Unit 5 · Applying Java OOP · 12 min

Putting it together: a small library system

A complete beginner project combining classes, encapsulation, inheritance, ArrayList and methods.

Why this matters

Concepts only click when you use several of them at once to solve a real problem.

Example

Java

import java.util.ArrayList;

class Item {
    private final String title;
    private boolean issued = false;

    Item(String title) {
        this.title = title;
    }

    public String getTitle() { return title; }
    public boolean isIssued() { return issued; }

    public void issue() {
        if (issued) {
            System.out.println(title + " is already issued");
        } else {
            issued = true;
            System.out.println("Issued: " + title);
        }
    }

    public void returnItem() {
        issued = false;
        System.out.println("Returned: " + title);
    }
}

class Book extends Item {
    private final String author;

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

    public void describe() {
        System.out.println(getTitle() + " by " + author
            + (isIssued() ? " [issued]" : " [available]"));
    }
}

public class Library {
    public static void main(String[] args) {
        ArrayList<Book> books = new ArrayList<>();
        books.add(new Book("Code Garden", "Mira Sen"));
        books.add(new Book("Object Trails", "Dev Nair"));

        books.get(0).issue();
        books.get(0).issue();

        for (Book b : books) {
            b.describe();
        }
    }
}

Output

Issued: Code Garden
Code Garden is already issued
Code Garden by Mira Sen [issued]
Object Trails by Dev Nair [available]

Step by step

1Item holds what every library item shares: a title and an issued flag, both private (encapsulation).
2Book extends Item and adds an author — inheritance.
3`super(title)` passes the title up to the parent constructor.
4Because title is private in Item, Book reads it through getTitle().
5issue() protects against issuing the same book twice — business rules live inside the class.
6ArrayList<Book> stores the collection and the for-each loop describes each one.

Key points

Design classes around real things: Item, Book, Member
Keep data private and rules inside methods
Inheritance removes duplicate code
A collection plus a loop is the backbone of most small projects

Common mistakes

Putting all the logic in main instead of in classes
Making fields public 'just to finish quickly'
Forgetting super() in the child constructor

Try it yourself

Add a Member class that can borrow up to 3 books.
Add a Magazine class that also extends Item.

Open the practice notebook