Core Java Interview Questions 2025

Java is one of the most widely used programming languages for building enterprise applications, web services, and Android applications. Preparing for a Java interview requires a strong grasp of fundamental concepts, object-oriented principles, multithreading, collections, exception handling, and Java 8+ features.

This guide provides a structured set of Core Java interview questions along with useful resources to help you prepare effectively.

Basic Java Concepts

• What is Java and what are its main features?
• Explain the difference between JDK, JRE, and JVM.
• What is platform independence in Java?
• What is the difference between path and classpath?
• What are the different types of variables in Java?
• What is the difference between local and instance variables?
• What are static variables?
• What is a constructor?
• What are the different types of constructors?
• What is the difference between break and continue statements?
• What is the difference between final, finally, and finalize?
• What is the difference between throw and throws?
• What is the difference between '==' and '.equals()'?
• What is the difference between String, StringBuilder, and StringBuffer?
• Why is String immutable in Java?
• What is the String pool?
• What are wrapper classes?
• What is the difference between primitive and reference types?
• What is the difference between stack and heap memory?
• What is the difference between shallow copy and deep copy?
• What is the difference between 'System.out.println()' and 'System.err.println()'?
• What is the difference between 'import' and 'static import' in Java?
• What is the difference between 'instanceof' and 'isInstance()'?

Object-Oriented Programming

• What are the main principles of OOP?
• What is the difference between a class and an object?
• What is inheritance?
• Why doesn't Java support multiple inheritance?
• What is method overloading?
• What is method overriding?
• What is polymorphism?
• What is the difference between compile-time and runtime polymorphism?
• What is abstraction?
• What is encapsulation?
• What is the difference between abstract class and interface?
• What is composition?
• What is aggregation?
• What is the difference between association, aggregation, and composition?
• What is the singleton pattern?
• What is the factory pattern?
• What is dependency injection?
• What is the difference between strategy and factory pattern?
• What is the observer pattern?
• What is the decorator pattern?
• What is the difference between tight coupling and loose coupling?
• What is the difference between a concrete class and an abstract class?
• What is the difference between a static method and an instance method?

Data Types and Variables

• What are primitive data types in Java?
• What is type casting?
• What is autoboxing and unboxing?
• What is the difference between '==' and '.equals()'?
• What is the difference between String, StringBuilder, and StringBuffer?
• Why is String immutable in Java?
• What is the String pool?
• What are wrapper classes?
• What is the difference between primitive and reference types?
• Explain final keyword with variables, methods, and classes.
• What is the difference between stack and heap memory?
• What is the difference between shallow copy and deep copy?
• What is the difference between 'int' and 'Integer'?
• What is the difference between 'float' and 'double'?
• What is the difference between 'char' and 'Character'?

Control Flow and Exception Handling

• What are the different types of loops in Java?
• What is the switch statement?
• What is exception handling?
• What is the difference between checked and unchecked exceptions?
• What is the try-with-resources statement?
• Explain the hierarchy of Exception classes.
• What is the difference between throw and throws?
• What is the finally block?
• Can we have try without catch?
• What is exception propagation?
• What is the difference between final, finally, and finalize?
• What is the difference between 'Error' and 'Exception'?
• What is the purpose of the 'assert' keyword?
• What is the difference between 'throw' and 'throws'?

Collections Framework

• What is the Collections Framework?
• What is the difference between List and Set?
• What is the difference between ArrayList and LinkedList?
• What is the difference between HashSet and TreeSet?
• How does HashMap work internally?
• What is the difference between HashMap and Hashtable?
• What is the difference between HashMap and ConcurrentHashMap?
• What is the load factor in HashMap?
• What is the difference between fail-fast and fail-safe iterators?
• How to make a collection read-only?
• What is the difference between shallow copy and deep copy?
• What is the difference between 'Iterator' and 'ListIterator'?
• What is the difference between 'Comparable' and 'Comparator'?
• What is the difference between 'Vector' and 'ArrayList'?

Multithreading

• What is multithreading?
• What is the difference between process and thread?
• What are different ways to create threads?
• What is synchronization?
• What is the difference between synchronized method and synchronized block?
• What is deadlock?
• What is thread starvation?
• What is the volatile keyword?
• What is the difference between 'wait()' and 'sleep()'?
• What is thread pool?
• What is the difference between final, finally, and finalize?
• What is the difference between 'notify()' and 'notifyAll()'?
• What is the difference between 'yield()' and 'join()'?
• What is the difference between 'Callable' and 'Runnable'?

Java 8 Features

• What are lambda expressions?
• What are functional interfaces?
• What is Stream API?
• What is the difference between intermediate and terminal operations in streams?
• What is Optional class?
• What are default methods?
• What is method reference?
• What is the difference between Comparator and Comparable?
• What are the new features in Java 8 Date/Time API?
• What is 'forEach()' method in Iterable interface?
• What is the difference between 'Predicate' and 'Function'?
• What is the difference between 'map()' and 'flatMap()' in Stream API?
• What is the purpose of 'Collectors' in Java 8?

Memory Management

• How is memory managed in Java?
• What is garbage collection?
• What are different types of references in Java?
• What is memory leak?
• What is the difference between stack and heap memory?
• What is the OutOfMemoryError?
• What are different types of garbage collectors?
• What is the 'finalize()' method?
• What is the difference between final, finally, and finalize?
• How to force garbage collection?
• What is the difference between 'SoftReference' and 'WeakReference'?
• What is the difference between 'Minor GC' and 'Major GC'?
• What is the purpose of the 'System.gc()' method?

File Handling and I/O

• What is the difference between FileInputStream and FileReader?
• What is serialization?
• What is the transient keyword?
• What is externalization?
• What is the difference between Reader/Writer and InputStream/OutputStream?
• What is BufferedReader?
• What is the difference between 'getPath()' and 'getAbsolutePath()'?
• What is NIO?
• What are channels and buffers?
• What is memory-mapped file I/O?
• What is the difference between 'FileInputStream' and 'BufferedInputStream'?
• What is the purpose of the 'RandomAccessFile' class?
• What is the difference between 'File' and 'Path' in Java NIO?

Advanced Concepts

• What is reflection in Java?
• What are annotations?
• What is marker interface?
• What is the difference between abstract class and interface?
• What is composition?
• What is aggregation?
• What is the difference between association, aggregation, and composition?
• What is the singleton pattern?
• What is the factory pattern?
• What is dependency injection?
• What is the difference between 'ClassNotFoundException' and 'NoClassDefFoundError'?
• What is the difference between 'ClassLoader' and 'Class'?
• What is the purpose of the 'Proxy' class in Java?

Java EE Basics

• What is JDBC?
• What is the difference between Statement and PreparedStatement?
• What is connection pooling?
• What is a servlet?
• What is JSP?
• What is the difference between forward and redirect?
• What is the MVC pattern?
• What are Java Beans?
• What is the difference between GET and POST methods?
• What are cookies and sessions?
• What is the difference between 'HttpServlet' and 'GenericServlet'?
• What is the purpose of the 'Filter' in Java EE?
• What is the difference between 'JSP' and 'JSF'?

Best Practices and Design

• What are SOLID principles?
• What is clean code?
• What is the difference between shallow copy and deep copy?
• What is immutable class?
• How to create an immutable class?
• What is the difference between strategy and factory pattern?
• What is the observer pattern?
• What is the decorator pattern?
• What are design patterns?
• What is the difference between composition and inheritance?
• What is the difference between 'Builder' and 'Factory' pattern?
• What is the purpose of the 'Adapter' pattern?
• What is the difference between 'Facade' and 'Proxy' pattern?