Integrated OOPS practice: Library System

Practical review day

1 Learn

Review day! This small library program uses an interface, an abstract class, inheritance, encapsulation, constructors and overriding together.

Read it slowly and find each concept — then extend it with the challenges below.

Want more detail? Open the full lesson

2 See the Code

Java

interface Borrowable {
    boolean borrow();
}

abstract class Item implements Borrowable {
    private final String title;
    private boolean out = false;

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

    public boolean borrow() {
        if (out) return false;
        out = true;
        return true;
    }
    abstract int loanDays();
}

class Book extends Item {
    Book(String t) { super(t); }
    int loanDays() { return 14; }
}

class Magazine extends Item {
    Magazine(String t) { super(t); }
    int loanDays() { return 3; }
}

public class Library {
    public static void main(String[] args) {
        Item[] items = { new Book("Code Garden"), new Magazine("Tech Weekly") };
        for (Item i : items) {
            System.out.println(i.getTitle() + ": " + i.loanDays() + " days, borrowed=" + i.borrow());
        }
        System.out.println("Again: " + items[0].borrow());
    }
}

Interface: Borrowable. Abstract class: Item. Inheritance: Book and Magazine extend Item.
Encapsulation: title and out are private and changed only through methods.
Polymorphism: loanDays() gives a different answer for each item type.

Output

Code Garden: 14 days, borrowed=true
Tech Weekly: 3 days, borrowed=true
Again: false

3 Practice

Challenge 1: Add a giveBack() method that sets out back to false.
Challenge 2: Add a DVD class with 7 loan days.
Challenge 3: Count how many items are currently borrowed using a loop.

Try it in your own Java setup or write it in the Practice Notebook.

4 Quick Check

1. Which concept makes loanDays() differ by item type?
2. Why is title private?
3. Why does the second borrow() return false?

Step 5

Answer and check the Quick Check above to unlock this.

Day 27