Why object-oriented programming is used in Java - See It in Action

Why Object-Oriented Programming is Used

📖 Scenario: Imagine you are building a simple program to manage a library. You want to keep track of books and their details like title, author, and number of pages.

🎯 Goal: You will create a basic Java program that shows why object-oriented programming (OOP) is useful by organizing book information into objects.

📋 What You'll Learn

Create a class called Book with three fields: title, author, and pages

Create an object of Book with specific values

Add a method to display the book details

Print the book details using the method

💡 Why This Matters

🌍 Real World

OOP is used to model real-world things like books, cars, or people as objects in programs. This makes programs easier to understand and maintain.

💼 Career

Many software jobs require understanding OOP concepts to build scalable and organized applications.

1

Create the Book class with fields

Create a class called Book with three fields: String title, String author, and int pages.

Java

// Your code here

Hint

Think of a class as a blueprint for creating book objects.

2

Create a Book object with values

Inside a Main class with a main method, create a Book object called myBook. Set title to "Java Basics", author to "Alice", and pages to 250.

Java

class Book {
    String title;
    String author;
    int pages;
}

public class Main {
    public static void main(String[] args) {
        // Create a Book object called myBook and set its fields
        
    }
}

Hint

Use new Book() to create the object and dot notation to set fields.

3

Add a method to display book details

Add a method called displayInfo inside the Book class that prints the book's title, author, and pages in one line.

Java

class Book {
    String title;
    String author;
    int pages;
    // Add displayInfo method here
}

public class Main {
    public static void main(String[] args) {
        Book myBook = new Book();
        myBook.title = "Java Basics";
        myBook.author = "Alice";
        myBook.pages = 250;
    }
}

Hint

The method should print all fields in one line separated by commas.

4

Print the book details using the method

In the main method, call myBook.displayInfo() to print the book details.

Java

class Book {
    String title;
    String author;
    int pages;
    
    void displayInfo() {
        System.out.println(title + ", " + author + ", " + pages + " pages");
    }
}

public class Main {
    public static void main(String[] args) {
        Book myBook = new Book();
        myBook.title = "Java Basics";
        myBook.author = "Alice";
        myBook.pages = 250;
        // Call displayInfo method here
    }
}

Hint

Use the object name myBook followed by dot and method name displayInfo().

Practice

1. Why do programmers use object-oriented programming (OOP) in Java?

easy

A. To organize code by grouping data and actions into objects

B. To write code only once without any changes

C. To make programs run faster by skipping data

D. To avoid using any variables in the program

Solution

1. Step 1: Understand what OOP does

   OOP groups related data and actions into objects, making code organized.

2. Step 2: Compare options with OOP purpose

   Only To organize code by grouping data and actions into objects correctly describes grouping data and actions into objects.

3. Final Answer:

   To organize code by grouping data and actions into objects -> Option A

4. Quick Check:

   OOP groups data and actions = D [OK]

Hint: OOP groups data and actions into objects [OK]

Common Mistakes:

• Thinking OOP only makes code faster
• Believing OOP avoids variables
• Confusing code reuse with skipping changes

2. Which of the following is the correct way to define a simple class in Java?

easy

A. class Car int speed; void drive() {}

B. Car class { int speed; void drive() {} }

C. class Car { int speed; drive() void {} }

D. class Car { int speed; void drive() {} }

Solution

1. Step 1: Check Java class syntax

   Java classes start with 'class ClassName { ... }' and methods have return type before name.

2. Step 2: Validate each option

   class Car { int speed; void drive() {} } uses correct syntax: class keyword, braces, field with type, method with return type and braces.

3. Final Answer:

   class Car { int speed; void drive() {} } -> Option D

4. Quick Check:

   Correct class syntax = C [OK]

Hint: Class syntax: class Name { fields and methods } [OK]

Common Mistakes:

• Omitting 'class' keyword
• Missing braces {} around class body
• Incorrect method declaration order

3. What will be the output of this Java code?

class Dog {
  String name;
  void bark() {
    System.out.println(name + " barks");
  }
}
public class Test {
  public static void main(String[] args) {
    Dog d = new Dog();
    d.name = "Max";
    d.bark();
  }
}

medium

A. Maxbarks

B. barks Max

C. Max barks

D. Compilation error

Solution

1. Step 1: Understand the Dog class and method

   Dog has a name field and bark() prints name + " barks" with a space.

2. Step 2: Trace main method execution

   Creates Dog object d, sets d.name = "Max", calls d.bark() which prints "Max barks".

3. Final Answer:

   Max barks -> Option C

4. Quick Check:

   Prints name + " barks" = A [OK]

Hint: Prints field + string exactly as coded [OK]

Common Mistakes:

• Forgetting space between name and 'barks'
• Mixing order of printed words
• Assuming compilation error without reason

4. Find the error in this Java code that uses OOP:

class Cat {
  String name;
  void meow() {
    System.out.println(name + " meows");
  }
}
public class Test {
  public static void main(String[] args) {
    Cat c;
    c.name = "Luna";
    c.meow();
  }
}

medium

A. Method meow() is missing return type

B. Variable 'c' is not initialized before use

C. Class Cat is missing a constructor

D. String name should be static

Solution

1. Step 1: Check object creation

   Variable 'c' is declared but not assigned a new Cat object before use.

2. Step 2: Understand consequences

   Using c.name or c.meow() without initializing 'c' causes a runtime error (NullPointerException).

3. Final Answer:

   Variable 'c' is not initialized before use -> Option B

4. Quick Check:

   Uninitialized object causes error = A [OK]

Hint: Always create objects with 'new' before use [OK]

Common Mistakes:

• Thinking method lacks return type (void is valid)
• Assuming constructor is mandatory
• Believing fields must be static

5. You want to reuse code for different types of vehicles in Java. Which OOP feature helps you write a base class Vehicle and create specific classes like Car and Bike that share common code but also have their own details?

hard

A. Inheritance to extend Vehicle class for Car and Bike

B. Encapsulation to hide Vehicle data from Car and Bike

C. Polymorphism to create unrelated classes Car and Bike

D. Abstraction to write all code only in Vehicle class

Solution

1. Step 1: Understand code reuse in OOP

   Inheritance allows new classes to reuse code from a base class and add their own features.

2. Step 2: Match feature to scenario

   Vehicle is base class; Car and Bike extend it to share common code and add details.

3. Final Answer:

   Inheritance to extend Vehicle class for Car and Bike -> Option A

4. Quick Check:

   Code reuse via inheritance = B [OK]

Hint: Use inheritance to share and extend code [OK]

Common Mistakes:

• Confusing encapsulation with code reuse
• Thinking polymorphism creates unrelated classes
• Believing abstraction means no subclass code