Top 100 Java Interview Questions & Answers (2026)

01 What is Java? 02 What is the difference between JDK, JRE, and JVM? 03 What are the eight primitive data types in Java? 04 What is the difference between int and Integer in Java? 05 What is autoboxing and unboxing in Java? 06 What is the difference between == and .equals() in Java? 07 Why is String immutable in Java? 08 What is the difference between String, StringBuilder, and StringBuffer? 09 What is a class in Java? 10 What is an object in Java? 11 What is a constructor in Java? 12 What is inheritance in Java? 13 What is method overloading in Java? 14 What is method overriding in Java? 15 What is encapsulation in Java? 16 What is polymorphism in Java? 17 What is an abstract class in Java? 18 What is an interface in Java? 19 What is the difference between abstract class and interface in Java? 20 What are access modifiers in Java? 21 What does the static keyword mean in Java? 22 What does the final keyword do in Java? 23 What is garbage collection in Java? 24 What is the difference between stack and heap memory in Java? 25 What is a package in Java? 26 What is the instanceof operator in Java? 27 What is type casting in Java? 28 What is the this keyword in Java? 29 What is the super keyword in Java? 30 What is a wrapper class in Java? 31 What is a try-catch block in Java? 32 What is the difference between checked and unchecked exceptions? 33 What is the difference between throw and throws in Java? 34 What is an array in Java? 35 What is the enhanced for loop (for-each) in Java? 36 What is the ternary operator in Java? 37 What is a default constructor in Java? 38 What is abstraction in Java? 39 What is the String pool in Java? 40 What is the difference between final, finally, and finalize in Java?

01 What is the Java Collections Framework? 02 What is the difference between ArrayList and LinkedList? 03 What is HashMap in Java and how does it work internally? 04 What is the difference between HashMap and HashTable in Java? 05 What is the difference between HashMap and LinkedHashMap? 06 What is the difference between HashSet and TreeSet? 07 What is an Iterator in Java? 08 What is the difference between fail-fast and fail-safe iterators? 09 What are generics in Java? 10 What is a thread in Java? 11 What is the synchronized keyword in Java? 12 What is the volatile keyword in Java? 13 What is deadlock in Java? 14 What is the Executor framework in Java? 15 What are lambda expressions in Java 8? 16 What is a functional interface in Java? 17 What is the Stream API in Java 8? 18 What is Optional in Java? 19 What is a method reference in Java? 20 What are default methods in Java interfaces? 21 What is the difference between Comparable and Comparator? 22 What is Java Reflection? 23 What is the difference between List.of() and Arrays.asList()? 24 What is try-with-resources in Java? 25 What is PriorityQueue in Java? 26 What is CountDownLatch in Java? 27 What is a BlockingQueue in Java? 28 What is the Semaphore class in Java? 29 What is the Deque interface in Java? 30 What is TreeMap in Java? 31 What is the Collections utility class in Java? 32 What is WeakReference and WeakHashMap in Java? 33 What is CopyOnWriteArrayList in Java?

01 What is the Java Memory Model (JMM)? 02 What is ConcurrentHashMap and how does it differ from HashMap? 03 What is CompletableFuture in Java? 04 What is the Fork/Join framework in Java? 05 What are Java records? 06 What are sealed classes in Java? 07 What is pattern matching for switch in Java? 08 What are text blocks in Java? 09 What is the ClassLoader in Java? 10 What is the Singleton design pattern and how is it implemented in Java? 11 What is the Builder design pattern in Java? 12 What is the Factory pattern in Java? 13 What is the Observer design pattern in Java? 14 What is the Strategy design pattern in Java? 15 What is Java NIO and how does it differ from traditional IO? 16 What is Java Serialization? 17 What are Java 9 modules (JPMS)? 18 What is the var keyword in Java? 19 What is the difference between Heap dump and Thread dump in Java? 20 What are the main garbage collection algorithms in Java? 21 What is the difference between String.valueOf() and toString() in Java? 22 What is Reactive Programming and how does Java support it? 23 What is the Decorator design pattern in Java? 24 What is a memory leak in Java and how do you prevent it? 25 What is the difference between Callable and Runnable in Java? 26 What are Java annotations and how do they work? 27 What is Virtual Threads in Java (Project Loom)?