Java
Interview Questions

1. What is the difference between == and equals() in Java?

This is a classic Java interview question because it verifies whether the candidate understands the difference between reference comparison and logical comparison.

The operator == compares references when used with objects. It checks whether two variables point to the same object in memory.

The method equals(), on the other hand, is used to compare logical equality between two objects. For example, two different String objects can contain the same text. In this case, equals() can return true, whereas == can return false.

2. Why override equals() and hashCode() together?

This is a very common Java interview question because it verifies whether the candidate understands how hash-based collections work, such as HashMap or HashSet.

The core rule is simple: if two objects are equal according to equals(), they must have the same hashCode().

In an interview, you should explain that equals() defines logical or business equality, while hashCode() helps certain collections retrieve objects quickly.

If hashCode() is poorly implemented, a HashSet or a HashMap might fail to correctly find an object even though it is equal according to equals().

3. What is the difference between an interface and an abstract class?

An interface primarily defines a contract. It specifies what a class must be able to do, without necessarily enforcing a complete implementation.

An abstract class allows you to factor out common behavior across multiple related classes. It can contain abstract methods, concrete methods, and shared state.

Since Java 8, interfaces can also contain methods default and static. Despite this, interfaces are still mainly used to express a capability or a contract.

4. What is the difference between overloading and overriding?

Overloading consists of defining multiple methods with the same name but different parameters within the same class.

Overriding consists of providing a new implementation for a method inherited from a parent class or an interface.

In an interview, make sure to emphasize that overloading is resolved at compile time based on the method signature, whereas overriding is resolved at runtime through polymorphism.

5. What is the difference between ArrayList and LinkedList?

ArrayList uses a dynamic array internally. It is generally very efficient for accessing an element by index because access is direct.

LinkedList uses a doubly linked list. Accessing an element by index is often less performant because it requires traversing the list.

In practice, ArrayList is the default choice in most cases. LinkedList can be useful in specific scenarios, but it is used much less frequently in standard applications.

6. How does a HashMap work in Java?

HashMap stores data as key-value pairs. To quickly retrieve a value, it uses the key's hashCode() to determine an internal bucket location.

When multiple keys resolve to the same location, it is called a collision. Java handles these collisions using a suitable internal data structure.

In an interview, you should explain that the proper functioning of a HashMap heavily depends on the consistency between equals() and hashCode().

7. What is the difference between a checked exception and an unchecked exception?

A checked exception is verified at compile time. The developer must either catch it with try/catch, or declare it with throws.

An unchecked exception inherits from RuntimeException. It is not enforced by the compiler and usually occurs at runtime.

For example, IOException is a checked exception, whereas NullPointerException or IllegalArgumentException are unchecked exceptions.

8. What is the difference between stack and heap in Java?

The stack notably contains method calls, local variables, and primitive local references.

The heap contains dynamically created objects, particularly those instantiated with new. This is also the memory area where the garbage collector operates to reclaim unused objects.

In an interview, avoid saying that everything is either strictly in the stack or strictly in the heap. A local variable reference may live on the stack, but the referenced object typically resides in the heap.

What are the most common Java interview questions?

The most common Java interview questions cover object-oriented programming, collections, equals() and hashCode(), exceptions, the JVM, Strings, streams, lambdas, and best development practices.

How to prepare for a junior Java interview?

To prepare for a junior Java interview, you need to master language fundamentals, understand core concepts, and practice explaining your answers clearly. Priority topics include OOP, collections, exceptions, String, HashMap, interfaces, and inheritance.

Do you need to know Spring Boot for a Java interview?

For a Java backend role, Spring Boot is often required. However, core Java fundamentals remain essential, as many Spring questions rely on Java concepts like interfaces, exceptions, annotations, collections, and object management.

How many questions should you review before a Java interview?

It is better to master 30 to 50 key questions with clear explanations than to read 200 questions without being able to explain them. In an interview, the quality of your explanation matters as much as raw knowledge.