OOP is all about designing software using objects, where data and behaviour come together. Unlike traditional procedural programming that focuses on sequences of tasks, OOP revolves around the interaction of objects to solve problems.<\/p>\n\n\n\n
Its four key principles, Encapsulation<\/strong>, Inheritance<\/strong>, Polymorphism<\/strong>, and Abstraction, <\/strong>form the building blocks of this approach.<\/p>\n\n\n\n Learning OOP in Java will help you write efficient, easily maintainable code that can be reused in other projects when developing large applications.<\/p>\n\n\n\n \n Java Programming Course<\/a>\n <\/p>\n Learn Java the right way! Our course teaches you essential programming skills, from coding basics to complex projects, setting you up for success in the tech industry.<\/p>\n Let's dive deep into these concepts and see how they work in Java!<\/p>\n\n\n\n A class is a blueprint for creating objects. It defines the attributes (data) and methods (behaviours) of the objects. Think of a class as a template.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n An object is an instance of a class. It represents a specific entity with its own values for the defined attributes.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Attributes (fields) store data, while methods define actions an object can perform. The brand and speed in the example are attributes, and drive() is a method.<\/p>\n\n\n\n Encapsulation bundles all the data (attributes) and method (behaviours) which are related in their functionality and role are grouped together or packaged in a single entity or class. It also limits the direct input and output of object data and allows input and output only by means of getters <\/strong>and setters<\/strong>.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Usage:<\/strong><\/p>\n\n\n Inheritance allows a class (child) to obtain its attributes and actions from another class (parent). It supports the reuse of code<\/strong> and also gives classes a well-defined relationship where one class is a specialized version of the other.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Usage:<\/strong><\/p>\n\n\n To learn more about this concept, check the free inheritance in the Java course<\/a>.<\/p>\n\n\n\n Polymorphism allows objects to take multiple forms, enabling the same method to behave differently based on context. It is achieved through:<\/p>\n\n\n\n Allows methods with the same name but different parameter lists.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Usage:<\/strong><\/p>\n\n\n Allows a subclass to provide a specific implementation of a method already defined in its superclass.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Usage:<\/strong><\/p>\n\n\n Explore more about Polymorphism in Java<\/strong><\/a> with this in-depth guide.<\/p>\n\n\n\n Abstraction focuses on what an object does<\/strong> rather than how it does it.<\/strong> It hides implementation details and exposes only essential features.<\/p>\n\n\n\n Usage:<\/strong><\/p>\n\n\n Java's OOP features rely on several key building blocks that enhance the creation, organization, and visibility of objects and their behaviour.<\/p>\n\n\n\n A constructor<\/a> is a special method invoked when an object is created. It initializes the object and sets its state.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Here, the constructor initializes brand and speed when the object myCar is created.<\/p>\n\n\n\n The this keyword is a reference to the current object and is used for:<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Access modifiers define the scope of variables, methods, and classes. Java provides four types of access modifiers:<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Learn more about Access Modifiers in Java<\/strong><\/a> with this insightful guide.<\/p>\n\n\n\n The static keyword denotes that a field or method belongs to the class, not any specific instance. These are shared across all instances of the class.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n An interface<\/a> is a blueprint of a class that contains abstract methods. Classes that implement an interface must provide implementations for its methods.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Inner classes are defined within another class and can access its private members. They are used for better encapsulation and logical grouping.<\/p>\n\n\n\n Types of Inner Classes:<\/strong><\/p>\n\n\n\n Example of Member Inner Class:<\/strong><\/p>\n\n\n Anonymous classes are unnamed inner classes, often used to override methods or provide specific functionality.<\/p>\n\n\n\n Example:<\/strong><\/p>\n\n\n Java supports Object Oriented Programming (OOP) which gives developers the chance to build scalable, reusable and maintainable software. Being able to use classes, objects, inheritance, and polymorphism are basic, and advanced features such as interfaces and design patterns will enable you to make robust, efficient applications.<\/p>\n\n\n\n OOP principles like encapsulation, abstraction and modularity help us handle complex problems by simplifying code and allowing us to manage and extend code. Experience of SOLID principles and design patterns will help you gain more experience.<\/p>\n\n\n\n Mastering OOP is the key to building scalable and robust software. However, if you try to learn these advanced features without a plan, it can feel like trying to build a house without a blueprint. The best way to turn these abstract concepts into real-world skill is with a hands-on start. To build your expertise the right way, join our free Java Course<\/a><\/strong>. It is the easiest way for absolute beginners to go from zero experience to a pro with a clear, step-by-step path.<\/p>\n\n\n\nUnderstanding the Basics of OOP<\/strong><\/h2>\n\n\n\n
