Java: Polymorphism and Abstraction
Polymorphism is a core OOP feature that makes code more flexible and extensible.
1. What is Polymorphism
Polymorphism means the same method behaves differently on different objects.
(1) Benefits of Polymorphism
| Benefit | Description |
|---|---|
| Flexibility | Same interface, different implementations |
| Extensibility | Adding new subclasses doesn't require modifying parent code |
| Simplified code | Use parent class references to operate on child objects |
2. Upcasting
A child object can be assigned to a parent class reference.
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Animal animal = new Dog(); // Upcasting
▶ Example: Upcasting
JAVA
public class Animal {
public void eat() {
System.out.println("Animal is eating");
}
}
public class Dog extends Animal {
@Override
public void eat() {
System.out.println("Dog is chewing a bone");
}
public void bark() {
System.out.println("Barking");
}
}
public class PolymorphismDemo {
public static void main(String[] args) {
Animal animal = new Dog(); // Upcasting
animal.eat(); // Dog is chewing a bone (calls Dog's eat)
// animal.bark(); // Compile error! Parent reference can only access parent methods
}
}
3. Dynamic Binding
At runtime, the method is called based on the actual object type, not the compile-time type.
▶ Example: Dynamic Binding
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public class Animal {
public void speak() {
System.out.println("Animal makes a sound");
}
}
public class Dog extends Animal {
@Override
public void speak() {
System.out.println("Woof woof");
}
}
public class Cat extends Animal {
@Override
public void speak() {
System.out.println("Meow meow");
}
}
public class DynamicBinding {
public static void makeSound(Animal animal) {
animal.speak(); // Decided at runtime which speak to call
}
public static void main(String[] args) {
makeSound(new Dog()); // Woof woof
makeSound(new Cat()); // Meow meow
}
}
4. Abstract Classes
Abstract classes cannot be instantiated—they can only be inherited.
(1) Abstract Methods
Methods without a body that subclasses must implement.
JAVA
public abstract class Shape {
// Abstract method: no body
public abstract double area();
// Regular method: has body
public void display() {
System.out.println("Area: " + area());
}
}
▶ Example: Abstract Class
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public abstract class Shape {
protected String name;
public Shape(String name) {
this.name = name;
}
// Abstract methods
public abstract double area();
public abstract double perimeter();
public void display() {
System.out.println(name + " - Area: " + area() + ", Perimeter: " + perimeter());
}
}
public class Circle extends Shape {
private double radius;
public Circle(double radius) {
super("Circle");
this.radius = radius;
}
@Override
public double area() {
return Math.PI * radius * radius;
}
@Override
public double perimeter() {
return 2 * Math.PI * radius;
}
}
public class Rectangle extends Shape {
private double width, height;
public Rectangle(double width, double height) {
super("Rectangle");
this.width = width;
this.height = height;
}
@Override
public double area() {
return width * height;
}
@Override
public double perimeter() {
return 2 * (width + height);
}
}
public class ShapeDemo {
public static void main(String[] args) {
Shape circle = new Circle(5);
Shape rect = new Rectangle(4, 6);
circle.display(); // Circle - Area: 78.54, Perimeter: 31.42
rect.display(); // Rectangle - Area: 24.0, Perimeter: 20.0
}
}
5. Interfaces
Interfaces define a set of rules that implementing classes must follow.
(1) Interface Definition
JAVA
public interface InterfaceName {
// Constants (default public static final)
int MAX_SIZE = 100;
// Abstract methods (default public abstract)
void method1();
int method2(int a);
}
(2) Interface Implementation
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public class ClassName implements InterfaceName {
@Override
public void method1() {
// Implementation
}
@Override
public int method2(int a) {
return a;
}
}
▶ Example: Interface
JAVA
public interface Flyable {
void fly();
}
public interface Swimmable {
void swim();
}
public class Duck implements Flyable, Swimmable {
@Override
public void fly() {
System.out.println("Duck is flying");
}
@Override
public void swim() {
System.out.println("Duck is swimming");
}
}
public class InterfaceDemo {
public static void main(String[] args) {
Duck duck = new Duck();
duck.fly(); // Duck is flying
duck.swim(); // Duck is swimming
// Interface reference
Flyable flyer = new Duck();
flyer.fly();
}
}
6. Interface vs Abstract Class
| Feature | Interface | Abstract Class |
|---|---|---|
| Keyword | interface |
abstract class |
| Implementation | implements |
extends |
| Multiple implementation | Supported | Not supported |
| Constructors | None | Has constructors |
| Member variables | Constants only | Can have regular variables |
| Methods | Abstract methods (pre-Java 8) | Can have regular methods |
7. Programming to Interfaces
JAVA
public interface Payment {
void pay(double amount);
}
public class Alipay implements Payment {
@Override
public void pay(double amount) {
System.out.println("Alipay payment: $" + amount);
}
}
public class WechatPay implements Payment {
@Override
public void pay(double amount) {
System.out.println("WeChat payment: $" + amount);
}
}
public class PaymentDemo {
// Program to interface
public static void checkout(Payment payment, double amount) {
payment.pay(amount);
}
public static void main(String[] args) {
checkout(new Alipay(), 100); // Alipay payment: $100.0
checkout(new WechatPay(), 200); // WeChat payment: $200.0
}
}
8. ❓ Frequently Asked Questions
Q When should I use an abstract class vs an interface?
A Use abstract classes when you have shared state and behavior. Use interfaces when you only need to define a contract.
Q Can interfaces have method bodies?
A Yes, since Java 8, you can use the
default keyword to define default methods.Q Can I call child-specific methods after upcasting?
A No, you need to downcast (explicit cast) first.
❓ FAQ
Q What is dynamic dispatch?
A The JVM determines which overridden method to call at runtime based on the actual object type.
Q Can I downcast?
A Yes, with explicit casting. Use instanceof to check before downcasting to avoid ClassCastException.
Q What is the difference between overriding and overloading?
A Overriding redefines a parent method with the same signature; overloading has the same name but different parameters.
📖 Summary
- Polymorphism: the same method behaves differently on different objects
- Upcasting: assigning a child object to a parent reference
- Dynamic binding: method called based on actual object type at runtime
- Abstract classes cannot be instantiated; can have abstract and regular methods
- Interfaces define contracts that implementing classes must follow
📝 Exercises
- Animal hierarchy: Define an abstract Animal class with Dog and Cat subclasses, implement polymorphic calls
- Shape calculations: Define a Shape interface, implement Circle and Rectangle, calculate areas
- Payment system: Define a Payment interface, implement multiple payment methods
9. Next Lesson
In the next lesson, we'll learn about Advanced Interfaces and Lambda — interface features and Lambda expressions.
