A PIE

What does it stand for?

Abstraction

We create abstractions to simplify our program domain. They allow us to focus on the problem we are solving rather than getting lost in minute details. You don't need to know how every subsystem in your car works in order to drive, do you?

Think about writing code as the process of creating a language to describe a problem we're solving. Abstractions are the building blocks of this language.

In our code

We are creating abstractions in the small whenever we:

• name a variable
• create (and name) a method
• create (and name) a class
• simplify an attribute by representing it as a number (hunger) or a label ("purple")

This is why naming is so important: we need to accurately convey the abstraction we're describing to those that follow (as well as our later selves).

We apply the other OO principles (PIE) to create meaningful and useful abstractions of more complex concepts.

Polymorphism

the quality or state of existing in or assuming different forms

Polymorphism allows us to represent intent, but allow the implementation to vary as needed based on context.

In our code

Method overloading

A method with the same name that accepts different argument types.

Examples:

• System.out's printLn() methods.
• jUnit's numerous assertEquals methods

Both of these methods accept strings, ints, decimals.

In our code

Method overriding

A method that redefines a method from a superclass is said to override that method. A great example of this are the toString, equals, and hashCode methods from java.lang.Object that we have discussed. (Remember, Object is the superclass of all classes.)

Superclasses and interfaces

We also implement polymorphism by:

• extending a parent class (a String isA Object)
• implementing an interface (an ArrayList isA List isA Collection isA Iterable)

Inheritance

Inheritance is the mechanism whereby a class inherits behavior from a superclass. We call this extending the superclass. Recall that all classes, whether we tell them to or not, implicitly extend Object.

Sometimes we create types (classes) that only exist so that they may be extended. We use the keyword abstract to create these abstract classes, which may also declare abstract methods.

The Java Collections Framework has several great examples of using inheritance and polymorphism: the Collections, Lists, and Maps that you know and love.

In our code

Inheritance is what allows String concatenation to work without us doing anything extra (though perhaps it ain't pretty). Like when we do this:

VirtualPet pet = new VirtualPet();
System.out.println("My pet is " + pet);

What happens behind the scenes is that the toString() method of Object (VirtualPet's parent class) is being called. VirtualPet has inherited this method from its parent.

Encapsulation

Encapsulation at its simplest is hiding away information that isn't necessary to share. The more knowledge we have about an object, the more complex our problem solving becomes.

Knowledge is power. Power corrupts.

In our code

public class Circle {
	private double radius;
	public double getRadius() {
		return radius;
	}
}

The radius instance variable above is private and exposed via an accessor method (getRadius). Not only does this avoid radius being manipulated externally and possibly resulting in an invalid state, but also gives us the flexibility to implement getRadius in a different way if necessary. The code that asks our Circle object for the radius doesn't need to know how the radius is being determined.

This is encapsulation. A more complex example of encapsulation would be an object responsible for calculating sales tax for a transaction. If the formula changes, yet it is encapsulated within the object, other objects requesting the tax calculation need not change. This would also likely involve polymorphism, since sales tax calculations vary by a number of factors, including location and product type.

Gratuitous Picture of Pie