What is a Class?<\/strong><\/h3>\n\n\n\n
\nclass Car {\n\n\u00a0\u00a0\u00a0\u00a0String brand;\n\n\u00a0\u00a0\u00a0\u00a0int speed;\n\n\u00a0\u00a0\u00a0\u00a0void drive() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println(brand + " is driving at " + speed + " km\/h.");\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n\nHere, Car is a class with attributes brand and speed, and a behavior drive().\n<\/pre><\/div>\n\n\nWhat is an Object?<\/strong><\/h3>\n\n\n\n
\nCar myCar = new Car();\n\nmyCar.brand = "Toyota";\n\nmyCar.speed = 120;\n\nmyCar.drive();\u00a0 \/\/ Output: Toyota is driving at 120 km\/h.\n<\/pre><\/div>\n\n\n
Attributes and Methods<\/strong><\/h3>\n\n\n\n
First Pillar of OOP: Encapsulation<\/strong><\/h2>\n\n\n\n
Definition:<\/strong><\/h3>\n\n\n\n
Benefits of Encapsulation:<\/strong><\/h3>\n\n\n\n
\n
\nclass Person {\n\n\u00a0\u00a0\u00a0\u00a0private String name;\u00a0 \/\/ Private attribute\n\n\u00a0\u00a0\u00a0\u00a0private int age;\n\n\u00a0\u00a0\u00a0\u00a0\/\/ Getter for name\n\n\u00a0\u00a0\u00a0\u00a0public String getName() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0return name;\n\n\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0\/\/ Setter for name\n\n\u00a0\u00a0\u00a0\u00a0public void setName(String name) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0this.name = name;\n\n\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0\/\/ Getter for age\n\n\u00a0\u00a0\u00a0\u00a0public int getAge() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0return age;\n\n\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0\/\/ Setter for age\n\n\u00a0\u00a0\u00a0\u00a0public void setAge(int age) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0if (age > 0) {\u00a0 \/\/ Validation\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0this.age = age;\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0} else {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Age must be positive.");\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n\nPerson p = new Person();\n\np.setName("John");\n\np.setAge(25);\n\nSystem.out.println(p.getName() + " is " + p.getAge() + " years old.");\n<\/pre><\/div>\n\n\nSecond Pillar of OOP: Inheritance<\/strong><\/h2>\n\n\n\n
Definition:<\/strong><\/h3>\n\n\n\n
Benefits of Inheritance:<\/strong><\/h3>\n\n\n\n
\n
\n\/\/ Parent class\n\nclass Animal {\n\n\u00a0\u00a0\u00a0\u00a0void eat() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("This animal eats food.");\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n\n\/\/ Child class\n\nclass Dog extends Animal {\n\n\u00a0\u00a0\u00a0\u00a0void bark() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("The dog barks.");\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n\nDog myDog = new Dog();\n\nmyDog.eat();\u00a0 \/\/ Output: This animal eats food.\n\nmyDog.bark(); \/\/ Output: The dog barks.\n<\/pre><\/div>\n\n\n
Third Pillar of OOP: Polymorphism<\/strong><\/h2>\n\n\n\n
Definition:<\/strong><\/h3>\n\n\n\n
\n
1. Method Overloading<\/a> (Compile-time Polymorphism):<\/strong><\/h3>\n\n\n\n
\nclass Calculator {\n\n\u00a0\u00a0\u00a0\u00a0int add(int a, int b) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0return a + b;\n\n\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0double add(double a, double b) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0return a + b;\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n\nCalculator calc = new Calculator();\n\nSystem.out.println(calc.add(2, 3)); \u00a0 \u00a0 \u00a0 \/\/ Output: 5\n\nSystem.out.println(calc.add(2.5, 3.5)); \u00a0 \/\/ Output: 6.0\n<\/pre><\/div>\n\n\n
2. Method Overriding (Runtime Polymorphism):<\/strong><\/h3>\n\n\n\n
\nclass Animal {\n\n\u00a0\u00a0\u00a0\u00a0void sound() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Some generic animal sound");\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n\nclass Cat extends Animal {\n\n\u00a0\u00a0\u00a0\u00a0@Override\n\n\u00a0\u00a0\u00a0\u00a0void sound() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Meow");\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n\nAnimal myAnimal = new Cat();\n\nmyAnimal.sound();\u00a0 \/\/ Output: Meow\n<\/pre><\/div>\n\n\n
Fourth Pillar of OOP: Abstraction<\/strong><\/h2>\n\n\n\n
Definition:<\/strong><\/h3>\n\n\n\n
Benefits of Abstraction:<\/strong><\/h3>\n\n\n\n
\n
Example Using Abstract Classes:<\/strong><\/h3>\n\n\n
\nabstract class Shape {\n\n\u00a0\u00a0\u00a0\u00a0abstract void draw();\u00a0 \/\/ Abstract method\n\n}\n\nclass Circle extends Shape {\n\n\u00a0\u00a0\u00a0\u00a0void draw() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Drawing a Circle");\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n\nclass Rectangle extends Shape {\n\n\u00a0\u00a0\u00a0\u00a0void draw() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Drawing a Rectangle");\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n\nShape s1 = new Circle();\n\nShape s2 = new Rectangle();\n\ns1.draw();\u00a0 \/\/ Output: Drawing a Circle\n\ns2.draw();\u00a0 \/\/ Output: Drawing a Rectangle\n<\/pre><\/div>\n\n\n
