✅ Top 50 Java Interview Questions and Answers (2025)

  

🔹 Core Java Basics

  1. Java kya hai?
    Answer: Java ek high-level, object-oriented programming language hai jo platform-independent hai, kyunki isme likha code JVM (Java Virtual Machine) par run hota hai.

  2. Java platform-independent kaise hai?
    Answer: Java code ko compile karne par bytecode banta hai, jo kisi bhi platform ke JVM par run ho sakta hai.

  3. JVM, JRE aur JDK mein kya antar hai?
    Answer:

  • JVM: Java Virtual Machine — Java code ko chalata hai.

  • JRE: Java Runtime Environment — JVM + libraries.

  • JDK: Java Development Kit — JRE + development tools.

  1. Object-Oriented Programming ke 4 pillars kya hain?
    Answer: Encapsulation, Inheritance, Polymorphism, Abstraction.

  2. Class aur Object mein kya farak hai?
    Answer: Class ek blueprint hai, jabki object us blueprint ka real-world instance hai.

🔹 OOP Concepts

  1. Encapsulation kya hai?
    Answer: Data ko wrap karna aur direct access se bachana, usually getters aur setters ke through.

  2. Inheritance ka kya use hai?
    Answer: Ek class doosri class ke properties aur methods inherit kar sakti hai, reusability ke liye.

  3. Polymorphism kya hota hai?
    Answer: Ek interface ke multiple implementations — overloading aur overriding ke zariye.

  4. Method Overloading kya hai?
    Answer: Same method name but different parameters (compile-time polymorphism).

  5. Method Overriding kya hai?
    Answer: Child class parent class ke method ko apne hisaab se redefine karta hai (run-time polymorphism).

🔹 Advanced Java Concepts

  1. Abstraction kya hai?
    Answer: Sirf essential details dikhana aur implementation details chhupana.

  2. Interface aur Abstract class mein kya difference hai?
    Answer: Interface mein sirf method declarations hote hain, abstract class mein method bodies bhi ho sakti hain.

  3. Final keyword ka use kya hai?
    Answer: Variable ko constant banane ke liye, method ko override se rokne ke liye, aur class ko extend hone se rokne ke liye.

  4. Static keyword ka kya role hai?
    Answer: Static members class level ke hote hain, kisi object se belong nahi karte.

  5. this aur super keyword mein antar?
    Answer: this current class ka reference deta hai, super parent class ka.

🔹 Exception Handling

  1. Exception kya hota hai?
    Answer: Runtime error jo normal flow ko interrupt karta hai.

  2. Checked aur Unchecked exception kya hote hain?
    Answer:

  • Checked: Compile time par handle karna padta hai (e.g., IOException).

  • Unchecked: Runtime par aate hain (e.g., NullPointerException).

  1. try-catch-finally kaise kaam karta hai?
    Answer: try block mein risky code, catch mein error handling, aur finally block hamesha execute hota hai.

  2. throw aur throws keyword mein kya difference hai?
    Answer: throw specific exception ko manually throw karta hai; throws method signature mein likha jaata hai.

  3. Custom Exception kaise banate hain?
    Answer: Ek class banai jaati hai jo Exception ya RuntimeException ko extend karti hai.

🔹 Collections Framework

  1. Collection aur Collections mein kya difference hai?
    Answer:

  • Collection: Interface

  • Collections: Utility class (helper methods ke liye)

  1. List, Set aur Map mein kya antar hai?
    Answer:

  • List: Duplicate allow karta hai (ArrayList, LinkedList)

  • Set: Duplicate allow nahi karta (HashSet, TreeSet)

  • Map: Key-value pair (HashMap, TreeMap)

  1. ArrayList vs LinkedList?
    Answer:

  • ArrayList: Fast for access, slow for insert/delete.

  • LinkedList: Slow for access, fast for insert/delete.

  1. HashMap aur Hashtable mein kya difference hai?
    Answer:

  • HashMap: Not synchronized, fast.

  • Hashtable: Synchronized, thread-safe.

  1. Iterator vs ListIterator?
    Answer:

  • Iterator: One-directional

  • ListIterator: Bi-directional traversal

🔹 Multithreading

  1. Thread kya hota hai?
    Answer: Program ka lightweight sub-process jo parallel kaam karta hai.

  2. Thread kaise banate hain?
    Answer:

  • Thread class extend kar ke

  • Runnable interface implement kar ke

  1. sleep() vs wait()?
    Answer:

  • sleep() Thread class ka method hai

  • wait() Object class ka method hai, notify ke saath kaam karta hai

  1. synchronized block kya karta hai?
    Answer: Multiple threads ke access ko control karta hai — thread safety ke liye.

