Interview Questions
Java Interview Questions
Java interview questions and answers, organized by topic, with examples and key points for quick revision.
Core Java
Java fundamentals — the JVM, JDK, JRE, bytecode, and how a Java program actually runs.
Open 12 questionsOOP
Object-oriented programming as a paradigm — the four pillars, class relationships, and design guidelines.
Open 3 questionsClasses & Objects
Classes, objects, instance vs static members, and object initialization.
Open 6 questionsConstructors
Constructors, overloading, chaining, and common constructor pitfalls.
Open 4 questionsInheritance
Inheritance, the super keyword, method hiding, and the diamond problem.
Open 4 questionsPolymorphism
Compile-time vs runtime polymorphism, dynamic dispatch, and upcasting/downcasting.
Open 11 questionsEncapsulation
Data hiding, access modifiers, and designing classes that protect their own state.
Open 2 questionsAbstraction
Hiding implementation detail behind abstract classes and interfaces.
Open 1 questionStrings
String immutability, the String Pool, StringBuilder/StringBuffer, and common String methods.
Open 4 questionsArrays
Array basics, multidimensional arrays, copying, sorting, and searching.
Open 2 questionsCollections
List/Set/Queue/Map implementations, Comparable vs Comparator, HashMap internals, and fail-fast iteration.
Open 12 questionsException Handling
Checked vs unchecked exceptions, try-with-resources, custom exceptions, and exception propagation.
Open 6 questionsMultithreading
Thread creation, synchronization, deadlock, volatile, locks, and the Executor framework.
Open 8 questionsJava 8+
Lambdas, functional interfaces, the Stream API, Optional, and default/static interface methods.
Open 8 questionsInterfaces
Interface contracts, multiple implementation, interface inheritance, and functional/marker interfaces.
Open 2 questionsAbstract Classes
Abstract class mechanics, constructors and fields, and a deep comparison with interfaces.
Open 1 questionGenerics
Generic classes and methods, bounded types, wildcards, PECS, and type erasure.
Open 4 questionsJVM
JVM architecture, class loading, runtime data areas, garbage collection, and common memory errors.
Open 7 questionsJDBC
Connecting Java to a database — drivers, statements, transactions, batching, and connection pooling.
Open 2 questionsSpring
Core Spring Framework ideas — dependency injection, inversion of control, and building loosely coupled components.
Open 2 questionsSpring Boot
Spring Boot fundamentals — auto-configuration on top of Spring, and building REST controllers quickly.
Open 2 questionsJPA / Hibernate
Object-relational mapping in Java — the JPA specification and its most common implementation, Hibernate.
Open 2 questionsREST API
REST API fundamentals for Java backends — resources, HTTP verbs, and stateless design.
Open 1 questionAdvanced Java
Object class internals, varargs, initializer blocks, type conversions, wrapper classes, and modern Java language features from records to text blocks.
Open 15 questions