Lab Manual IV · CSE 202

Inheritance, Polymorphism & Overriding

Build a Vehicle hierarchy with Car, Motorcycle and ElectricCar: reuse code with extends and super, override display(), and let one loop drive every kind of vehicle.

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Before we start

Why this lab

A car, a motorcycle and an electric car all accelerate and brake the same way, but each shows different details. Do we write accelerate() three times?

No: write it once in Vehicle and inherit it. Each subclass then overrides only what differs, and one Vehicle[] array can hold them all.

Lab 4 of 10

Lab at a glance

After this lab you can

Explain inheritance as code reuse and an is-a relationship
Create subclasses with extends
Call superclass constructors with super(…)
Override methods with @Override
Distinguish overriding from overloading
Use superclass references to subclass objects (polymorphism)
Cast safely with instanceof

Before you start

Labs 2–3: classes, constructors, encapsulation, constructor chaining, overloading

Plan for the session

Theory recap
Walk through the program, section by section, and run it
In-lab tasks, shown to the instructor
Report exercises at home

Lab 4 of 10

Select a part to jump to it.

Theory recap
Program design
Code walkthrough
Step-through traces: stack, heap and output
Expected output
Interactive demos
In-lab tasks
Lab report exercises
Viva questions and quiz

Theory 1 of 5

Inheritance and extends

class Car extends Vehicle: a Car is a Vehicle and inherits its accessible fields and methods.
Java classes have single inheritance, but chains are allowed: ElectricCar → Car → Vehicle.

Theory 2 of 5

super and protected

public Car(String make, int year, int numDoors, double fuelLevel) {
    super(make, year);          // Vehicle part first
    this.numDoors = numDoors;
    ...
}

super(…) runs the superclass constructor and must be the first statement.
protected fields (make, year, speed) are visible to subclasses; subclass-only fields stay private.

Theory 3 of 5

Overriding vs overloading

	Overloading	Overriding
Where	same class	subclass
Signature	different parameters	same name, parameters, return type
Chosen	at compile time	at run time (object type)
Example	display() / display(String)	Car.display() replaces Vehicle.display()

@Override makes the compiler check that you really override something.

Theory 4 of 5

Polymorphism: one call, many behaviours

Vehicle[] vehicles = {vehicle, car, motorcycle, electricCar};
for (Vehicle v : vehicles) {
    v.display();   // Car, Motorcycle or ElectricCar version
}

The reference type (Vehicle) decides what you may call; the object type decides which version runs.
This is dynamic method dispatch.

Theory 5 of 5

instanceof and safe casting

Vehicle v = new Car("Honda Civic", 2021, 2, 60.0);
if (v instanceof Car) {
    Car c = (Car) v;
    System.out.println(c.getNumDoors());
}

A Vehicle reference cannot call getNumDoors(); cast it first.
Check with instanceof before casting to avoid a ClassCastException.

Implementation design

Execution flow

Create a Vehicle, a Car, a Motorcycle and an ElectricCar
Call display() on each directly
Store them in a Vehicle[] and display them polymorphically
Call inherited accelerate() and brake()
Call subclass-only refuel(), wheelie() and charge()
Test instanceof and cast a Vehicle reference to Car
Verify overriding through Vehicle references
Show brake() never makes speed negative

Common state in Vehicle; each subclass adds fields and overrides display().

Code walkthrough 1 of 10 · Vehicle.java

The superclass

// Protected fields are accessible in subclasses
protected String make;
protected int year;
protected int speed;

public Vehicle(String make, int year) {
    this.make = make;
    this.year = year;
    this.speed = 0;
}

protected fields are visible to subclasses.

Code walkthrough 2 of 10 · Vehicle.java

Methods every vehicle inherits

public void accelerate(int delta) {
    if (delta > 0) {
        this.speed = this.speed + delta;
    }
}

public void brake(int delta) {
    if (delta > 0) {
        this.speed = Math.max(0, this.speed - delta);
    }
}

Code walkthrough 2 of 10 · Vehicle.java

Methods every vehicle inherits (cont.)

public void display() {
    System.out.println("------- Vehicle -------");
    System.out.println("Make  : " + make);
    System.out.println("Year  : " + year);
    System.out.println("Speed : " + speed + " km/h");
    System.out.println("-----------------------");
}

Car, Motorcycle and ElectricCar get these without writing them again.

