Inheritance - Single Inheritance

Inheritance is a fundamental concept in object-oriented programming that allows one class to inherit properties and methods from another class. This mechanism promotes code reusability and establishes a relationship between the parent class (superclass) and the child class (subclass).

In single inheritance, a class inherits from only one superclass. This is the most basic form of inheritance.

Here's a simple Java program to demonstrate single inheritance:

// Superclass
class Animal {
    // Method in the superclass
    void eat() {
        System.out.println("This animal eats food.");
    }
}

// Subclass
class Dog extends Animal {
    // Method in the subclass
    void bark() {
        System.out.println("The dog barks.");
    }
}

// Main class to test the inheritance
public class Main {
    public static void main(String[] args) {
        // Create an object of the subclass
        Dog myDog = new Dog();
        
        // Call the methods of the superclass and the subclass
        myDog.eat();  // Inherited method from Animal class
        myDog.bark(); // Method of the Dog class
    }
}

Explanation:

  1. Superclass (Animal):

    • It has a method eat() which prints a message.

  2. Subclass (Dog):

    • It extends the Animal class, inheriting its properties and methods.
    • It has an additional method bark() which prints a message.

  3. Main class:

    • It creates an object of the Dog class.
    • It calls the eat() method from the Animal class and the bark() method from the Dog class.

This example demonstrates how the Dog class inherits the eat() method from the Animal class, allowing the Dog object to use both the inherited method and its own method.

In Java, the super keyword is used in two main contexts:

  1. To call the superclass's constructor:

    • It is used to invoke the constructor of the superclass from the subclass.
    • This is particularly useful for reusing the initialization code of the superclass and ensuring that the superclass is properly initialized before the subclass's constructor code is executed.

  2. To call the superclass's methods and access the superclass's fields:

    • It can be used to call a method or access a field that is defined in the superclass from within a subclass.
    • This is useful when the subclass overrides a method of the superclass and still needs to call the overridden method.
Example:
// Superclass
class Animal {
    String name;

    // Superclass constructor
    Animal(String name) {
        this.name = name;
    }

    void displayInfo() {
        System.out.println("Animal Name: " + name);
    }
}

// Subclass
class Dog extends Animal {
    String breed;

    // Subclass constructor
    Dog(String name, String breed) {
        super(name); // Call to the superclass constructor
        this.breed = breed;
    }

    // Overriding the displayInfo method
    @Override
    void displayInfo() {
        super.displayInfo(); // Call to the superclass method
        System.out.println("Breed: " + breed);
    }
}

// Main class to test the inheritance
public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog("Buddy", "Golden Retriever");
        dog.displayInfo();
    }
}

Program to try

// Superclass
class Shape {
    // Properties of the shape
    double length;
    double width;

    // Default constructor
    Shape() {
        length = 0;
        width = 0;
    }

    // Parameterized constructor
    Shape(double l, double w) {
        length = l;
        width = w;
    }

    // Method to display the dimensions
    void displayDimensions() {
        System.out.println("Length: " + length + ", Width: " + width);
    }
}

// Subclass
class Rectangle extends Shape {
    // Default constructor
    Rectangle() {
        super();
    }

    // Parameterized constructor
    Rectangle(double l, double w) {
        super(l, w);
    }

    // Method to calculate the area of the rectangle
    double calculateArea() {
        return length * width;
    }

    // Method to calculate the perimeter of the rectangle
    double calculatePerimeter() {
        return 2 * (length + width);
    }
}

// Main class to test the Rectangle class
public class Main {
    public static void main(String[] args) {
        // Create a Rectangle object using the default constructor
        Rectangle rect1 = new Rectangle();
        System.out.println("Default Rectangle:");
        rect1.displayDimensions();
        System.out.println("Area: " + rect1.calculateArea());
        System.out.println("Perimeter: " + rect1.calculatePerimeter());

        // Create a Rectangle object using the parameterized constructor
        Rectangle rect2 = new Rectangle(5, 3);
        System.out.println("\nParameterized Rectangle:");
        rect2.displayDimensions();
        System.out.println("Area: " + rect2.calculateArea());
        System.out.println("Perimeter: " + rect2.calculatePerimeter());
    }
}

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