Question 1

What are the main differences between the JDK, JRE, and JVM?

Accepted Answer

JVM (Java Virtual Machine) executes bytecode and provides platform independence. JRE (Java Runtime Environment) includes JVM plus core libraries needed to run Java applications. JDK (Java Development Kit) includes JRE plus development tools (compiler, debugger, javadoc). Developers need JDK; end users only need JRE.

Question 2

What are the four main principles of OOP in Java?

Accepted Answer

Encapsulation: bundling data with methods and restricting access via access modifiers. Inheritance: creating new classes from existing ones using 'extends'. Polymorphism: same interface with different implementations through method overriding/overloading. Abstraction: hiding complexity using abstract classes and interfaces to expose only essential features.

Question 3

What is the difference between == and equals() when comparing objects?

Accepted Answer

== compares references (memory addresses), checking if two variables point to the same object. equals() compares content/values, checking if two objects are logically equivalent. For primitives, use ==. For objects like String, always use equals() for value comparison. Override equals() and hashCode() together for custom classes.

Question 4

What are abstract classes and interfaces, and when do you use each?

Accepted Answer

Abstract classes can have both abstract and concrete methods, instance variables, and constructors—use for shared base functionality with 'is-a' relationships. Interfaces define contracts with abstract methods (and default methods since Java 8)—use for defining capabilities across unrelated classes. Classes extend one abstract class but implement multiple interfaces.

Question 5

How does Java garbage collection work?

Accepted Answer

Garbage collection automatically reclaims memory from objects no longer referenced. The heap is divided into Young Generation (Eden + Survivor spaces) and Old Generation. New objects go to Eden; survivors move through Survivor spaces then to Old Generation. Minor GC cleans Young Gen; Major/Full GC cleans entire heap. Common collectors include G1, ZGC, and Parallel GC.

Question 6

What is the difference between ArrayList and LinkedList?

Accepted Answer

ArrayList uses a dynamic array—fast O(1) random access and iteration, but slow O(n) insertions/deletions in the middle due to shifting. LinkedList uses doubly-linked nodes—fast O(1) insertions/deletions when position is known, but slow O(n) random access. ArrayList is generally preferred; use LinkedList for frequent insertions/deletions at known positions.

Question 7

How does HashMap work internally?

Accepted Answer

HashMap stores key-value pairs in an array of buckets. Keys are hashed to determine bucket index. Collisions (same hash) are handled using linked lists or balanced trees (Java 8+). When load factor exceeds threshold (default 0.75), the array resizes. Time complexity is O(1) average for get/put. Keys must properly implement hashCode() and equals().

Question 8

What is the difference between checked and unchecked exceptions?

Accepted Answer

Checked exceptions (IOException, SQLException) must be declared with 'throws' or caught—compiler enforces handling. Unchecked exceptions extend RuntimeException (NullPointerException, IllegalArgumentException)—no compile-time handling required. Use checked exceptions for recoverable conditions; unchecked for programming errors. Error class represents serious JVM problems, not meant to be caught.

Question 9

How do you create threads in Java?

Accepted Answer

Two main approaches: 1) Extend Thread class and override run() method. 2) Implement Runnable interface (preferred—allows extending other classes). Modern approach: use ExecutorService for thread pool management. Start threads with thread.start() (not run()). Use Callable with Future when you need return values from threads.

Question 10

What is the synchronized keyword and how does it work?

Accepted Answer

synchronized ensures only one thread can execute a block/method at a time by acquiring an intrinsic lock. On methods, it locks 'this' (instance) or Class object (static). On blocks, specify the lock object. Prevents race conditions but can cause deadlocks and performance issues. Consider java.util.concurrent alternatives like ReentrantLock for more control.

Question 11

What are Lambda expressions and functional interfaces?

Accepted Answer

Lambda expressions are anonymous functions enabling functional programming: (parameters) -> expression. Functional interfaces have exactly one abstract method (annotated @FunctionalInterface). Common examples: Runnable, Callable, Comparator, Consumer, Function, Predicate, Supplier. Lambdas provide cleaner syntax than anonymous classes for functional interface implementations.

Question 12

What is the Stream API and how does it differ from collections?

Accepted Answer

Streams process sequences of elements with functional operations—they don't store data like collections. Streams are lazy (intermediate operations like filter, map execute only when terminal operation is called), support parallel processing, and can't be reused. Terminal operations include collect, forEach, reduce, count. Streams enable declarative, pipeline-style data processing.

Question 13

What is Optional and how should it be used?

Accepted Answer

Optional<T> is a container that may or may not contain a non-null value, introduced to reduce NullPointerExceptions. Use orElse(), orElseGet(), orElseThrow() to handle absence. Use map(), flatMap(), filter() for transformations. Best for return types; avoid as method parameters or fields. Never call get() without checking isPresent() or using safer alternatives.

Question 14

What is the difference between Statement and PreparedStatement in JDBC?

Accepted Answer

Statement executes static SQL strings—vulnerable to SQL injection and slower for repeated queries. PreparedStatement uses parameterized queries with placeholders (?)—prevents SQL injection, allows precompilation for better performance, and handles data type conversion. Always use PreparedStatement for queries with user input. CallableStatement is for stored procedures.