Code walkthrough 3 of 10 · Car.java

Car extends Vehicle

public class Car extends Vehicle {

    private int numDoors;
    private double fuelLevel;

    public Car(String make, int year, int numDoors, double fuelLevel) {
        super(make, year);
        if (numDoors > 0) {
            this.numDoors = numDoors;
        } else {
            this.numDoors = 4;

Code walkthrough 3 of 10 · Car.java

Car extends Vehicle (cont.)

        }
        if (fuelLevel >= 0.0 && fuelLevel <= 100.0) {
            this.fuelLevel = fuelLevel;
        } else {
            this.fuelLevel = 0.0;
        }
    }

super(make, year) builds the Vehicle part first.
Then Car validates its own fields.

Code walkthrough 4 of 10 · Car.java

Overriding display()

@Override
public void display() {
    System.out.println("------- Car -------");
    System.out.println("Make      : " + make);
    System.out.println("Year      : " + year);
    System.out.println("Speed     : " + speed + " km/h");
    System.out.println("Doors     : " + numDoors);
    System.out.printf("Fuel Level: %.1f%%%n", fuelLevel);
    System.out.println("-------------------");
}

Same signature as Vehicle.display(); @Override lets the compiler check it.
make, year and speed are inherited protected fields.

Code walkthrough 5 of 10 · Motorcycle.java

Motorcycle: another subclass

public class Motorcycle extends Vehicle {

    private boolean hasStorage;

    public Motorcycle(String make, int year, boolean hasStorage) {
        super(make, year);
        this.hasStorage = hasStorage;
    }

    public void wheelie() {
        System.out.println(make + " is performing a wheelie!");
    }

Adds its own field and a method only motorcycles have.
Its display() override follows the same pattern as Car’s.

Code walkthrough 6 of 10 · ElectricCar.java

ElectricCar: a chain of inheritance

public class ElectricCar extends Car {

    private double batteryCapacity;
    private double chargeLevel;

    public ElectricCar(String make, int year, int numDoors,
                       double batteryCapacity, double chargeLevel) {
        super(make, year, numDoors, 100.0);
        if (batteryCapacity > 0.0) {
            this.batteryCapacity = batteryCapacity;
        } else {

Code walkthrough 6 of 10 · ElectricCar.java

ElectricCar: a chain of inheritance (cont.)

            this.batteryCapacity = 50.0;
        }
        if (chargeLevel >= 0.0 && chargeLevel <= 100.0) {
            this.chargeLevel = chargeLevel;
        } else {
            this.chargeLevel = 0.0;
        }
    }

ElectricCar → Car → Vehicle. super(…) calls Car’s constructor.

Code walkthrough 7 of 10 · Lab4Demo.java

Polymorphic dispatch

// 3. Polymorphism with Superclass References
System.out.println("\n=== Polymorphic Dispatch ===");
Vehicle[] vehicles = {vehicle, car, motorcycle, electricCar};
for (int i = 0; i < vehicles.length; i++) {
    System.out.println("vehicles[" + i + "]:");
    displayVehicle(vehicles[i]);
}

One Vehicle[] holds every kind of vehicle.
Each element runs its own display().

Code walkthrough 8 of 10 · Lab4Demo.java

Inherited and subclass-only methods

// 5. Subclass-Specific Methods
System.out.println("\n=== Subclass-Specific Methods ===");
car.refuel(20.0);
System.out.println("After car.refuel(20.0):");
car.display();
motorcycle.wheelie();
electricCar.charge(15.0);
System.out.println("After electricCar.charge(15.0):");
electricCar.display();

refuel, wheelie and charge exist only in their subclasses.

