java, and when would you use each?

Why they ask: This is one of the most frequently asked Java questions and shows whether you understand design, not just syntax.

2. How does the equals and hashCode contract work in Java, and what goes wrong if you break it?

Why they ask: Breaking this contract causes subtle bugs in collections, so it is a classic test of core Java knowledge.

3. Explain how Spring Boot works and what dependency injection gives you.

Why they ask: Spring Boot dominates UK Java employers, so they need to see you understand it and not just copy annotations.

4. How does garbage collection work in Java, and how would you diagnose a memory leak?

Why they ask: Production Java developers need to understand memory behaviour, because leaks and long pauses cause real outages.

5. What are the main features introduced in Java 8, and which newer features such as records or virtual threads have you used?

Why they ask: They want to confirm you write modern Java and keep up with LTS releases.

6. A production Java service is running out of memory and restarting every few hours. How do you investigate?

Why they ask: This tests structured diagnosis of a live incident, which Java developers in support rotas must do.

7. Tell me about a time you improved the performance of a Java application.

Why they ask: Performance work shows technical depth and a measurement-driven approach.

8. Why do you want to work as a Java developer, and what do you enjoy about the language and its ecosystem?

Why they ask: They want to understand your motivation and whether you are committed to Java rather than just any language.

A sample answer to question 1

What is the difference between an abstract class and an interface in Java, and when would you use each?

An interface defines a contract: a set of methods a class promises to provide, without saying how. Since Java 8, interfaces can include default and static methods, and a class can implement many interfaces. An abstract class can hold state through fields, constructors and a mix of concrete and abstract methods, but a class can extend only one. So I use an interface when I'm describing a capability that unrelated classes might share, such as Comparable or a PaymentProvider that could be implemented by Stripe or PayPal adapters. I use an abstract class when closely related classes share common state and behaviour, like a base report generator that handles formatting while subclasses supply the data. In modern Java I lean towards interfaces and composition, because they make testing and mocking easier and avoid rigid inheritance trees. Sealed interfaces, added in Java 17, are also useful when I want a closed set of implementations.

Common mistake to avoid: Saying interfaces cannot have any method bodies; Not giving a practical example

Want the rest?

Full model answers, key points and common mistakes for all 25 questions are on the Java Developer interview questions page. The first five are free, no sign-up needed.