  2. Deadlock kya hota hai?
    Answer: Jab do threads ek doosre ka lock hold kar lein aur kisi ko access na mil paaye.

🔹 Java Keywords & Concepts

  1. Transient keyword kya hai?
    Answer: Transient fields serialization mein ignore kiye jaate hain.

  2. Volatile keyword kya hai?
    Answer: Variable ko multiple threads ke liye fresh rakhne ke liye use hota hai.

  3. Immutable class ka example?
    Answer: String class — ek baar object bana to uska state change nahi ho sakta.

  4. Garbage Collection kya hota hai?
    Answer: JVM ka automatic memory cleanup process.

  5. finalize() method kya karta hai?
    Answer: Object ke destruction ke pehle JVM is method ko call karta hai.

🔹 Input/Output & Serialization

  1. Serialization kya hai?
    Answer: Object ko byte stream mein convert karna — store ya transfer ke liye.

  2. Serializable interface ka role kya hai?
    Answer: Java class ko serializable banata hai — bina kisi method ke (marker interface).

  3. FileReader aur FileInputStream mein antar?
    Answer:

  • FileReader — character streams ke liye

  • FileInputStream — byte streams ke liye

  1. BufferedReader ka use?
    Answer: Line-by-line text reading ke liye — fast performance ke saath.

  2. Scanner class ka role kya hai?
    Answer: User se input lene ke liye use hoti hai (console input).

🔹 Java 8 Features

  1. Lambda Expression kya hai?
    Answer: Ek short syntax function likhne ke liye — functional interface ke saath.

  2. Functional Interface kya hai?
    Answer: Ek interface jisme sirf ek abstract method ho (e.g., Runnable, Comparator).

  3. Stream API kya hai?
    Answer: Collection data par functional-style operations perform karne ka tarika.

  4. Optional class ka use kya hai?
    Answer: NullPointerException se bachne ke liye — safe value wrapping.

  5. Default methods in interface kya hote hain?
    Answer: Interface ke andar method body ke saath likhe ja sakte hain — Java 8 se.

🔹 Miscellaneous

  1. Java me main method kyun zaroori hota hai?
    Answer: JVM sabse pehle public static void main(String[] args) ko call karta hai.

  2. Can we overload main method?
    Answer: Haan, lekin JVM default main method hi call karega.

  3. Java me memory kaise manage hoti hai?
    Answer: JVM heap aur stack ke through memory manage karta hai — garbage collector ke saath.

  4. Wrapper classes kya hoti hain?
    Answer: Primitive data types ko object mein convert karne ke liye (e.g., int → Integer).

  5. Difference between == and .equals()?
    Answer:

  • == object references compare karta hai

  • .equals() object values


🔹 Java Fundamentals

  1. Java bytecode kya hota hai?
    Answer: Java source code ko compile karne ke baad jo intermediate code banta hai usse bytecode kehte hain. Ye JVM ke liye executable hota hai.

  2. Java me memory leak kya hota hai?
    Answer: Jab unused objects heap memory me bane rehte hain aur unka reference delete nahi hota, to use memory leak kehte hain.

  3. Primitive data types kitne hote hain?
    Answer: Java me 8 primitive types hote hain: byte, short, int, long, float, double, char, boolean.

  4. Java ke features kya hain?
    Answer: Simple, Secure, Portable, Platform Independent, Object-Oriented, Robust, Multithreaded, High Performance.

  5. Java me type casting kya hoti hai?
    Answer: Ek data type ko doosre me badalne ko type casting kehte hain (implicit & explicit).

🔹 Classes and Objects

  1. Constructors kya hote hain?
    Answer: Constructor ek special method hai jo object banate waqt call hota hai. Iska naam class ke naam jaisa hota hai.

  2. Default constructor kya hota hai?
    Answer: Java compiler automatically ek no-argument constructor provide karta hai agar koi constructor define na kiya ho.

  3. Parameterized constructor kya hai?
    Answer: Constructor jisme arguments diye jaate hain — alag values ke sath object banane ke liye.

  4. Can we overload constructors?
    Answer: Haan, ek class me multiple constructors ho sakte hain with different parameters.

  5. Object cloning kya hota hai?
    Answer: Ek object ka exact copy banane ke process ko cloning kehte hain (clone() method se).

