Runtime polymorphism in Java

Introduction

Runtime polymorphism lets a program decide which method to use while it is running. This helps make programs flexible and easy to change.

When you want different objects to respond differently to the same method call.

When you want to write code that works with a general type but behaves differently for specific types.

When you want to change or extend behavior without changing existing code.

When you want to use a common interface but different implementations.

When you want to simplify code by using method overriding.

Syntax

Java

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

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

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

The method to call is decided at runtime based on the actual object type.

Use method overriding to achieve runtime polymorphism.

Examples

Different objects (Dog, Cat) respond differently to the same method call sound().

Java

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

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

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

public class Test {
    public static void main(String[] args) {
        Animal a1 = new Dog();
        Animal a2 = new Cat();
        a1.sound();
        a2.sound();
    }
}

The run() method of Bike is called even though the reference is of type Vehicle.

Java

class Vehicle {
    void run() {
        System.out.println("Vehicle is running");
    }
}

class Bike extends Vehicle {
    @Override
    void run() {
        System.out.println("Bike is running fast");
    }
}

public class Demo {
    public static void main(String[] args) {
        Vehicle v = new Bike();
        v.run();
    }
}

Sample Program

This program shows how the method greet() behaves differently depending on the actual object type at runtime.

Java

class Parent {
    void greet() {
        System.out.println("Hello from Parent");
    }
}

class Child extends Parent {
    @Override
    void greet() {
        System.out.println("Hello from Child");
    }
}

public class RuntimePolymorphismDemo {
    public static void main(String[] args) {
        Parent p = new Parent();
        Parent c = new Child();

        p.greet();  // Calls Parent's greet
        c.greet();  // Calls Child's greet because of runtime polymorphism
    }
}

Important Notes

Runtime polymorphism requires inheritance and method overriding.

The reference type decides what methods are accessible, but the object type decides which method runs.

Final, static, and private methods cannot be overridden, so they do not support runtime polymorphism.

Summary

Runtime polymorphism lets a program choose the right method to run while it is running.

It is done using method overriding in inheritance.

This helps write flexible and reusable code.

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