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".

Practice

1. What is runtime polymorphism in Java?

easy

A. Creating multiple objects of the same class

B. Using multiple classes with the same name

C. Choosing which method to call during program execution based on object type

D. Writing methods with different names in the same class

Solution

1. Step 1: Understand polymorphism concept

   Polymorphism means many forms; in Java, it allows methods to behave differently based on object type.

2. Step 2: Identify runtime polymorphism

   Runtime polymorphism happens when the program decides which overridden method to call during execution, not before.

3. Final Answer:

   Choosing which method to call during program execution based on object type -> Option C

4. Quick Check:

   Runtime polymorphism = method choice at runtime [OK]

Hint: Runtime polymorphism means method choice happens while running [OK]

Common Mistakes:

• Confusing compile-time and runtime polymorphism
• Thinking it means multiple classes with same name
• Believing it is about method overloading

2. Which syntax correctly shows method overriding for runtime polymorphism in Java?

easy

A. class Parent { void show() {} } class Child extends Parent { void show() {} }

B. class Parent { void show() {} } class Child extends Parent { void display() {} }

C. class Parent { void show() {} } class Child { void show() {} }

D. class Parent { void show() {} } class Child extends Parent { void show(int x) {} }

Solution

1. Step 1: Check method overriding rules

   Method overriding requires same method name and parameters in subclass extending superclass.

2. Step 2: Match options with overriding

   class Parent { void show() {} } class Child extends Parent { void show() {} } shows subclass overriding show() method correctly; others differ in method name or parameters.

3. Final Answer:

   class Parent { void show() {} } class Child extends Parent { void show() {} } -> Option A

4. Quick Check:

   Same method name and parameters in subclass = overriding [OK]

Hint: Overriding needs same method name and parameters in subclass [OK]

Common Mistakes:

• Changing method name in subclass instead of overriding
• Changing method parameters (overloading, not overriding)
• Not extending the parent class

3. What is the output of this code?

class Animal {
  void sound() { System.out.println("Animal sound"); }
}
class Dog extends Animal {
  void sound() { System.out.println("Bark"); }
}
public class Test {
  public static void main(String[] args) {
    Animal a = new Dog();
    a.sound();
  }
}

medium

A. Bark

B. Animal sound

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand object and reference types

   Reference is of type Animal, but object is Dog, so overridden method in Dog is called.

2. Step 2: Identify method called at runtime

   Due to runtime polymorphism, sound() of Dog runs, printing "Bark".

3. Final Answer:

   Bark -> Option A

4. Quick Check:

   Overridden method runs based on object type [OK]

Hint: Method called depends on object type, not reference type [OK]

Common Mistakes:

• Thinking reference type decides method called
• Expecting superclass method output
• Confusing compile-time and runtime behavior

4. Find the error in this code related to runtime polymorphism:

class Parent {
  void show() { System.out.println("Parent"); }
}
class Child extends Parent {
  void show(int x) { System.out.println("Child " + x); }
}
public class Test {
  public static void main(String[] args) {
    Parent p = new Child();
    p.show();
  }
}

medium

A. Parent class method show() is private

B. Child class does not override show() method correctly

C. Cannot assign Child object to Parent reference

D. Missing main method

Solution

1. Step 1: Check method overriding in Child class

   Child defines show(int x), which is overloading, not overriding show().

2. Step 2: Understand method call on Parent reference

   Parent reference calls show() with no arguments, but Child has no overriding method, so Parent's method runs.

3. Final Answer:

   Child class does not override show() method correctly -> Option B

4. Quick Check:

   Overriding needs exact method signature match [OK]

Hint: Overriding needs same method signature, not just same name [OK]

Common Mistakes:

• Thinking overloading is overriding
• Expecting Child's show(int) to override show()
• Ignoring method parameters in overriding

5. Given these classes:

class Vehicle {
  void start() { System.out.println("Vehicle starts"); }
}
class Car extends Vehicle {
  void start() { System.out.println("Car starts"); }
}
class Bike extends Vehicle {
  void start() { System.out.println("Bike starts"); }
}
public class Test {
  public static void main(String[] args) {
    Vehicle[] vehicles = {new Car(), new Bike(), new Vehicle()};
    for (Vehicle v : vehicles) {
      v.start();
    }
  }
}

What is the output when this program runs?

hard

A. Vehicle starts
Vehicle starts
Vehicle starts

B. Compilation error due to array initialization

C. Car starts
Vehicle starts
Bike starts

D. Car starts
Bike starts
Vehicle starts

Solution

1. Step 1: Analyze array elements and their types

   Array holds objects: Car, Bike, Vehicle, all as Vehicle references.

2. Step 2: Understand method calls in loop

   Each start() call runs overridden method of actual object type due to runtime polymorphism.

3. Step 3: Determine output lines

   Car prints "Car starts", Bike prints "Bike starts", Vehicle prints "Vehicle starts" in order.

4. Final Answer:

   Car starts Bike starts Vehicle starts -> Option D

5. Quick Check:

   Overridden methods run per object type in array [OK]

Hint: Loop calls overridden methods based on actual object type [OK]

Common Mistakes:

• Expecting all calls to run Vehicle's method
• Confusing array reference type with object type
• Thinking array initialization causes error