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Inheritance - Extending Classes

Inheritance lets a class (subclass) acquire fields and methods from another class (superclass) using the extends keyword. It models IS-A relationships - an EBook IS-A Book. Inheritance enables code reuse and sets the foundation for polymorphism. However, it also introduces tight coupling - use it judiciously.

15 min•By Priygop Team•Updated 2026

extends - IS-A Relationship

// Superclass - the general concept
public class LibraryItem {
    private final String id;
    private String title;
    private boolean available;

    public LibraryItem(String id, String title) {
        this.id        = id;
        this.title     = title;
        this.available = true;
    }

    public boolean borrow() {
        if (!available) return false;
        available = false;
        return true;
    }

    public void returnItem() { available = true; }

    // Getters
    public String getId()        { return id; }
    public String getTitle()     { return title; }
    public boolean isAvailable() { return available; }

    @Override
    public String toString() {
        return String.format("[%s] %s (%s)", id, title, available ? "Available" : "Borrowed");
    }
}

// Subclass - inherits all LibraryItem behaviour, adds its own
public class Book extends LibraryItem {
    private final String isbn;
    private final String author;
    private double price;

    public Book(String isbn, String title, String author, double price) {
        super(isbn, title);     // CALLS LibraryItem constructor - must be FIRST
        this.isbn   = isbn;
        this.author = author;
        this.price  = price;
    }

    // Book-specific method
    public String getCitation() {
        return String.format("%s. "%s." (ISBN: %s)", author, getTitle(), isbn);
    }

    @Override
    public String toString() {
        // super.toString() reuses parent's implementation
        return super.toString() + String.format(" - %s, $%.2f", author, price);
    }
}

// Subclass - digital book, different attributes
public class EBook extends LibraryItem {
    private final String isbn;
    private final String downloadUrl;
    private int maxSimultaneousReaders;
    private int currentReaders;

    public EBook(String isbn, String title, String downloadUrl, int maxReaders) {
        super(isbn, title);
        this.isbn = isbn;
        this.downloadUrl = downloadUrl;
        this.maxSimultaneousReaders = maxReaders;
        this.currentReaders = 0;
    }

    // Override borrow - EBook can be borrowed by multiple readers simultaneously
    @Override
    public boolean borrow() {
        if (currentReaders >= maxSimultaneousReaders) return false;
        currentReaders++;
        return true;  // Note: isAvailable() still true if under limit
    }

    @Override
    public void returnItem() {
        if (currentReaders > 0) currentReaders--;
    }
}

Inheritance Rules

Java supports single inheritance only - a class can extend ONE superclass (unlike C++).
All Java classes implicitly extend Object if no explicit superclass defined.
super() calls the parent constructor - must be first statement in subclass constructor.
super.method() calls the parent's version of an overridden method.
final class - cannot be extended: String, Integer, Math are all final.
final method - cannot be overridden in a subclass.

Quick Quiz

Tip

Practice Inheritance Extending Classes in small, isolated examples before integrating into larger projects. Breaking concepts into small experiments builds genuine understanding faster than reading alone.

Diagram

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Visual architecture and concepts for Inheritance - Extending Classes

Practice Task

Note

Practice Task - (1) Write a working example of Inheritance Extending Classes from scratch without looking at notes. (2) Modify it to handle an edge case (empty input, null value, or error state). (3) Share your solution in the Priygop community for feedback.

Common Mistake

Warning

A common mistake with Inheritance Extending Classes is skipping edge case testing - empty inputs, null values, and unexpected data types. Always validate boundary conditions to write robust, production-ready java code.

Key Takeaways

Inheritance lets a class (subclass) acquire fields and methods from another class (superclass) using the extends keyword.
Java supports single inheritance only - a class can extend ONE superclass (unlike C++).
All Java classes implicitly extend Object if no explicit superclass defined.
super() calls the parent constructor - must be first statement in subclass constructor.

Topics in This Module

extends - IS-A Relationship

// Superclass - the general concept
public class LibraryItem {
    private final String id;
    private String title;
    private boolean available;

    public LibraryItem(String id, String title) {
        this.id        = id;
        this.title     = title;
        this.available = true;
    }

    public boolean borrow() {
        if (!available) return false;
        available = false;
        return true;
    }

    public void returnItem() { available = true; }

    // Getters
    public String getId()        { return id; }
    public String getTitle()     { return title; }
    public boolean isAvailable() { return available; }

    @Override
    public String toString() {
        return String.format("[%s] %s (%s)", id, title, available ? "Available" : "Borrowed");
    }
}

// Subclass - inherits all LibraryItem behaviour, adds its own
public class Book extends LibraryItem {
    private final String isbn;
    private final String author;
    private double price;

    public Book(String isbn, String title, String author, double price) {
        super(isbn, title);     // CALLS LibraryItem constructor - must be FIRST
        this.isbn   = isbn;
        this.author = author;
        this.price  = price;
    }

    // Book-specific method
    public String getCitation() {
        return String.format("%s. "%s." (ISBN: %s)", author, getTitle(), isbn);
    }

    @Override
    public String toString() {
        // super.toString() reuses parent's implementation
        return super.toString() + String.format(" - %s, $%.2f", author, price);
    }
}

// Subclass - digital book, different attributes
public class EBook extends LibraryItem {
    private final String isbn;
    private final String downloadUrl;
    private int maxSimultaneousReaders;
    private int currentReaders;

    public EBook(String isbn, String title, String downloadUrl, int maxReaders) {
        super(isbn, title);
        this.isbn = isbn;
        this.downloadUrl = downloadUrl;
        this.maxSimultaneousReaders = maxReaders;
        this.currentReaders = 0;
    }

    // Override borrow - EBook can be borrowed by multiple readers simultaneously
    @Override
    public boolean borrow() {
        if (currentReaders >= maxSimultaneousReaders) return false;
        currentReaders++;
        return true;  // Note: isAvailable() still true if under limit
    }

    @Override
    public void returnItem() {
        if (currentReaders > 0) currentReaders--;
    }
}

Inheritance Rules

Java supports single inheritance only - a class can extend ONE superclass (unlike C++).

All Java classes implicitly extend Object if no explicit superclass defined.

super() calls the parent constructor - must be first statement in subclass constructor.

super.method() calls the parent's version of an overridden method.

final class - cannot be extended: String, Integer, Math are all final.

final method - cannot be overridden in a subclass.

Tip

Diagram

Loading diagram…

Visual architecture and concepts for Inheritance - Extending Classes

Topics in This Module