Week 2 - Core Java, OOP, Exceptions, Testing & Agile

[Java Overview] What is Java?

A statically typed, object-oriented programming language compiled to bytecode that runs on a JVM.

[Java Overview] What are JDK, JRE and JVM?

JVM executes bytecode; JRE supplies the runtime; JDK includes tools such as javac for developing Java programs.

[Java Overview] What is the JIT compiler?

The Just-In-Time compiler translates frequently executed bytecode into native machine code at runtime.

[Java Overview] What is Java bytecode?

Platform-independent intermediate instructions produced by Java compilation and executed by a JVM.

[Java Overview] Why is Java called platform independent?

Compiled bytecode can run on compatible JVMs across operating systems.

[Java Overview] What does public static void main(String[] args) mean?

public makes it accessible, static allows class-level invocation, void returns nothing, main is the entry method, and args receives command-line arguments.

[Java Basics] What are primitive Java types?

byte, short, int, long, float, double, char and boolean.

[Java Basics] What is a wrapper class?

An object type representing a primitive, such as Integer for int and Boolean for boolean.

[Java Basics] What is autoboxing and unboxing?

Automatic conversion between primitives and corresponding wrapper objects.

[Java Basics] What is the difference between == and equals()?

== compares primitive values or object references; equals() can compare logical object content.

[Java Basics] Why is String immutable?

A String object's contents cannot change after creation; operations create new String values.

[Java Basics] What does static mean in Java?

A member belongs to the class rather than an individual instance.

[Java Basics] What does final mean?

A final variable cannot be reassigned, a final method cannot be overridden, and a final class cannot be extended.

[Java Basics] What is the difference between final, finally and finalize?

final is a modifier; finally is a cleanup block; finalize was an unreliable deprecated finalization mechanism.

[Java Basics] What is the difference between stack and heap?

Stack frames hold method-local execution state; the heap holds objects managed by garbage collection.

[Classes and Objects] What is a class?

A blueprint defining an object's fields, constructors and methods.

[Classes and Objects] What is an object?

A runtime instance of a class with identity, state and behavior.

[Classes and Objects] What is a constructor?

A special initializer invoked when creating an object, usually to establish valid initial state.

[Classes and Objects] What is constructor overloading?

Defining multiple constructors with different parameter lists.

[Classes and Objects] What is this in Java?

A reference to the current object, often used to distinguish instance fields from parameters.

[Classes and Objects] What is a singleton?

A design pattern ensuring only one instance of a class is available through controlled access.

[Classes and Objects] What are Java access modifiers?

public, protected, package-private and private control member visibility.

[Methods] What is method overloading?

Multiple methods share a name but differ in parameter types or counts; resolved at compile time.

[Methods] What is method overriding?

A subclass supplies a compatible implementation of an inherited instance method; selected dynamically.

[Methods] What is the difference between parameters and arguments?

Parameters are method declaration variables; arguments are values supplied at a call site.

[Methods] What is pass-by-value in Java?

Java passes copies of values, including copies of object references, to methods.

[Methods] What is a static method?

A method associated with the class and callable without an instance.

[Methods] Can static methods be overridden?

No; subclass static methods hide superclass methods rather than overriding them.

[Object-Oriented Programming] What are the four pillars of OOP?

Encapsulation, inheritance, polymorphism and abstraction.

[Object-Oriented Programming] What is encapsulation?

Bundling state and behavior and controlling access to internal implementation details.

[Object-Oriented Programming] What is inheritance?

A subclass extends a superclass to reuse and specialize accessible behavior.

[Object-Oriented Programming] What is polymorphism?

The same interface or method call can behave differently depending on the actual object type.

[Object-Oriented Programming] What is abstraction?

Exposing essential behavior while hiding implementation details.

[Object-Oriented Programming] What is an abstract class versus an interface?

An abstract class can hold shared state and implementation; an interface defines a contract and may include default or static methods.

[Object-Oriented Programming] Can Java classes extend multiple classes?

No; Java has single class inheritance, but a class can implement multiple interfaces.

[Object-Oriented Programming] What is the difference between composition and inheritance?

Composition builds behavior using contained objects; inheritance establishes an is-a relationship.

[Exceptions] What is an exception?

An object representing an abnormal condition that interrupts normal program execution.

[Exceptions] What are checked and unchecked exceptions?

Checked exceptions must be caught or declared; unchecked exceptions extend RuntimeException and need not be declared.

[Exceptions] How do try, catch and finally work?

try encloses risky code, catch handles matching exceptions, and finally generally runs cleanup logic.

[Exceptions] What is the difference between throw and throws?

throw raises an exception; throws declares possible exceptions in a method signature.

[Exceptions] How do you create a custom exception?

Define a class extending Exception for checked or RuntimeException for unchecked behavior.

[Exceptions] What is try-with-resources?

A try statement that automatically closes AutoCloseable resources when the block finishes.

[Exceptions] How do Error and Exception differ?

Errors generally signal serious JVM or system problems; exceptions represent conditions applications may handle.

[Java IO] What is Java I/O?

APIs for reading and writing bytes, characters and files.

[Java IO] What is an InputStream versus Reader?

InputStream handles bytes; Reader handles decoded characters.

[Java IO] What is an OutputStream versus Writer?

OutputStream writes bytes; Writer writes characters.

[Java IO] What is buffering?

Temporarily storing data to reduce costly underlying read and write operations.

[Java IO] What is the NIO Files API?

java.nio.file provides Path and Files utilities for file and directory operations.

[Java IO] How should files be closed safely?

Use try-with-resources so streams and readers close even if an exception occurs.

[Logging] What is logging?

Recording diagnostic and operational events to understand application behavior and failures.

[Logging] What are common logging levels?

TRACE, DEBUG, INFO, WARN and ERROR, from fine detail to serious failures.

[Logging] Why use SLF4J instead of System.out.println?

SLF4J offers consistent logging APIs, configurable levels and integration with logging backends.

[Logging] What should not be logged?

Passwords, access tokens, private keys and unnecessary sensitive user information.

[Logging] What is structured logging?

Emitting consistent key-value fields to make logs searchable and analyzable.

[Testing] What is unit testing?

Testing a small unit of behavior in isolation, typically using JUnit in Java.

[Testing] What is an integration test?

Testing interactions between components or with infrastructure such as a database.

[Testing] What is JUnit?

A Java testing framework with annotations and assertions for automated tests.

[Testing] What is Mockito?

A mocking framework used to replace collaborators and verify interactions in tests.

[Testing] What is the testing pyramid?

Many fast unit tests, fewer integration tests and a smaller number of end-to-end tests.

[Testing] What is the difference between assertion and mock?

An assertion checks an observed result; a mock simulates or verifies a dependency's behavior.

[Agile and Scrum] What is Agile software development?

An iterative approach emphasizing collaboration, feedback, adaptation and frequent delivery of value.

[Agile and Scrum] What is Scrum?

An Agile framework using time-boxed Sprints and defined accountabilities, events and artifacts.

[Agile and Scrum] What is a Sprint?

A fixed-length iteration of one month or less producing a usable Increment.

[Agile and Scrum] What are Scrum roles?

Product Owner, Scrum Master and Developers form the Scrum Team.

[Agile and Scrum] What is a daily Scrum?

A short daily event where Developers inspect progress toward the Sprint Goal and adapt their plan.

[Agile and Scrum] What is the difference between Agile and Waterfall?

Agile delivers iteratively and adapts to change; Waterfall generally follows sequential phases with more upfront planning.

[Agile and Scrum] What is a retrospective?

A Scrum event to inspect how the Sprint went and identify improvements.