Code walkthrough 9 of 10 · Lab4Demo.java

instanceof and a safe cast

// 6. instanceof and Safe Casting
System.out.println("\n=== instanceof and Safe Casting ===");
Vehicle v = new Car("Honda Civic", 2021, 2, 60.0);
System.out.println("v instanceof Vehicle    : " + (v instanceof Vehicle));
System.out.println("v instanceof Car        : " + (v instanceof Car));
System.out.println("v instanceof Motorcycle : " + (v instanceof Motorcycle));
if (v instanceof Car) {
    Car castedCar = (Car) v;
    System.out.println("Number of doors after casting: " + castedCar.getNumDoors());
}

Check the real type, then cast to reach Car-only methods.

Code walkthrough 10 of 10 · Lab4Demo.java

Overriding through Vehicle references

// 7. Method Overriding Verification
System.out.println("\n=== Method Overriding Verification ===");
Vehicle ref1 = new Car("Ford Mustang", 2020, 2, 50.0);
Vehicle ref2 = new Motorcycle("Harley-Davidson", 2022, false);
Vehicle ref3 = new ElectricCar("Nissan Leaf", 2023, 4, 62.0, 70.0);
ref1.display();
ref2.display();
ref3.display();

The reference type is Vehicle, but each object’s own display() runs.

Step through the code · 1 of 2

Trace: super(…) and dynamic dispatch

Watch super(…) build the inherited part, then one call site run two different display() methods.

Step through the code · 1 of 2

Trace: super(…) and dynamic dispatch (cont.)

Step through the code · 2 of 2

Trace: instanceof before every cast

One loop over a Vehicle[]: the check decides whether the cast is safe. The last line shows what happens without it.

Step through the code · 2 of 2

Trace: instanceof before every cast (cont.)

For reference · 5 files

The complete program

public class Vehicle {

    // Protected fields are accessible in subclasses
    protected String make;
    protected int year;
    protected int speed;

    public Vehicle(String make, int year) {
        this.make = make;
        this.year = year;
        this.speed = 0;
    }

    public void accelerate(int delta) {
        if (delta > 0) {
            this.speed = this.speed + delta;
        }
    }

    public void brake(int delta) {
        if (delta > 0) {
            this.speed = Math.max(0, this.speed - delta);
        }
    }

    public void display() {
        System.out.println("------- Vehicle -------");
        System.out.println("Make  : " + make);
        System.out.println("Year  : " + year);
        System.out.println("Speed : " + speed + " km/h");
        System.out.println("-----------------------");
    }

    public int getYear() {
        return year;
    }
}

public class Car extends Vehicle {

    private int numDoors;
    private double fuelLevel;

    public Car(String make, int year, int numDoors, double fuelLevel) {
        super(make, year);
        if (numDoors > 0) {
            this.numDoors = numDoors;
        } else {
            this.numDoors = 4;
        }
        if (fuelLevel >= 0.0 && fuelLevel <= 100.0) {
            this.fuelLevel = fuelLevel;
        } else {
            this.fuelLevel = 0.0;
        }
    }

    public void refuel(double amount) {
        if (amount > 0.0) {
            this.fuelLevel = Math.min(100.0, this.fuelLevel + amount);
        }
    }

    @Override
    public void display() {
        System.out.println("------- Car -------");
        System.out.println("Make      : " + make);
        System.out.println("Year      : " + year);
        System.out.println("Speed     : " + speed + " km/h");
        System.out.println("Doors     : " + numDoors);
        System.out.printf("Fuel Level: %.1f%%%n", fuelLevel);
        System.out.println("-------------------");
    }

    public int getNumDoors() { return numDoors; }
    public double getFuelLevel() { return fuelLevel; }
}

public class Motorcycle extends Vehicle {

    private boolean hasStorage;

    public Motorcycle(String make, int year, boolean hasStorage) {
        super(make, year);
        this.hasStorage = hasStorage;
    }

    public void wheelie() {
        System.out.println(make + " is performing a wheelie!");
    }

