Interfaces, Abstract Classes, and Polymorphism

Required tools

• JDK (21 LTS or newer)
• A terminal (any)

Setup

1. Create a project folder named library-loans.
2. Inside it, create src/ for your .java files.
3. You'll add: a Borrowable interface, two classes implementing it (Book, Magazine), an encapsulated Loan class composing a Borrowable, and a Main that demonstrates polymorphism.

Project structure

library-loans/
  src/
    Borrowable.java
    Book.java
    Magazine.java
    Loan.java
    Main.java

Starter files

public interface Borrowable {
    // TODO: declare a method String describe() and a method int loanPeriodDays()
}

public class Book implements Borrowable {
    private final String title;
    private final String author;

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

    // TODO: implement describe() (e.g. "title by author") and loanPeriodDays() (return 21)
}

public class Magazine implements Borrowable {
    private final String name;
    private final int issueNumber;

    public Magazine(String name, int issueNumber) {
        this.name = name;
        this.issueNumber = issueNumber;
    }

    // TODO: implement describe() (e.g. "name, issue #N") and loanPeriodDays() (return 7)
}

public class Loan {
    private final Borrowable item; // composition: Loan HAS-A Borrowable, it doesn't extend it
    private final String borrowerName;
    private boolean returned;

    public Loan(Borrowable item, String borrowerName) {
        // TODO: store item and borrowerName; returned starts false
    }

    // TODO: add a public method summary() returning something like
    // "<borrowerName> borrowed <item.describe()> for <item.loanPeriodDays()> days"

    // TODO: add markReturned() (sets returned = true) and isReturned() (getter)
}

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Loan> loans = new ArrayList<>();
        // TODO: create at least one Book loan and one Magazine loan, add both to loans

        // TODO: loop over loans polymorphically -- print each summary() -- with
        // NO instanceof checks or branching on whether the item is a Book or Magazine
    }
}

Requirements

• Borrowable is an interface with describe() and loanPeriodDays().
• Book and Magazine both implement Borrowable with genuinely different internal fields.
• Loan composes a Borrowable (a field, not inheritance) plus a borrower name and a returned flag, all private.
• Loan exposes summary(), markReturned(), and isReturned() as its only public surface.
• Main builds a List<Loan> containing at least one Book-backed and one Magazine-backed loan, then prints every summary() via one polymorphic loop with no type-checking branches.

Commands to run

• Compile every source file into out/

  javac -d out src/Borrowable.java src/Book.java src/Magazine.java src/Loan.java src/Main.java

• Run the program

  java -cp out Main

Expected behavior

The program compiles with no errors or warnings, and running it prints one summary line per loan — one mentioning a book title/author and a 21-day period, one mentioning a magazine name/issue and a 7-day period — produced by the same loop, with no branch checking which concrete type each loan wraps.

Verify it yourself

• javac -d out src/*.java

  Expected: Compiles cleanly with no errors

• java -cp out Main

  Expected: Prints at least two summary lines, one for a Book-backed loan and one for a Magazine-backed loan

• grep -n instanceof src/Main.java

  Expected: No output — Main.java should contain no instanceof checks in the printing loop

Troubleshooting

• `error: Loan is not abstract and does not override abstract method describe() in Borrowable` — Book or Magazine is missing an implementation of one of the interface's methods — every method declared in an interface must be implemented by a non-abstract implementing class.
• Compiles, but the loop in Main only prints Book-shaped output — Check that both a Book and a Magazine were actually constructed and added to the loans list before the loop runs.
• `cannot find symbol: method summary()` — Confirm Loan's summary() method is declared public, and that you're calling loan.summary() (a Loan method) rather than trying to call it on the Borrowable directly.

Extension challenge

Add a third Borrowable implementation, DvdBoxSet, with its own fields and a 14-day loan period, and confirm the existing Main loop prints it correctly with zero changes to Main.java itself — that's the payoff of coding against the interface.