Building Blocks in Java OOP<\/strong><\/h2>\n\n\n\n
1. Constructors: Special Methods to Initialize Objects<\/strong><\/h3>\n\n\n\n
\n
\n
\nclass Car {\n\n\u00a0\u00a0\u00a0\u00a0String brand;\n\n\u00a0\u00a0\u00a0\u00a0int speed;\n\n\u00a0\u00a0\u00a0\u00a0\/\/ Constructor\n\n\u00a0\u00a0\u00a0\u00a0Car(String brand, int speed) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0this.brand = brand;\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0this.speed = speed;\n\n\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0void display() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Brand: " + brand + ", Speed: " + speed);\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n\npublic class Main {\n\n\u00a0\u00a0\u00a0\u00a0public static void main(String[] args) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0Car myCar = new Car("Toyota", 120); \/\/ Constructor called\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0myCar.display(); \/\/ Output: Brand: Toyota, Speed: 120\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n2. this Keyword: Refers to the Current Object<\/strong><\/h3>\n\n\n\n
\n
\nclass Employee {\n\n\u00a0\u00a0\u00a0\u00a0String name;\n\n\u00a0\u00a0\u00a0\u00a0Employee(String name) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0this.name = name; \/\/ Refers to the class attribute\n\n\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0void display() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Employee Name: " + this.name);\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n3. Access Modifiers: Control the Visibility of Attributes and Methods<\/strong><\/h3>\n\n\n\n
\n
\nclass Example {\n\n\u00a0\u00a0\u00a0\u00a0private int privateValue = 10; \/\/ Private: restricted to this class\n\n\u00a0\u00a0\u00a0\u00a0public int publicValue = 20;\u00a0 \/\/ Public: accessible anywhere\n\n\u00a0\u00a0\u00a0\u00a0public int getPrivateValue() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0return privateValue; \/\/ Controlled access via a public method\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n4. Static Members: Define Shared Variables and Methods<\/strong><\/h3>\n\n\n\n
\nclass Counter {\n\n\u00a0\u00a0\u00a0\u00a0static int count = 0; \/\/ Static variable shared by all objects\n\n\u00a0\u00a0\u00a0\u00a0Counter() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0count++;\n\n\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0static void displayCount() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Count: " + count); \/\/ Static method accessing static field\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n\npublic class Main {\n\n\u00a0\u00a0\u00a0\u00a0public static void main(String[] args) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0new Counter();\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0new Counter();\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0Counter.displayCount(); \/\/ Output: Count: 2\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\nAdvanced OOP Features<\/strong><\/h2>\n\n\n\n
1. Interfaces: Define Contracts for Implementing Classes<\/strong><\/h3>\n\n\n\n
\ninterface Printable {\n\n\u00a0\u00a0\u00a0\u00a0void print(); \/\/ Abstract method\n\n}\n\nclass Document implements Printable {\n\n\u00a0\u00a0\u00a0\u00a0public void print() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Printing Document...");\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n\npublic class Main {\n\n\u00a0\u00a0\u00a0\u00a0public static void main(String[] args) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0Printable doc = new Document();\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0doc.print(); \/\/ Output: Printing Document...\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n2. Inner Classes: Enable Modular Organization<\/strong><\/h3>\n\n\n\n
\n
\nclass Outer {\n\n\u00a0\u00a0\u00a0\u00a0private String message = "Hello from Outer class";\n\n\u00a0\u00a0\u00a0\u00a0class Inner {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0void display() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println(message); \/\/ Accessing private member of Outer class\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n\npublic class Main {\n\n\u00a0\u00a0\u00a0\u00a0public static void main(String[] args) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0Outer outer = new Outer();\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0Outer.Inner inner = outer.new Inner();\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0inner.display(); \/\/ Output: Hello from Outer class\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\n3. Anonymous Classes: Simplify Short-lived Object Creation<\/strong><\/h3>\n\n\n\n
\ninterface Greeting {\n\n\u00a0\u00a0\u00a0\u00a0void sayHello();\n\n}\n\npublic class Main {\n\n\u00a0\u00a0\u00a0\u00a0public static void main(String[] args) {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0Greeting greet = new Greeting() { \/\/ Anonymous class\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0public void sayHello() {\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0System.out.println("Hello, Anonymous World!");\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0}\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0};\n\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0greet.sayHello(); \/\/ Output: Hello, Anonymous World!\n\n\u00a0\u00a0\u00a0\u00a0}\n\n}\n<\/pre><\/div>\n\n\nConclusion<\/strong><\/h2>\n\n\n\n