Runtime polymorphism in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of overridden method call

What is the output of this Java program demonstrating runtime polymorphism?

Java

class Animal {
    void sound() {
        System.out.println("Animal makes a sound");
    }
}

class Dog extends Animal {
    @Override
    void sound() {
        System.out.println("Dog barks");
    }
}

public class Test {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.sound();
    }
}

A Dog barks

B Animal makes a sound

C Compilation error

D Runtime error

💡 Hint

Remember that the method called depends on the actual object type, not the reference type.

The reference variable a is of type Animal, but it points to an object of type Dog. At runtime, the overridden sound() method in Dog is called, so the output is "Dog barks".

❓ Predict Output

intermediate

Output with multiple overridden methods

What will be printed when this Java code runs?

Java

class Vehicle {
    void start() {
        System.out.println("Vehicle starts");
    }
}

class Car extends Vehicle {
    @Override
    void start() {
        System.out.println("Car starts");
    }
}

class SportsCar extends Car {
    @Override
    void start() {
        System.out.println("SportsCar starts quickly");
    }
}

public class Test {
    public static void main(String[] args) {
        Vehicle v = new SportsCar();
        v.start();
    }
}

A Vehicle starts

B Car starts

C Compilation error

D SportsCar starts quickly

💡 Hint

Which class's start() method is called depends on the actual object type.

The reference v is of type Vehicle, but it points to a SportsCar object. The most specific overridden method start() in SportsCar is called at runtime, printing "SportsCar starts quickly".

❓ Predict Output

advanced

Output with casting and overridden methods

What is the output of this Java program?

Java

class Parent {
    void show() {
        System.out.println("Parent show");
    }
}

class Child extends Parent {
    @Override
    void show() {
        System.out.println("Child show");
    }
    void childOnly() {
        System.out.println("Child only method");
    }
}

public class Test {
    public static void main(String[] args) {
        Parent p = new Child();
        p.show();
        ((Child) p).childOnly();
    }
}

A

Child show
Compilation error

B Runtime error

C

Child show
Child only method

D

Parent show
Child only method

💡 Hint

Check which method is overridden and which method is only in the child class.

The show() method is overridden, so Child show is printed. The cast to Child allows calling childOnly(), so "Child only method" is printed next.

❓ Predict Output

advanced

Output with overridden method and variable hiding

What will this Java program print?

Java

class Base {
    String name = "Base";
    void printName() {
        System.out.println(name);
    }
}

class Derived extends Base {
    String name = "Derived";
    @Override
    void printName() {
        System.out.println(name);
    }
}

public class Test {
    public static void main(String[] args) {
        Base b = new Derived();
        System.out.println(b.name);
        b.printName();
    }
}

A

Derived
Derived

B

Base
Derived

C

Base
Base

D

Derived
Base

💡 Hint

Remember that variables are not polymorphic, but methods are.

The variable name accessed by b.name uses the reference type Base, so it prints "Base". The method printName() is overridden, so the Derived version runs and prints "Derived".

❓ Predict Output

expert

Output with abstract class and runtime polymorphism

What is the output of this Java program using an abstract class and runtime polymorphism?

Java

abstract class Shape {
    abstract void draw();
}

class Circle extends Shape {
    @Override
    void draw() {
        System.out.println("Drawing Circle");
    }
}

class Square extends Shape {
    @Override
    void draw() {
        System.out.println("Drawing Square");
    }
}

public class Test {
    public static void main(String[] args) {
        Shape s = new Circle();
        s.draw();
        s = new Square();
        s.draw();
    }
}

A

Drawing Circle
Drawing Square

B

Drawing Square
Drawing Circle

C Runtime error

D Compilation error: cannot instantiate abstract class

💡 Hint

Abstract classes cannot be instantiated directly, but references can point to subclass objects.

The reference s is of type Shape. It first points to a Circle object, so draw() prints "Drawing Circle". Then it points to a Square object, so draw() prints "Drawing Square".