🔹 Method & Variables

  1. Java me pass-by-value kya hai?
    Answer: Java me sab kuch pass-by-value hota hai, lekin objects ke reference ka value pass hota hai.

  2. Instance variable kya hota hai?
    Answer: Class ke object level par define hota hai — har object ka apna hota hai.

  3. Class variable kya hota hai?
    Answer: static keyword se defined hota hai — sabhi objects ke liye common.

  4. Method hiding kya hai?
    Answer: Jab static method subclass me re-define kiya jata hai, to use method hiding kehte hain.

  5. Can we define a method inside another method?
    Answer: Java me nested methods directly allowed nahi hain.

🔹 Inheritance and Polymorphism

  1. Multilevel inheritance kya hai?
    Answer: Jab ek class ek aur class se inherit hoti hai, aur wo bhi kisi aur se, to use multilevel inheritance kehte hain.

  2. Hierarchical inheritance kya hai?
    Answer: Jab ek parent class ko multiple subclasses inherit karte hain.

  3. Can we override a static method?
    Answer: Nahi, static methods override nahi hote. Ye class level ke hote hain.

  4. Can constructors be inherited?
    Answer: Nahi, constructors inherit nahi hote, lekin subclass super() se parent constructor ko call kar sakti hai.

  5. Covariant return types kya hote hain?
    Answer: Jab overridden method ka return type subclass of original return type ho.

🔹 Exception & Error Handling

  1. Error aur Exception me kya farak hai?
    Answer:

  • Error: JVM related issues (e.g., OutOfMemoryError)

  • Exception: Code level issues (e.g., FileNotFoundException)

  1. try-with-resources kya hota hai?
    Answer: Java 7 ka feature jisme auto-closeable resources try block me declare kiye jaate hain.

  2. Multiple catch blocks ka use?
    Answer: Ek se zyada catch blocks use karke specific exceptions handle kiye ja sakte hain.

  3. finally block kab execute hota hai?
    Answer: finally block hamesha execute hota hai, chahe exception aaye ya nahi.

  4. Can we catch multiple exceptions in one block?
    Answer: Haan, Java 7 se multi-catch block use kiya ja sakta hai: catch(IOException | SQLException e)

🔹 Collections & Generics

  1. TreeSet kya hota hai?
    Answer: Sorted set jo elements ko natural ya custom order me store karta hai — duplicates allow nahi karta.

  2. LinkedHashMap kya hai?
    Answer: Map jo insertion order ko maintain karta hai — predictable iteration.

  3. Comparable vs Comparator?
    Answer:

  • Comparable: compareTo() method implement karta hai (natural order).

  • Comparator: compare() method implement karta hai (custom order).

  1. Generic classes ka use?
    Answer: Type-safe collections aur reusable code likhne ke liye.

  2. Stack class kya hai?
    Answer: Last-In-First-Out (LIFO) data structure — push(), pop() methods ke saath.

🔹 Multithreading & Concurrency

  1. What is thread priority?
    Answer: Thread ke execution preference ko dikhata hai (1 to 10). Default value 5 hoti hai.

  2. yield() method ka use?
    Answer: Current thread ko temporary pause karke dusre thread ko chance deta hai.

  3. join() method kya karta hai?
    Answer: Ek thread ko complete hone tak dusre thread ko wait karne ko kehta hai.

  4. Runnable vs Callable?
    Answer:

  • Runnable: koi return value nahi deta

  • Callable: return value deta hai aur exception throw kar sakta hai

  1. Future interface ka use kya hai?
    Answer: Callable task ka result future me retrieve karne ke liye use hota hai.

🔹 String Handling

  1. StringBuilder vs StringBuffer?
    Answer:

  • StringBuilder: Not thread-safe but fast

  • StringBuffer: Thread-safe but slow

  1. String pool kya hai?
    Answer: JVM ka memory area jahan same string literals store hote hain — memory optimization ke liye.

  2. String immutable kyun hai?
    Answer: Security, caching, synchronization ke liye string ko immutable banaya gaya hai.

  3. Intern() method kya karta hai?
    Answer: String ko pool me daal deta hai agar wo pehle se exist nahi karta ho.

  4. split() method ka use?
    Answer: String ko delimiter ke basis par tod kar array banata hai.

🔹 Java APIs & Libraries

  1. Math class kya hai?
    Answer: Java ka utility class jo mathematical operations provide karta hai — sqrt, pow, abs, etc.

