README.md

Session 6

Table of Contents

1. Abstract Classes
2. Interfaces
3. Dynamic Method Dispatch
4. Assignments
   1. HackerRank
   2. Miscellaneous

Abstract Classes

Abstract classes are classes that are declared abstract, and may or may not have abstract methods.

Important points:

1. Abstract classes cannot be instantiated, only subclassed.
2. An abstract method is a method with the abstract modifier, and no implementation.
3. If you extend an abstract class, but don't override ALL the abstract methods, then your class MUST be abstract as well.

abstract class BankAccount {
	abstract void deposit(double amount);
	abstract void withdraw(double amount);
	abstract void getBalance();

	// A default implementation since 4% is a standard interest rate
	double getInterestRate() {
		return 4.0;
	}
}

class UCUCUBankAccount extends BankAccount {
	private double balance;

	void deposit(double amount) {
		if (amount < 1000) {
			System.out.println("Cannot deposit less than 1000");
		}
		else {
			balance += amount;
		}
	}

	void withdraw(double amount) {
		// Ugh, 1% transaction charge
		balance = balance - amount - 0.01 * amount;
	}

	void getBalance() {
		return balance;
	}

	// Does not override getInterestRate, gives the usual interest rate of 4%
}

class LenaBankAccount extends BankAccount {
	private double balance;

	void deposit(double amount) {
		// More lenient :)
		if (amount < 200) {
			System.out.println("Cannot deposit less than 200");
		}
		else {
			balance += amount;
		}
	}

	void withdraw(double amount) {
		// No transaction charge :)
		balance = balance - amount;
	}

	void getBalance() {
		return balance;
	}

	// Override getInterestRate to get a better interest rate
	double getInterestRate() {
		return 6.0;
	}
}

An excellent resource is the Oracle Javadoc for abstract classes

Interfaces

An interface establishes a contract between software developers developing two separate pieces of software. It allows a developer to use a library without worrying about the internal implementation.

Important points:

1. All methods in an interface are implicitly public and abstract.
2. All variables in an interface are implicitly public, static and final.
3. A class can implement one or more interfaces.

interface Shape {
	// This is effectively a constant
	// Since it is final and static
	int NO_OF_DIMENSIONS = 2;

	double getArea();
	double getPerimeter();
}

class Circle implements Shape {
	private int radius;

	public Circle(int radius) {
		this.radius = radius;
	}

	double getArea() {
		return Math.PI * radius * radius;
	}

	double getPerimeter() {
		return 2 * Math.PI * radius;
	}
}

class Square implements Shape {
	private int length;

	public Square(int length) {
		this.length = length;
	}

	double getArea() {
		return length * length;
	}

	double getPerimeter() {
		return 4 * length;
	}
}

class VolumeCalculator {
	// This accepts a Shape s
	// And since every Shape subclass MUST override the getArea method
	// This can calculate the volume of any uniform solid, like a cylinder or cuboid
	// It doesn't care what the actual type of shape is, i.e.
	// It doesn't care whether it is given a circle or square or whatever else
	// As long as it implements the Shape interface
	public static double getVolume(Shape s, double h) {
		return s.getArea() * h;
	}

	public static void main(String[] args) {
		Circle c = new Circle(10);
		System.out.println(getVolume(c, 10));

		Square s = new Square(10);
		System.out.println(getVolume(s, 10));
	}
}

Again, an excellent resource is the Oracle Javadoc for interfaces

Dynamic Method Dispatch

Dynamic Method Dispatch is a mechanism by which a call to an overridden method on a parent reference is resolved at runtime.

For instance, simplifying the above example, we have:

class DmdExample {
	public static void main(String[] args) {
		Shape c = new Circle(10);
		System.out.println(c.getArea());

		Shape s = new Square(10);
		System.out.println(s.getArea());
	}
}

As you can see, the reference is of the parent Shape, and the instance is of the child Circle or Square. When the method getArea is called, the runtime checks for an overridden method in the instance.

This now clarifies the two types of polymorphism and their implementations in Java:

1. Static Polymorphism = Overloading (Decided at Compile time )
2. Dynamic Polymorphism = Overriding (Decided at Runtime)

Assignments

HackerRank

1. https://www.hackerrank.com/challenges/game-of-stones-1
2. https://www.hackerrank.com/challenges/equality-in-an-array
3. https://www.hackerrank.com/challenges/picking-numbers
4. https://www.hackerrank.com/challenges/beautiful-triplets
5. https://www.hackerrank.com/challenges/missing-numbers
6. https://www.hackerrank.com/challenges/sam-and-substrings
7. https://www.hackerrank.com/challenges/the-birthday-bar
8. https://www.hackerrank.com/challenges/electronics-shop