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ABall.updateState() is abstract because at the abstraction level of ABall , one only knows that the ball will definitely do something with regards to modifying (perhaps) its internal field values(its "state"). Each subclass will do it differently however. The StraightBall will do nothing in its updateState method because a ball with a constant (unchanging) velocity will travel in a straight line. Remember, doing nothing is doing something ! The CurveBall 's updateState method uses sines and cosines to turn the velocity by a fixed (though randomly chosen) angle at every update event. You can imagine that other possible subclassses could do things such as randomly change the velocity or change the radius of the ball or change the color of the ball.

There is no code in the entire Ballworld system that explicitly references any of the concrete ABall subclasses. All of the code runs at the level of abstraction of an abstract ball. The differences in behavior of the various balls made on the screen using the different concrete subclasses is strictly due to polymorphism . New types of balls can be added to the system without recompiling any of the existing code. In fact, new types of balls can be added without even stopping the Ballworld program!

Abstract classes vs. interfaces

Subclasses have a different relationship between interfaces and abstract superclasses. A subclass that implements an interface is saying simply that it "acts like" that specified by the interface. The class makes no statements however about fundamentally what it actually is. An actor implements a fiercesome alien from a distant plantet in one movie and a fickle feline in another. But an actor is actually neither. Just because the actor protrayed a interplanetary alien, doesn't mean that the actor fundamentally possessed all the abilities of such an alien. All it says is that in so far the context in which the actor was utilized as the alien, the actor did implement all the necessary behaviors of the alien.

A subclass is fundamentally an example of its superclass. A subclass automatically contains all the behaviors of its superclass because it fundamentally is the superclass. The subclass doesn't have to implement the behaviors of its superclass, it already has them. An actor is a human and by that right, automatically possesses all that which makes a human: one head, two arms, 10 toes, etc. Note that this is true even if the abstract class has 100% abstract methods--it still enforces a strict taxonomical hierarchy.

implements is about behaving , extends is about being .

Variant vs. invariant behaviors

A crucial observation is that the the Ballworld code that manages the GUI ( BallGUI ) and the ball management ( BallControl , Dispatcher , etc.) only deal with the abstract ball, ABall . That is, they represent invariant behavior at the abstract ball level. The display, creation and managment of the balls is independent of the particular kinds of concrete balls that is being handled. The main Ballworld framework can thus handle any type of ABall , past, present and future.

Questions & Answers

important of enocomic
Adu Reply
what is division of labour
Dennis Reply
division of labour can be defined as the separation of task to individuals in any economic system to specialize on it.
Ahmad
what is demand curve
Victoria Reply
demand curve is a downward sloping economic graph that shows the relationship between the price of product and the quantity of the product demanded.
Ahmad
What is demand
Frank Reply
It refers to the quantity of a commodity purchased in the market at a price and at a point of time.
Basanta
refers to amount of commodities a consumer is willing and able to buy at particular price within a period of time
Clifford
It is the ability and willingness a customer buys a product or service at a particular price, place and time while other things remaining constant or the same
kum
In which case is opportunity cost is zero
Francis Reply
where no alternative is available
Bhartendu
who is the father of economic
Omar Reply
Adam Smith
Suraj
ok
Tony
Adam Smith
Francis
Adam smith
Opana
Adam Smith
Basanta
What is monopoly
Mauthoor Reply
it an economic situation where one individual controls the essential commodities or value product for maximum profit
James
monopoly is a market situation in which there is only one producer of a good or service which has no close substitutes
eliano
is where only one person is solely the price taker
Francis
what is Monopoly
Dauda Reply
The word Monopoly is a Latin word. it is the combination of two words-Mono means single and Poly means seller. thus Monopoly means single seller. but this is not the full meaning of Monopoly. Monopoly must produce a product which does not have close substitute in the market.
Basanta
Monopoly is define as a firm in an industry with very high barriers to entry.
Favour
If close substitute is available, Monopoly will be a king without a crown.
Basanta
what does it array
Cbdishakur Reply
what are the differences between monopoly and.oligopoly
Onome Reply
what are the difference between monopoly and oligopoly
Cbdishakur
The deference between Monopoly and Oligopoly: Monopoly means:A single-firm-Industry producing and selling a product having no close business and Oligopoly means:A market structure where a few sellers compete with each other and each controls a significant portion of market .
Basanta
so that the price-output policy one affects the other.
Basanta
what are difference between physical policy and monotory policy
hon
what is economic
Emakpor Reply
what is economic
Cbdishakur
the word economic was derived from the Greek word oikos (a house)and mein(to manage) which in effect meant managing a household with the limited funds available 🙂.
Basanta
good excample about scarsity
hon
An Enquiry into the nature and causes of wealth Nations, this book clearly defined what economic is🙂🙂🙏🙏 thank you...
Basanta
good example about scarcity: money,time, energy, human or natural resources. Scarcity of resources implies that there supply is very much limited in relation to demand.
Basanta
equilibrium is a situation in which economic forces such as demand and supply are balanced and in the absence of external influences,the value of economic variables will not change
Onome Reply
hmnn
Emakpor
marginal cost and marginal revenue is equilibrium .
Kho
yessss
Basanta
what is equilibrium
Rodrice Reply
policy prescriptions for unemployment
Jeslyne Reply
Am working on it
Blacks
Study
Janelle
study
simeon
what are the factors effecting demand sedule
Kalimu Reply
we should talk about more important topics, you can search it on Google n u will find your answer we should try to focus on how we can improve our society using economics
shubham
so good night
hon
Why do people buy more grapes in December than in July?
lungi
because at time know money
Adu
Researchers demonstrated that the hippocampus functions in memory processing by creating lesions in the hippocampi of rats, which resulted in ________.
Mapo Reply
The formulation of new memories is sometimes called ________, and the process of bringing up old memories is called ________.
Mapo Reply
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Source:  OpenStax, Principles of object-oriented programming. OpenStax CNX. May 10, 2013 Download for free at http://legacy.cnx.org/content/col10213/1.37
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