Inheritance Principle

Inheritance is a way of extending class. The new class (child class) builds logic on the top of the parent class (super class).

Here's an example:

// Base class:
class Vehicle
{
    protected String make;
    protected String model;

public void representation()
    {
        System.out.println(this.make + " " + this.model);
    }

public double getInsuranceCost(int months)
    {
        return 0;
    }

public String getMake()
    {
        return this.make;
    }

public String getModel()
    {
        return this.model;
    }
}

// Car inherits Vehicle:
class Car extends Vehicle
{
    // Adds class member:
    private int doorsCount;

// Overwriting method:
    public double getInsuranceCost(int months)
    {
        return months * 25;
    }

// Adding constructor:
    Car(String makeValue, String modelValue,
    int doorsCountValue)
    {
        this.make = makeValue;
        this.model = modelValue;
        this.doorsCount = doorsCountValue;
    }

// Method extension:
    public void representation()
    {
        super.representation();
        System.out.println( "Doors count: " + doorsCount);
    }

// Adding new method:
    public int getDoorsCount()
    {
        return this.doorsCount;
    }
}

public class Main {

public static void main(String[] args) {
        Car car = new Car("Toyota", "Corolla", 4);

// Method that was extended:
        car.representation();

System.out.println("------");
        // Method that was added:
        System.out.println(car.getDoorsCount());
        System.out.println("------");

// Methods that weren't changed:
        System.out.println("Make: " + car.getMake());
        System.out.println("Model: " + car.getModel());
        System.out.println("------");

// Method that was overwritten:
        System.out.println("Insurance cost: " +
                car.getInsuranceCost(12) + " $");
    }
}

Output:

Basically the child class can add, extend, rewrite or use the inherited logic from the parent.

For this example:

Added logic:
- member - doorsCount (line 32)
- parametrized constructor (line 41)
- getter for doorsCount (line 57)
Extended logic:
- representation method (line 50)
Entirely rewritten logic:
- getInsuranceCost (line 35)
Not modified logic:
- getMake (line 17)
- getModel (line 22)