    @Override
    public void display() {
        System.out.println("------- Motorcycle -------");
        System.out.println("Make        : " + make);
        System.out.println("Year        : " + year);
        System.out.println("Speed       : " + speed + " km/h");
        System.out.println("Has Storage : " + (hasStorage ? "Yes" : "No"));
        System.out.println("--------------------------");
    }

    public boolean hasStorage() { return hasStorage; }
}

public class ElectricCar extends Car {

    private double batteryCapacity;
    private double chargeLevel;

    public ElectricCar(String make, int year, int numDoors,
                       double batteryCapacity, double chargeLevel) {
        super(make, year, numDoors, 100.0);
        if (batteryCapacity > 0.0) {
            this.batteryCapacity = batteryCapacity;
        } else {
            this.batteryCapacity = 50.0;
        }
        if (chargeLevel >= 0.0 && chargeLevel <= 100.0) {
            this.chargeLevel = chargeLevel;
        } else {
            this.chargeLevel = 0.0;
        }
    }

    public void charge(double amount) {
        if (amount > 0.0) {
            this.chargeLevel = Math.min(100.0, this.chargeLevel + amount);
        }
    }

    @Override
    public void display() {
        System.out.println("------- Electric Car -------");
        System.out.println("Make             : " + make);
        System.out.println("Year             : " + year);
        System.out.println("Speed            : " + speed + " km/h");
        System.out.println("Doors            : " + getNumDoors());
        System.out.printf("Battery Capacity : %.1f kWh%n", batteryCapacity);
        System.out.printf("Charge Level     : %.1f%%%n", chargeLevel);
        System.out.println("----------------------------");
    }

    public double getBatteryCapacity() { return batteryCapacity; }
    public double getChargeLevel() { return chargeLevel; }
}

public class Lab4Demo {

    static void displayVehicle(Vehicle vehicle) {
        vehicle.display();
    }

    public static void main(String[] args) {

        // 1. Object Creation
        System.out.println("=== Object Creation ===");
        Vehicle vehicle = new Vehicle("Generic Vehicle", 2020);
        Car car = new Car("Toyota Camry", 2022, 4, 75.0);
        Motorcycle motorcycle = new Motorcycle("Yamaha YZF", 2021, true);
        ElectricCar electricCar = new ElectricCar("Tesla Model 3", 2023, 4, 75.0, 85.0);

        // 2. Direct Method Calls
        System.out.println("\n=== Direct Method Calls ===");
        vehicle.display();
        car.display();
        motorcycle.display();
        electricCar.display();

        // 3. Polymorphism with Superclass References
        System.out.println("\n=== Polymorphic Dispatch ===");
        Vehicle[] vehicles = {vehicle, car, motorcycle, electricCar};
        for (int i = 0; i < vehicles.length; i++) {
            System.out.println("vehicles[" + i + "]:");
            displayVehicle(vehicles[i]);
        }

        // 4. Inherited Methods
        System.out.println("\n=== Inherited Methods ===");
        car.accelerate(50);
        System.out.println("After car.accelerate(50):");
        car.display();
        motorcycle.accelerate(80);
        System.out.println("After motorcycle.accelerate(80):");
        motorcycle.display();
        electricCar.accelerate(40);
        System.out.println("After electricCar.accelerate(40):");
        electricCar.display();

        // 5. Subclass-Specific Methods
        System.out.println("\n=== Subclass-Specific Methods ===");
        car.refuel(20.0);
        System.out.println("After car.refuel(20.0):");
        car.display();
        motorcycle.wheelie();
        electricCar.charge(15.0);
        System.out.println("After electricCar.charge(15.0):");
        electricCar.display();

        // 6. instanceof and Safe Casting
        System.out.println("\n=== instanceof and Safe Casting ===");
        Vehicle v = new Car("Honda Civic", 2021, 2, 60.0);
        System.out.println("v instanceof Vehicle    : " + (v instanceof Vehicle));
        System.out.println("v instanceof Car        : " + (v instanceof Car));
        System.out.println("v instanceof Motorcycle : " + (v instanceof Motorcycle));
        if (v instanceof Car) {
            Car castedCar = (Car) v;
            System.out.println("Number of doors after casting: " + castedCar.getNumDoors());
        }

        // 7. Method Overriding Verification
        System.out.println("\n=== Method Overriding Verification ===");
        Vehicle ref1 = new Car("Ford Mustang", 2020, 2, 50.0);
        Vehicle ref2 = new Motorcycle("Harley-Davidson", 2022, false);
        Vehicle ref3 = new ElectricCar("Nissan Leaf", 2023, 4, 62.0, 70.0);
        ref1.display();
        ref2.display();
        ref3.display();

