Basic Java Interview Questions

• Q: What is Java?

  A: Java is a platform-independent, object-oriented programming language.

• Q: Explain JVM, JRE, and JDK.

  A: JVM runs bytecode, JRE = JVM + libraries, JDK = JRE + development tools.

• Q: What is a class?

  A: Blueprint for objects.

• Q: What is an object?

  A: Instance of a class.

• Q: What are access modifiers?

  A: public, private, protected, default.

• Q: What is a constructor?

  A: Initializes object during creation.

• Q: Difference between == and equals()

  A: == compares references; equals() compares values.

• Q: What is method overloading?

  A: Same method name, different parameters.

• Q: What is method overriding?

  A: Child class redefines parent method.

• Q: What is inheritance?

  A: Child class inherits parent class properties.

• Q: What is polymorphism?

  A: Same method, different behavior.

• Q: What is abstraction?

  A: Hiding internal implementation.

• Q: What is encapsulation?

  A: Hiding data with private fields and public getters/setters.

• Q: What is an interface?

  A: Abstract type defining methods a class must implement.

• Q: What is an abstract class?

  A: Class with abstract methods (no body).

• Q: What is final keyword?

  A: Prevents modification of class, method, or variable.

• Q: Difference between static and instance methods?

  A: Static belongs to class; instance belongs to object.

• Q: What is a package?

  A: Group of related classes.

• Q: What is exception handling?

  A: Handling runtime errors with try-catch.

• Q: Difference between checked and unchecked exceptions?

  A: Checked at compile-time, unchecked at runtime.

• Q: What is a thread?

  A: Lightweight process.

• Q: How to create a thread?

  A: Extend Thread class or implement Runnable.

• Q: What is synchronization?

  A: Prevents multiple threads from accessing critical section.

• Q: What is String in Java?

  A: Immutable sequence of characters.

• Q: Difference between String, StringBuffer, StringBuilder?

  A: String immutable, StringBuffer thread-safe, StringBuilder faster.

• Q: What is ArrayList?

  A: Resizable array.

• Q: Difference between ArrayList and LinkedList?

  A: ArrayList faster for indexing, LinkedList better for inserts/deletes.

• Q: What is HashMap?

  A: Key-value pair collection.

• Q: What is garbage collection?

  A: Automatic memory cleanup.

• Q: What is wrapper class?

  A: Converts primitive to object.

• Q: What is autoboxing/unboxing?

  A: Auto conversion between primitives and objects.

• Q: What is this keyword?

  A: Refers to current object.

• Q: What is super keyword?

  A: Refers to parent class.

• Q: Can a class have multiple constructors?

  A: Yes (constructor overloading).

• Q: What is the main method signature?

  A: public static void main(String[] args)

• Q: Can you override static method?

  A: No.

• Q: Can abstract class have constructor?

  A: Yes.

• Q: What is instanceof operator?

  A: Checks object type at runtime.

• Q: What is a static block?

  A: Executes once when class loads.

• Q: What is a singleton class?

  A: Class with single instance.

• Q: What are varargs?

  A: Variable-length arguments.

• Q: Can interface have constructors?

  A: No.

• Q: What is default method in interface?

  A: Method with body in interface (Java 8+).

• Q: What is toString()?

  A: Converts object to string.

• Q: What is equals() and hashCode()?

  A: Defines object equality and hash.

• Q: Difference between throw and throws?

  A: throw for exceptions; throws for method signature.

• Q: What is try-with-resources?

  A: Automatic resource management (ARM).

• Q: What is Enum?

  A: Special class with fixed set of constants.

• Q: What is functional interface?

  A: Interface with exactly one abstract method.

• Q: What is lambda expression?

  A: Concise way to pass code as parameter.

• Q: What is stream in Java?

  A: Sequence of elements supporting functional operations.