Question 15

What is dependency injection in Spring?

Accepted Answer

Dependency injection (DI) is a design pattern where objects receive dependencies from external sources rather than creating them. Spring's IoC container manages object creation and wiring. Types: constructor injection (preferred—immutable, testable), setter injection, field injection (@Autowired). Benefits: loose coupling, easier testing, better maintainability. Configure via annotations or XML.

Java Basics(10 questions)

What are the main differences between the JDK, JRE, and JVM?

Why is Java considered platform-independent?

How do you define the main method in a Java class, and what is its significance?

What is the difference between byte, short, int, and long data types?

What are the default values of primitive data types in Java?

What is the difference between "==" and equals() when comparing objects and primitives?

How do you convert a String to an int and vice versa?

What is a package in Java, and how do you import classes from it?

What is the use of the "final" keyword in Java?

How does Java handle memory allocation for primitive and reference types?

Object-Oriented Programming in Java(10 questions)

What does it mean that Java is an object-oriented programming language?

What are the four main principles of OOP, and how are they implemented in Java?

How do you achieve encapsulation in Java?

What is inheritance, and how do you implement it?

What is polymorphism, and how does method overriding differ from method overloading?

What is the importance of the "this" keyword, and when is "super" used?

How do you prevent a class from being inherited?

What are abstract classes and interfaces, and when do you use each?

How do default methods in interfaces work in Java?

Can you explain the concept of composition versus inheritance?

Java Memory Management and Garbage Collection(10 questions)

What is the structure of the JVM's memory areas (heap, stack, metaspace, etc.)?

Can you describe how Java garbage collection works internally?

What is the difference between a reference that is strongly referenced, weakly referenced, softly referenced, and phantom referenced?

How do you explicitly call garbage collection, and is that recommended in production?

What is the purpose of the finalize() method, and why is it generally avoided?

What happens if an OutOfMemoryError occurs, and how do you troubleshoot it?

Can you compare Serial, Parallel, CMS, and G1 garbage collectors?

How can you force a minor GC and a major GC?

What is the Eden space, and what roles do Survivor spaces play?

How would you tune your application if you see frequent full GCs causing performance issues?

Java Collections Framework Interview Questions(10 questions)

What are the main interfaces of the Java Collections Framework?

Can you explain the differences between List, Set, and Map?

What are the characteristics of an ArrayList compared to a LinkedList?

How does HashSet ensure uniqueness, and how does it compare to TreeSet?

What happens if you override equals() but not hashCode()?

How does a HashMap internally store key-value pairs, and what is a hash collision?

What is the difference between ConcurrentHashMap and HashMap?

When might you use a LinkedHashMap instead of a HashMap?

How does a PriorityQueue work, and what interfaces does it implement?

How does the Collections utility class help with unmodifiable or synchronized collections?

Exception Handling in Java(10 questions)

How do checked exceptions differ from unchecked exceptions in Java?

How does the "try-catch-finally" block work, and in what order are statements executed?

What is the purpose of the "throws" keyword, and how does it differ from throw?

How do you create a custom exception?

Which exception is thrown when you attempt to access an element outside of an array's bounds?

What is the difference between Error and Exception in Java?

Can you explain the concept of a "chained exception"?

How do you handle multiple exceptions in the same try-catch block?

What is a resource leak, and how does the "try-with-resources" statement help?

How should exceptions ideally be used in methods that might fail?

Multithreading and Concurrency Interview Questions(10 questions)

How do you create a thread in Java using Thread class versus Runnable interface?

What is the difference between concurrent programming and parallel programming?

How does the synchronized keyword work, and what are the drawbacks of using it?

Can you explain the concept of locks and the ReentrantLock class?

What are the differences between volatile variables and atomic variables?

What is the Executor framework, and how does it improve thread management?

How do you use the ThreadPoolExecutor and ScheduledThreadPoolExecutor?

What are the differences between wait(), notify(), and notifyAll()?

What are common concurrency pitfalls or race conditions, and how do you avoid them?

How do the Concurrent collections (e.g., ConcurrentLinkedQueue) differ from standard collections?

Java 8 and Newer Features - Interview Questions &amp; Answers(10 questions)

What are Lambda expressions, and how do they work under the hood?

What is a functional interface, and can you give some examples from the standard library?

How does the stream API differ from collections, and what are some of its advantages?

What is the difference between a stream's intermediate and terminal operations?

How do you perform parallel stream operations, and what are the potential issues?

Can you explain method references, and give an example usage?

What is the Optional class, and how do you use it to handle null values?

How do default methods in interfaces work, and why were they introduced?

What is the purpose of the java.time package, and how does it improve upon Date?

Can you explain the new switch expressions introduced in later Java versions?

Java I/O and NIO - Interview Questions &amp; Answers(10 questions)

How do you read and write files using traditional I/O (java.io package)?

What is the difference between byte streams and character streams?

Java 8 and Newer Features - Interview Questions & Answers(10 questions)

Java I/O and NIO - Interview Questions & Answers(10 questions)