        // 8. Inherited brake() Method
        System.out.println("\n=== Inherited brake() Method ===");
        Car testCar = new Car("Subaru Outback", 2019, 5, 45.0);
        testCar.accelerate(100);
        System.out.println("After testCar.accelerate(100):");
        testCar.display();
        testCar.brake(30);
        System.out.println("After testCar.brake(30):");
        testCar.display();
        testCar.brake(200);
        System.out.println("After testCar.brake(200):");
        testCar.display();
    }
}

Copy or download each file, save it under the same name, then compile and run: javac *.java → java Lab4Demo

Implementation

Expected output

=== Object Creation ===

=== Direct Method Calls ===
------- Vehicle -------
Make  : Generic Vehicle
Year  : 2020
Speed : 0 km/h
-----------------------
------- Car -------
Make      : Toyota Camry
Year      : 2022
Speed     : 0 km/h
Doors     : 4
Fuel Level: 75.0%
-------------------
------- Motorcycle -------
Make        : Yamaha YZF
Year        : 2021
Speed       : 0 km/h
Has Storage : Yes
--------------------------
------- Electric Car -------
Make             : Tesla Model 3
Year             : 2023
Speed            : 0 km/h
Doors            : 4
Battery Capacity : 75.0 kWh
Charge Level     : 85.0%
----------------------------

=== Polymorphic Dispatch ===
vehicles[0]:
------- Vehicle -------
  … (the same four blocks again, one per array element)

=== Inherited Methods ===
After car.accelerate(50):            Speed : 50 km/h
After motorcycle.accelerate(80):     Speed : 80 km/h
After electricCar.accelerate(40):    Speed : 40 km/h

=== Subclass-Specific Methods ===
After car.refuel(20.0):              Fuel Level: 95.0%
Yamaha YZF is performing a wheelie!
After electricCar.charge(15.0):      Charge Level : 100.0%

=== instanceof and Safe Casting ===
v instanceof Vehicle    : true
v instanceof Car        : true
v instanceof Motorcycle : false
Number of doors after casting: 2

=== Method Overriding Verification ===
------- Car -------          (Ford Mustang)
------- Motorcycle -------   (Harley-Davidson, Has Storage : No)
------- Electric Car -------  (Nissan Leaf, 62.0 kWh, 70.0%)

=== Inherited brake() Method ===
After testCar.accelerate(100):  Speed : 100 km/h
After testCar.brake(30):        Speed : 70 km/h
After testCar.brake(200):       Speed : 0 km/h

Shortened: repeated display blocks are summarised. The full output is in the lab manual, section 4.5.2.

Interactive 1 of 2

Reference type vs object type

Interactive 2 of 2

When is a cast safe?

In-lab task 1 of 3

Task 1: Add a Bus class

A new Vehicle subclass that sells tickets.

Requirements

Private fields numSeats and ticketPrice with getters
Constructor that calls super(make, year)
Override display()
calculateRevenue(int passengersBoarded)
Demonstrate accelerate/brake, display and revenue in Lab4Demo

Show a hint

Cap revenue at the number of seats: Math.min(passengersBoarded, numSeats) * ticketPrice.

In-lab task 2 of 3

Task 2: Create a HybridCar class

A Car subclass that can run on fuel or battery.

Requirements

Fields batteryLevel and currentMode
useBattery(double) and switchMode(String) ("Fuel" / "Battery")
Override display()
Store it in a Vehicle reference and call display()
Cast to HybridCar to call its own methods

Show a hint

Calling v.switchMode("Battery") on a Vehicle reference will not compile — that is the point of the cast.

In-lab task 3 of 3

Task 3: Test instanceof and casting

Process a mixed array safely.

Requirements

Array with Car, Motorcycle, ElectricCar and Bus objects
Loop over it
Identify each actual type with instanceof
Call type-specific methods after a safe cast
Print clear output

Show a hint

Test the most specific type first: an ElectricCar is also a Car.