  2. System.out.println() ka meaning?
    Answer: System ek class hai, out ek PrintStream object hai, aur println() output print karta hai.

  3. Date vs Calendar class?
    Answer:

  • Date: Basic date/time support

  • Calendar: Advanced manipulation (add, set, get)

  1. UUID class kya karta hai?
    Answer: Random unique identifiers generate karta hai.

  2. Pattern and Matcher class ka use?
    Answer: Regular expressions ke through pattern matching perform karte hain.

🔹 Java 8+ Features (Continued)

  1. Method reference kya hota hai?
    Answer: Lambda expressions ko short karne ka tarika — ClassName::methodName

  2. Predicate functional interface ka use?
    Answer: Ek input leta hai aur boolean return karta hai — filtering ke liye useful.

  3. Consumer functional interface kya hai?
    Answer: Ek input leta hai, return kuch nahi karta — mainly printing/logging ke liye.

  4. Supplier functional interface kya hai?
    Answer: Koi input nahi leta, sirf value return karta hai — lazily produce karne ke liye.

  5. Stream filter and map me antar?
    Answer:

  • filter(): condition apply karta hai

  • map(): elements ko transform karta hai


🔹 JVM Internals & Performance

  1. JVM Architecture ke main components kya hain?
    Answer: Class Loader, Runtime Data Areas (Heap, Stack, Method Area, etc.), Execution Engine, Native Interface, Garbage Collector.

  2. Heap aur Stack memory me kya antar hai?
    Answer: Heap me objects store hote hain, Stack me method call frames. Stack thread-specific hoti hai, Heap shared hoti hai.

  3. Java me Metaspace kya hota hai?
    Answer: Java 8 ke baad PermGen ko replace karta hai. Metaspace JVM ke bahar native memory use karta hai.

  4. JVM tuning kaise kiya jata hai?
    Answer: Flags jaise -Xmx, -Xms, -XX:+UseG1GC, -XX:+PrintGCDetails use karke memory aur GC behavior optimize kiya jata hai.

  5. Garbage Collection ke types kya hain?
    Answer: Serial, Parallel, CMS, G1, ZGC, Shenandoah (Java 11+).

🔹 Java Memory Model & Concurrency

  1. Java Memory Model kya define karta hai?
    Answer: Thread ke beech shared memory access ke rules ko define karta hai — reordering aur visibility ka control karta hai.

  2. volatile keyword ka kya role hai?
    Answer: Shared variable ki changes ko sabhi threads ke liye visible banata hai.

  3. synchronized vs ReentrantLock me kya difference hai?
    Answer:

  • synchronized simple hai, automatic unlock hota hai.

  • ReentrantLock me tryLock(), fairness, interruptible locks jaise advanced features milte hain.

  1. Thread starvation kya hota hai?
    Answer: Jab ek thread continuously CPU resources se vanchit rehta hai due to higher priority threads.

  2. Deadlock detect aur avoid kaise karein?
    Answer: Lock ordering, timeout, tryLock() se avoid kar sakte hain. Thread dump analysis se detect kar sakte hain.

🔹 Java 8–17 Advanced Features

  1. Optional class ka purpose kya hai?
    Answer: NullPointerException avoid karne ke liye Optional container use hota hai.

  2. Streams API lazy evaluation kaise karta hai?
    Answer: Intermediate operations lazy hote hain, actual execution terminal operation par hoti hai.

  3. Collectors.groupingBy ka use?
    Answer: Stream me data ko kisi key ke basis par group karne ke liye.

  4. default aur static methods interface me kyun laaye gaye?
    Answer: Interface backward compatibility maintain karne ke liye.

  5. Sealed classes kya hoti hain (Java 15)?
    Answer: Aisi classes jinke subclasses ko restrict kiya jata hai using permits keyword.

🔹 Design Patterns in Java

  1. Singleton pattern ka thread-safe implementation kaise hota hai?
    Answer: Using enum, double-checked locking, or static inner class.

  2. Factory vs Abstract Factory pattern?
    Answer:

  • Factory: Ek object banata hai

  • Abstract Factory: Related objects ke families banata hai

  1. Builder pattern ka use kab hota hai?
    Answer: Complex objects banane ke liye jisme multiple optional fields hote hain.

  2. Strategy pattern kya hota hai?
    Answer: Behavior ko runtime pe switch karne ka pattern (via interfaces).

  3. Observer pattern Java me kaise implement hota hai?
    Answer: Observer interface, Observable class (deprecated), ya custom listeners se.

