Class definition in Java - Mini Project: Build & Apply

Class definition

📖 Scenario: You are creating a simple program to represent a book in a library system. Each book has a title and an author.

🎯 Goal: Build a Java class called Book with two fields: title and author. Then create an object of this class and print its details.

📋 What You'll Learn

Create a class named Book

Add two String fields: title and author

Create an object of Book with title "The Alchemist" and author "Paulo Coelho"

Print the book's title and author using System.out.println

💡 Why This Matters

🌍 Real World

Classes like Book help organize data about real things in programs, such as books in a library or products in a store.

💼 Career

Understanding how to define classes and create objects is a fundamental skill for any Java developer.

1

Create the Book class with fields

Write a Java class named Book with two String fields called title and author.

Java

// Your code here

Hint

Use public class Book {} and declare two variables inside.

2

Create a Book object with values

Create a Book object named myBook. Set its title to "The Alchemist" and author to "Paulo Coelho".

Java

public class Book {
    String title;
    String author;
}

// Your code here

Hint

Create the object with new Book() and assign values to fields.

3

Add code to print the book details

Inside the main method, add two System.out.println statements to print myBook.title and myBook.author.

Java

public class Book {
    String title;
    String author;
}

public class Main {
    public static void main(String[] args) {
        Book myBook = new Book();
        myBook.title = "The Alchemist";
        myBook.author = "Paulo Coelho";
        // Your code here
    }
}

Hint

Use System.out.println to show the values stored in the object.

4

Run the program to see the output

Run the program and observe the output. It should print the book's title and author on separate lines.

Java

public class Book {
    String title;
    String author;
}

public class Main {
    public static void main(String[] args) {
        Book myBook = new Book();
        myBook.title = "The Alchemist";
        myBook.author = "Paulo Coelho";
        System.out.println(myBook.title);
        System.out.println(myBook.author);
    }
}

// Run the program

Hint

The program should print exactly the title and author on two lines.

Practice

1. What is a class in Java?
class Car { }

easy

A. A blueprint to create objects with data and actions

B. A type of variable that stores numbers

C. A method that runs automatically

D. A special kind of loop

Solution

1. Step 1: Understand the role of a class

   A class defines a template or blueprint for creating objects that hold data and actions.

2. Step 2: Match the definition to options

   A blueprint to create objects with data and actions correctly describes a class as a blueprint for objects.

3. Final Answer:

   A blueprint to create objects with data and actions -> Option A

4. Quick Check:

   Class = blueprint for objects [OK]

Hint: Remember: class = blueprint for objects [OK]

Common Mistakes:

• Confusing class with variable
• Thinking class is a method
• Mixing class with loops

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

easy

A. Person class { }

B. class = Person { }

C. class Person { }

D. define class Person { }

Solution

1. Step 1: Recall Java class syntax

   In Java, a class is defined using the keyword class followed by the class name and braces.

2. Step 2: Check each option

   class Person { } matches the correct syntax: class Person { }. Others have incorrect order or keywords.

3. Final Answer:

   class Person { } -> Option C

4. Quick Check:

   Correct class syntax = class Name { } [OK]

Hint: Use 'class ClassName { }' to define a class [OK]

Common Mistakes:

• Swapping 'class' and class name
• Using '=' sign in class definition
• Using wrong keywords like 'define'

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

class Dog {
  String name = "Buddy";
}

public class Main {
  public static void main(String[] args) {
    Dog d = new Dog();
    System.out.println(d.name);
  }
}

medium

A. Buddy

B. null

C. Dog

D. Compilation error

Solution

1. Step 1: Understand object creation and field access

   The code creates a Dog object and accesses its field 'name' which is set to "Buddy".

2. Step 2: Predict the printed output

   Printing d.name outputs the string "Buddy" stored in the object.

3. Final Answer:

   Buddy -> Option A

4. Quick Check:

   Object field value = Buddy [OK]

Hint: Access object fields with dot notation: object.field [OK]

Common Mistakes:

• Expecting class name instead of field value
• Thinking uninitialized fields print 'null'
• Confusing syntax causing compile errors

4. Identify the error in this class definition:

class Animal {
  String type;
  void Animal() {
    type = "Mammal";
  }
}

medium

A. Class name should be lowercase

B. Constructor has void return type

C. Missing semicolon after type declaration

D. Field 'type' must be static

Solution

1. Step 1: Check constructor syntax

   Constructors in Java must not have a return type, not even void.

2. Step 2: Identify the error

   The method void Animal() is treated as a regular method, not a constructor, causing no constructor defined.

3. Final Answer:

   Constructor has void return type -> Option B

4. Quick Check:

   Constructor = no return type [OK]

Hint: Constructors never have a return type, not even void [OK]

Common Mistakes:

• Adding void to constructor
• Thinking semicolon needed after field
• Believing class names must be lowercase
• Assuming fields must be static

5. You want to create a class Book with a field title and a method printTitle() that prints the title. Which code correctly implements this?

hard

A. class Book {
String title;
void printTitle() {
System.out.println("title");
}
}

B. class Book {
String title;
void printTitle() {
print(title);
}
}

C. class Book {
String title;
void printTitle() {
System.out.print("title");
}
}

D. class Book {
String title;
void printTitle() {
System.out.println(title);
}
}

Solution

1. Step 1: Check method to print field value

   Method should use System.out.println with the field variable title to print its value.

2. Step 2: Evaluate each option

   Options printing the literal "title" (with or without newline) are incorrect. Calling undefined print(title) causes an error. Only System.out.println(title) correctly prints the field value.

3. Final Answer:

   class Book { String title; void printTitle() { System.out.println(title); } } -> Option D

4. Quick Check:

   Print field with System.out.println(field) [OK]

Hint: Use System.out.println(field) to print variable content [OK]

Common Mistakes:

• Using print() instead of println()
• Printing string literal instead of variable
• Calling undefined print() method