Object interaction in Java - Mini Project: Build & Apply

Object Interaction in Java

📖 Scenario: You are creating a simple program to manage a library. The library has books, and each book has a title and an author. You want to show how books and the library can work together.

🎯 Goal: Build two classes, Book and Library, where the library holds multiple books. You will create book objects, add them to the library, and then print the list of books in the library.

📋 What You'll Learn

Create a Book class with title and author fields

Create a Library class with a list to hold Book objects

Add a method in Library to add a Book

Add a method in Library to print all books with their titles and authors

💡 Why This Matters

🌍 Real World

Managing collections of items like books, movies, or products often requires objects that hold other objects. This project shows how to organize such data.

💼 Career

Understanding object interaction is key for software development jobs where you build systems with multiple parts working together.

1

Create the Book class

Create a class called Book with two String fields: title and author. Add a constructor that sets these fields.

Java

class Book {
    // Your code here
}

Hint

Think of a book as having a name and a writer. Use a constructor to set these when you create a book.

2

Create the Library class with a list of books

Create a class called Library with a field books that is an ArrayList<Book>. Initialize this list in the constructor.

Java

import java.util.ArrayList;

public class Library {
    // Your code here
}

Hint

Use ArrayList<Book> to hold many books. Initialize it inside the constructor.

3

Add method to add books to the library

In the Library class, add a method called addBook that takes a Book object as a parameter and adds it to the books list.

Java

import java.util.ArrayList;

public class Library {
    ArrayList books;

    public Library() {
        books = new ArrayList();
    }

    // Add a method to add a Book to books list
    // Your code here
}

Hint

Use the add method of ArrayList to add the book.

4

Print all books in the library

In the Library class, add a method called printBooks that loops over the books list and prints each book's title and author in the format: Title: [title], Author: [author]. Then, in the main method, create a Library object, add two Book objects with titles "1984" and "Animal Farm" by author "George Orwell", and call printBooks.

Java

class Book {
    String title;
    String author;

    public Book(String title, String author) {
        this.title = title;
        this.author = author;
    }
}

import java.util.ArrayList;

public class Library {
    ArrayList books;

    public Library() {
        books = new ArrayList();
    }

    public void addBook(Book book) {
        books.add(book);
    }

    // Add printBooks method and main method below
    // Your code here
}

Hint

Use a for-each loop to go through each book and print its details. Then create the library and books in main.

Practice

1.

What does it mean when two objects in Java interact?

Choose the best explanation.

easy

A. One object calls a method of another object to perform a task.

B. Two objects share the same memory location.

C. Objects are created using the same class.

D. Objects are stored in the same variable.

Solution

1. Step 1: Understand object interaction meaning

   Object interaction means objects communicate by calling each other's methods to work together.

2. Step 2: Evaluate options

   Only One object calls a method of another object to perform a task. describes calling methods between objects, which is how interaction happens.

3. Final Answer:

   One object calls a method of another object to perform a task. -> Option A

4. Quick Check:

   Object interaction = method calls between objects [OK]

Hint: Objects interact by calling methods on each other [OK]

Common Mistakes:

• Thinking objects share memory to interact
• Confusing object creation with interaction
• Assuming variables store multiple objects

2.

Which of the following is the correct syntax to call a method start() on an object car in Java?

easy

A. start(car);

B. car.start();

C. car->start();

D. start.car();

Solution

1. Step 1: Recall Java method call syntax

   In Java, to call a method on an object, use objectName.methodName();.

2. Step 2: Check each option

   car.start(); matches correct syntax. Options A, B, and C are invalid Java syntax.

3. Final Answer:

   car.start(); -> Option B

4. Quick Check:

   Method call = object.method() [OK]

Hint: Use objectName.methodName() to call methods [OK]

Common Mistakes:

• Using arrow (->) like in C++
• Reversing method and object order
• Calling method like a function with object as argument

3.

Consider the following Java code:

class Light {
    boolean isOn = false;
    void toggle() {
        isOn = !isOn;
    }
    boolean status() {
        return isOn;
    }
}

class Room {
    Light light = new Light();
    void switchLight() {
        light.toggle();
    }
    boolean lightStatus() {
        return light.status();
    }
}

public class Main {
    public static void main(String[] args) {
        Room room = new Room();
        System.out.println(room.lightStatus());
        room.switchLight();
        System.out.println(room.lightStatus());
    }
}

What is the output when this program runs?

medium

A. false
true

B. true
false

C. false
false

D. true
true

Solution

1. Step 1: Check initial light status

   Initially, isOn is false, so room.lightStatus() prints false.

2. Step 2: Toggle light and check status again

   Calling room.switchLight() toggles isOn to true. Then room.lightStatus() prints true.

3. Final Answer:

   false true -> Option A

4. Quick Check:

   Initial false, toggled true = false then true [OK]

Hint: Track boolean changes step-by-step [OK]

Common Mistakes:

• Assuming toggle sets true first without initial check
• Confusing method calls and variable values
• Ignoring initial value of isOn

4.

Find the error in this Java code snippet involving object interaction:

class Printer {
    void print(String message) {
        System.out.println(message);
    }
}

public class Main {
    public static void main(String[] args) {
        Printer printer;
        printer.print("Hello World");
    }
}

medium

A. The print statement syntax is incorrect.

B. The method 'print' is not defined in the Printer class.

C. The object 'printer' is declared but not initialized before use.

D. The class Printer should be public.

Solution

1. Step 1: Check object declaration and initialization

   The object 'printer' is declared but never assigned a new Printer instance.

2. Step 2: Understand consequences of uninitialized object

   Calling a method on an uninitialized object causes a NullPointerException at runtime.

3. Final Answer:

   The object 'printer' is declared but not initialized before use. -> Option C

4. Quick Check:

   Uninitialized object causes runtime error [OK]

Hint: Always initialize objects before calling methods [OK]

Common Mistakes:

• Assuming declaration equals initialization
• Thinking method absence causes error here
• Ignoring runtime NullPointerException

5.

You have two classes, BankAccount and Customer. A Customer has a BankAccount object. You want to add a method transferTo in BankAccount that transfers money to another BankAccount. Which of the following best shows how objects interact to perform this transfer?

class BankAccount {
    double balance;
    void deposit(double amount) { balance += amount; }
    void withdraw(double amount) { balance -= amount; }
    void transferTo(BankAccount other, double amount) {
        // Fill in this method
    }
}

class Customer {
    BankAccount account = new BankAccount();
}

hard

A. deposit(amount); withdraw(amount);

B. other.withdraw(amount); deposit(amount);

C. balance -= amount; other.balance += amount;

D. withdraw(amount); other.deposit(amount);

Solution

1. Step 1: Understand transfer logic

   To transfer money, withdraw from current account and deposit into the other account.

2. Step 2: Check method calls for interaction

   Calling withdraw(amount) on this object and deposit(amount) on the other object shows interaction.

3. Final Answer:

   withdraw(amount); other.deposit(amount); -> Option D

4. Quick Check:

   Transfer = withdraw from one, deposit to another [OK]

Hint: Transfer = withdraw from self, deposit to other [OK]

Common Mistakes:

• Withdrawing from the wrong account
• Directly changing balance without methods
• Depositing before withdrawing