Viva Questions

20 most-asked Java viva Q&A — short, exam-ready answers.

Common Java Viva Questions and Answers

What is Java? Object-oriented, platform-independent, secure, multithreaded language by Sun Microsystems (now Oracle).
What is bytecode? Intermediate code (.class) produced by javac and executed by the JVM.
What is JVM? Java Virtual Machine — runs bytecode, manages memory, garbage collection.
JDK vs JRE? JDK = JRE + dev tools (compiler, debugger). JRE only runs.
What is OOP? Programming style based on objects (data + behaviour). 4 pillars: Inheritance, Encapsulation, Polymorphism, Abstraction.
What is a class and an object? Class = blueprint; object = instance of a class.
What is constructor? Special method with same name as class, no return type, called automatically when object is created.
this vs super? this → current object; super → parent class.
Method overloading vs overriding? Overloading: same method name, different params (compile-time). Overriding: subclass changes parent's method (run-time).
Difference between final, finally, finalize? final keyword for constants; finally block always executes; finalize() called by GC before object is destroyed.
What is exception handling? try / catch / finally / throw / throws — handle runtime errors gracefully.
Checked vs unchecked exception? Checked: compile-time enforced (IOException). Unchecked: runtime (NullPointerException).
What is a thread? Lightweight subprocess. Create via extends Thread or implements Runnable.
String vs StringBuilder vs StringBuffer? String — immutable. StringBuilder — mutable, fast, NOT thread-safe. StringBuffer — mutable + thread-safe.
What is collection framework? Set of interfaces & classes (List, Set, Map, Queue) for storing groups of objects.
Why is Java not 100% OOP? Because it has primitive types (int, char, double) — not objects.
What is package? Group of related classes — gives namespace + access protection.
What is JDBC? Java DataBase Connectivity API — connects Java code to databases.
What is static keyword? Belongs to the class, not any instance — shared across all objects.
Is Java pass-by-value or pass-by-reference? Always pass-by-value (but for objects, the value is a reference copy).

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