Object-Oriented Programming

Object-Oriented Programming (OOP) is a programming paradigm that organises software around objects — self-contained units that combine data (fields) and behaviour…

A class is the fundamental building block of Java.

An object is a runtime instance of a class.

Object creation in Java is the process of allocating memory, initialising fields, and running constructor logic to bring an object into existence.

A default constructor is a constructor that takes no parameters.

A parameterized constructor is a constructor that accepts one or more arguments, allowing the caller to supply initial values for the object's fields at the moment of…

Constructor overloading means defining multiple constructors in the same class, each with a different parameter list.

A copy constructor creates a new object as a duplicate of an existing object of the same type.

The this keyword in Java is a reference to the current object — the instance on which a method or constructor is currently executing.

The static keyword declares that a member belongs to the class itself rather than to any specific instance of the class.

The final keyword in Java means that something cannot be changed after it is first set.

Access modifiers control the visibility of classes, fields, constructors, and methods.

Encapsulation is the principle of bundling data and the methods that operate on that data into a single unit — the class — and controlling access to the internals…

Single inheritance is the most fundamental relationship between classes in Java — one class extends exactly one other class, inheriting its fields and methods.

Multilevel inheritance forms a chain of classes where each class extends the one above it — A extends B, B extends C, creating a three-level hierarchy.

Hierarchical inheritance occurs when multiple classes extend a single superclass — one parent, many children.

Method overriding occurs when a subclass provides its own implementation of a method that is already defined in its superclass.

The super keyword in Java is a reference to the superclass portion of the current object.

Compile-time polymorphism, also called static polymorphism or early binding, is the form of polymorphism where the method to be called is determined by the compiler at…

Runtime polymorphism, also called dynamic polymorphism or late binding, is the form of polymorphism where the method implementation to execute is determined at runtime…

Abstraction is the process of exposing only the essential features of a concept while hiding the irrelevant implementation details.

An abstract class is a class that cannot be instantiated — it exists only to be extended.

An interface is a pure contract — a named set of method signatures that any implementing class promises to support.

Java does not allow a class to extend more than one class, but a class can implement any number of interfaces.

Association is a relationship between two classes where one class uses or interacts with another, but neither class owns the other and both can exist independently.

Aggregation is a specialised form of association that represents a whole-part relationship where the part can exist independently of the whole.

Composition is the strongest form of the has-a relationship, where the whole object creates, owns, and is entirely responsible for the lifecycle of its parts.

java.lang.Object is the root of the entire Java class hierarchy.

An anonymous object is an object that is created and used without being assigned to a named reference variable.

Initialization blocks are code blocks that run during object or class creation, supplementing constructors and field initializers.

A nested class is a class declared inside another class.

An inner class is a non-static class declared inside another class.

An anonymous inner class is a class that is declared and instantiated in a single expression.

A static nested class is a class declared with the static modifier inside another class.