🔹 Multithreading Deep Dive

  1. Thread-safe collection classes kaun si hain?
    Answer: ConcurrentHashMap, CopyOnWriteArrayList, BlockingQueue, ConcurrentLinkedQueue.

  2. ThreadLocal ka use kya hai?
    Answer: Har thread ke liye alag instance maintain karne ke liye.

  3. Fork/Join framework kya hai?
    Answer: Parallel task execution ke liye divide and conquer approach. Introduced in Java 7.

  4. CountDownLatch vs CyclicBarrier?
    Answer:

  • CountDownLatch: One-time latch

  • CyclicBarrier: Reusable barrier for fixed number of threads

  1. ExecutorService me shutdown vs shutdownNow?
    Answer:

  • shutdown(): graceful termination

  • shutdownNow(): forcefully interrupt running tasks

🔹 Annotations & Reflection

  1. Custom annotation kaise banate hain?
    Answer: @interface se define karte hain aur @Retention, @Target set karte hain.

  2. Reflection API se kya kar sakte hain?
    Answer: Runtime par classes, methods, fields ko inspect aur modify kar sakte hain.

  3. Annotation retention policies kya hoti hain?
    Answer: SOURCE, CLASS, RUNTIME — define karta hai annotation kitni der tak available hoga.

  4. Marker interface kya hota hai?
    Answer: Interface without methods — jiska use JVM ya frameworks special behavior ke liye karte hain (e.g., Serializable).

  5. Spring me annotations ka role kya hota hai?
    Answer: Dependency injection, configuration, mapping, lifecycle management ke liye (@Autowired, @Component, etc.)

🔹 Serialization & Networking

  1. transient keyword ka kya role hai?
    Answer: Serializable objects ke aise fields ko exclude karta hai jo serialize nahi hone chahiye.

  2. Externalizable vs Serializable?
    Answer:

  • Serializable: default serialization

  • Externalizable: custom logic for read/write

  1. Socket programming me ServerSocket kya karta hai?
    Answer: Client se connection accept karta hai aur socket return karta hai.

  2. ObjectInputStream/ObjectOutputStream ka use kya hai?
    Answer: Java objects ko stream ke through read/write karne ke liye.

  3. Java RMI kya hai?
    Answer: Remote Method Invocation — remote JVM me object methods call karne ka mechanism.

🔹 Security, Modules, Miscellaneous

  1. Java me ClassLoader hierarchy kya hoti hai?
    Answer: Bootstrap → Extension → Application → Custom. Ye JVM me classes load karte hain.

  2. SecurityManager ka kya role hai?
    Answer: Java application ke liye runtime security policies enforce karta hai.

  3. JDK 9 module system kya hai?
    Answer: Java Platform Modular System (JPMS) — application ko modules me tod kar encapsulation improve karta hai.

  4. Why Java removed Applets?
    Answer: Security issues aur browser support ke khatam hone ke kaaran Applets deprecated ho gaye.

  5. Lombok library kya karti hai?
    Answer: Java classes me boilerplate code (getters, setters, constructors) ko avoid karti hai via annotations.

🔹 Best Practices & Debugging

  1. Immutable class kaise banate hain?
    Answer: Final class, private final fields, no setters, defensive copies in constructor.

  2. Memory leaks in Java applications kaise detect karte hain?
    Answer: Tools jaise VisualVM, JProfiler, YourKit, MAT se heap dump analyze karke.

  3. equals() and hashCode() override karne ke rules?
    Answer:

  • Equal objects must have same hashCode

  • hashCode consistency maintain hona chahiye

  1. Fail-fast vs Fail-safe iterators?
    Answer:

  • Fail-fast: Concurrent modification detect karta hai (e.g., ArrayList)

  • Fail-safe: Concurrent modification tolerate karta hai (e.g., CopyOnWriteArrayList)

  1. Final vs finally vs finalize()?
    Answer:

  • final: variable/method/class ke liye constant

  • finally: exception handling me block

  • finalize(): object destruction se pehle call hota hai (deprecated)

🔹 Newer Java Features (Java 11–17)

  1. var keyword ka use kya hai?
    Answer: Local variable ke liye type inference provide karta hai (Java 10+).

  2. Text Blocks (""") kya hain?
    Answer: Multi-line string literals ko simple banata hai (Java 13+).

  3. Records kya hain?
    Answer: Immutable data carriers with minimal boilerplate — record Point(int x, int y)

  4. Pattern Matching for instanceof kya hai?
    Answer: Simplifies casting:


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