AP CSA Unit 7 | Inheritance

Unit 7: Inheritance

Inheritance lets a subclass reuse and specialize behavior from a superclass. AP questions often test constructor chaining, overridden methods, and dynamic dispatch.

extendssuperoverridepolymorphismis-a

AP habit: trace values by hand before trusting your eyes. Most AP mistakes are loop bounds, type conversion, and reference mistakes.

Core Examples

Superclass

public class Athlete {
    private String name;
    public Athlete(String n) { name = n; }
    public String getName() { return name; }
}

Shared data and behavior belong in the superclass.

Subclass

public class Runner extends Athlete {
    private double bestTime;
    public Runner(String n, double t) {
        super(n);
        bestTime = t;
    }
}

Use super(...) to call the superclass constructor.

Override

public String toString() {
    return getName() + " " + bestTime;
}

A subclass can provide its own version.

Try It in Java

Run and modify the examples below. Add print statements, change values, and test edge cases.

Code Tracing Drop-Down Practice

Which constructor runs first?

The superclass constructor.

What does extends mean?

The subclass inherits from the superclass.

If a subclass overrides a method, which version runs?

The object type determines the runtime method.

Debugging Practice

Bug: forgetting super(...).

Java needs a valid superclass constructor call.

Bug: accessing private superclass fields directly.

Use inherited public methods such as getters.

AP-Style Multiple Choice

Which keyword calls a superclass constructor?

super

What relationship should inheritance model?

An is-a relationship.

Can a subclass add fields?

Yes.

FRQ Practice

Create a superclass PhysicsObject and subclasses Projectile and SpringMass. Override toString() in each subclass.

Scoring focus

Look for the correct method signature, correct data use, clear control flow, and boundary-case handling.

Physics / Real-World Mini Project

Model athletes using inheritance: Athlete superclass with Runner, Jumper, and Thrower subclasses. Rank them using event-specific score methods.