OOP principles overview in Java - Mini Project: Build & Apply

OOP Principles Overview in Java

📖 Scenario: Imagine you are creating a simple program to manage information about cars in a car rental service. You want to organize the data and behavior of cars using basic Object-Oriented Programming (OOP) principles.

🎯 Goal: Build a Java program that demonstrates the four main OOP principles: encapsulation, inheritance, polymorphism, and abstraction by creating classes and using them step-by-step.

📋 What You'll Learn

Create a class with private variables and public methods (encapsulation)

Create a subclass that inherits from a superclass (inheritance)

Use method overriding to show polymorphism

Use an abstract class with an abstract method (abstraction)

Print outputs to show how each principle works

💡 Why This Matters

🌍 Real World

Organizing code for vehicle management systems in rental or sales companies using OOP makes the code easier to maintain and extend.

💼 Career

Understanding OOP principles is essential for Java developers working on real-world applications, as it helps create clean, reusable, and scalable code.

1

Create the Car class with encapsulation

Create a class called Car with a private String variable model and a private int variable year. Add a public constructor that takes model and year as parameters and sets the variables. Add public getter methods getModel() and getYear() to access these variables.

Java

public class Car {
    // Your code here
}

Need a hint?

Use private for variables and public for methods to follow encapsulation.

2

Create a subclass ElectricCar that inherits from Car

Create a class called ElectricCar that extends Car. Add a private int variable batteryCapacity. Create a constructor that takes model, year, and batteryCapacity and calls the superclass constructor for model and year. Add a public method getBatteryCapacity() to return the battery capacity.

Java

public class Car {
    private String model;
    private int year;

    public Car(String model, int year) {
        this.model = model;
        this.year = year;
    }

    public String getModel() {
        return model;
    }

    public int getYear() {
        return year;
    }
}

// Your code here

Need a hint?

Use extends to inherit and super() to call the parent constructor.

3

Override a method to show polymorphism

In the Car class, add a public method public String start() that returns the String "Car is starting". In the ElectricCar class, override the start() method to return "Electric car is starting silently". This shows polymorphism by changing behavior in the subclass.

Java

public class Car {
    private String model;
    private int year;

    public Car(String model, int year) {
        this.model = model;
        this.year = year;
    }

    public String getModel() {
        return model;
    }

    public int getYear() {
        return year;
    }

    // Your code here
}

public class ElectricCar extends Car {
    private int batteryCapacity;

    public ElectricCar(String model, int year, int batteryCapacity) {
        super(model, year);
        this.batteryCapacity = batteryCapacity;
    }

    public int getBatteryCapacity() {
        return batteryCapacity;
    }

    // Your code here
}

Need a hint?

Use the same method name in the subclass and add @Override above it.

4

Create an abstract Vehicle class to show abstraction and print outputs

Create an abstract class called Vehicle with an abstract method public abstract String fuelType(). Make Car extend Vehicle and implement fuelType() to return "Gasoline". Make ElectricCar override fuelType() to return "Electricity". In a Main class with main method, create a Car object and an ElectricCar object. Print the results of calling start() and fuelType() on both objects.

Java

public class Car extends Vehicle {
    // Your code here
}

public class ElectricCar extends Car {
    // Your code here
}

public class Main {
    public static void main(String[] args) {
        // Your code here
    }
}

Need a hint?

Remember to declare Vehicle as abstract and implement fuelType() in subclasses. Use System.out.println() to print outputs.