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L3.1 — Inheritance introduces why inheritance exists (avoiding code duplication between similar classes), the superclass/subclass relationship, the extends keyword, inheritance hierarchies, the four access modifiers (public, protected, default, private), how constructors work with super(), substitution (upcasting), and the Object class at the root of every Java class.

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📋 At a Glance

• [ ] Explain why inheritance solves code duplication between similar classes
• [ ] Define superclass and subclass
• [ ] Use the extends keyword to create a subclass
• [ ] Draw and interpret an inheritance hierarchy (class diagram with hollow arrows)
• [ ] List all four access modifiers in order: public, protected, default, private
• [ ] Know that protected includes subclasses even across packages; default does not
• [ ] Use super() to call the superclass constructor for proper encapsulation
• [ ] Know that super() must be the first statement in the subclass constructor
• [ ] Explain substitution (upcasting) and its limitation on method calls
• [ ] Know that all Java classes implicitly inherit from the Object class

1. Why Inheritance?

The Problem: Code Duplication

Even with proper encapsulation, similar classes end up duplicating code. Consider music CDs and film DVDs — both have title, comment, gotIt, plus getters/setters and a print() method.

public class DVD {
    private String title;
    private String director;
    private String comment;
    // constructor, getters, setters, print()...
}

public class CD {
    private String title;
    private String artist;
    private String comment;
    // constructor, getters, setters, print()...
}

Duplicated code is bad because:

• Creates opportunities for inconsistency and errors