In-lab tasks

Show your work during the lab

For each task, show the instructor:

the source code
a successful compilation
the running output
a short explanation of the logic you used

Lab report · exercise 1 of 2

Employee hierarchy and payroll

Employee with Manager, Developer and Intern subclasses.

Requirements

Employee: protected empId, name, salary, department; getters/setters, display(), calculateBonus() = 5% of salary
Manager: teamSize, responsibilityBonus; override bonus and display; add/remove team member
Developer: programmingLanguage, yearsExperience; override bonus and display; upgradeLanguage, gainExperience
Intern: university, monthsRemaining; bonus 0.0; decrementMonths
≥2 objects of each subclass in an Employee[]
Print total salary, total bonus and average bonus using polymorphic calls

Lab report · exercise 2 of 2

Zoo animal classification

Animal → Mammal (Lion, Elephant) and Bird (Eagle).

Requirements

Animal: species, age, weight, enclosure; eat, sleep, makeSound, display
Mammal (hasFur) and Bird (canFly) override makeSound and display
Lion "ROAR!" + hunt(prey); Elephant "TRUMPET!" + spray(); Eagle "SCREECH!" + dive()
≥2 objects of each concrete class in an Animal[]
Simulate a day: everyone eats, sleeps and makes a sound
Call class-specific methods safely with instanceof

Lab report

Writing the lab report

Problem statement and your design (classes, fields, methods)
Complete source code, formatted and commented
Sample output or screenshots for normal and edge cases
A short discussion: what each OOP concept did in your program

Academic integrity

Copying from the internet, classmates, seniors or any other unauthorized source is strictly prohibited. Full marks may be deducted if plagiarism, copied work or academic dishonesty is detected. Complete the tasks, implementation, output analysis and report yourself.

Discussion and analysis

Viva questions (1 of 2)

Why does Car’s constructor start with super(make, year)?

Show the answer

The Vehicle part of the object (make, year, speed) must be initialised first, and only Vehicle’s constructor knows how.

Why are Vehicle’s fields protected instead of private?

Show the answer

So subclasses such as Car can use make, year and speed directly in their display() methods.

Discussion and analysis

Viva questions (2 of 2)

Vehicle v = new Motorcycle(…); v.display(); — which display runs?

Show the answer

Motorcycle’s. The object type decides at run time (dynamic dispatch).

Why can’t v.wheelie() compile?

Show the answer

The compiler only knows v is a Vehicle, and Vehicle has no wheelie(). Check instanceof and cast to Motorcycle first.

Check yourself

Quiz

Summary

extends creates an is-a relationship and reuses superclass code.
super(…) initialises the inherited part first.
Overriding replaces an inherited method; @Override lets the compiler check it.
Superclass references + overriding = polymorphism (dynamic dispatch).
Use instanceof before downcasting.

Next: Lab 5 · Abstract classes and interfaces All labs

Inheritance, Polymorphism & Overriding

Why this lab

Lab at a glance

After this lab you can

Before you start

Plan for the session

Contents

Inheritance and extends

super and protected

Overriding vs overloading

Polymorphism: one call, many behaviours

instanceof and safe casting

Execution flow

The superclass

Methods every vehicle inherits

Methods every vehicle inherits (cont.)

Car extends Vehicle

Car extends Vehicle (cont.)

Overriding display()

Motorcycle: another subclass

ElectricCar: a chain of inheritance

ElectricCar: a chain of inheritance (cont.)

Polymorphic dispatch

Inherited and subclass-only methods

instanceof and a safe cast

Overriding through Vehicle references

Trace: super(…) and dynamic dispatch

Trace: super(…) and dynamic dispatch (cont.)

Trace: instanceof before every cast

Trace: instanceof before every cast (cont.)

The complete program

Expected output

Reference type vs object type

When is a cast safe?

Task 1: Add a Bus class

Task 2: Create a HybridCar class

Task 3: Test instanceof and casting

Show your work during the lab

Employee hierarchy and payroll

Zoo animal classification

Writing the lab report

Academic integrity

Viva questions (1 of 2)

Viva questions (2 of 2)

Quiz

Summary