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From inheritance to composition

Coming back to Ballworld, we see that the updateState method in ABall is an abstract algorithm to update the state of the ball. So, just as in the pizza example, we can represent this algorithm, and just this algorithm , as an object. We can say that a ball has an algorithm to update its state. Another wa of saying this is to say that the ball has a strategy to update its state. We can represent this by using composition. Instead of having an abstract method to update the state, we model a ball as having a reference to an IUpdateStrategy object. The code for update thus becomes public void update(Observable o, Object g) {_strategy.updateState(this); // update this ball's state using the strategy location.translate (velocity.x, velocity.y); // move the ballbounce(); // bounce the ball off the wall if necessary paint((Graphics) g); // paint the ball onto the container} The ball hands a reference to itself, this , to the strategy so that the strategy knows which ball to update. The variant updating behaviors are now represented by concrete implementations of the IUpdateStrategy interface.

Composition-based ballworld

Ball is now a concrete class and its subclasses have been eliminated in favor of being composed with an abstract strategy.
(Note that the Randomizer class has been redesigned to eliminate its static methods. One new method has been added as well.)

There are a number of very important points to notice about this new formulation:

  • The modified ABall class now contains 100% concrete code and thus should not be abstract anymore.
    • ABall has been renamed to simply Ball .
    • Accessor methods for the strategy ( getStrategy and setStrategy ) have been added.
    • The Ball class is still 100% invariant code.
  • The CurveBall , StraightBall , etc. subclasses are no longer needed as their variant behaviors have been moved to the IUpdateStrategy subclasses.
    • Effectively what has happened is that the updateState method has been moved from the ABall subclasses and embodied into their own classes.
    • The IUpdateStrategy subclasses do not inherit anything from Ball , hence they do not contain any invariant code.
    • The strategies are thus 100% variant code.
  • The reference to the ball during the updating process has gone from a peristent communication link (implicitly, this ), to a transient communication link ( host ).
  • This composition-based model divides the code exactly between the variant and invariant behaviors--this is the key to the power and flexibility it generates.

This new composition-based model of Ballworld is an example of the Strategy Design Pattern . The strategy design pattern allows us to isolate variant behaviors on a much finer level than simple inheritance models.

Composing behaviors

So far, all of our redesigning has resulted in a system that behaves exactly as it did when we started. But what one finds very often in developing systems is that in order to make two steps forward, one must first make one step backwards in order to fundmentally change the direction in which they are going. So, even though it looks like our system has not progressed because it still does exactly the same thing, we are indeed in a very different position, architecturally. By freeing the variant behaviors from the invariant ones, we have generated a tremendous amount of flexibility.

Questions & Answers

show that the set of all natural number form semi group under the composition of addition
Nikhil Reply
explain and give four Example hyperbolic function
Lukman Reply
_3_2_1
felecia
⅗ ⅔½
felecia
_½+⅔-¾
felecia
The denominator of a certain fraction is 9 more than the numerator. If 6 is added to both terms of the fraction, the value of the fraction becomes 2/3. Find the original fraction. 2. The sum of the least and greatest of 3 consecutive integers is 60. What are the valu
SABAL Reply
1. x + 6 2 -------------- = _ x + 9 + 6 3 x + 6 3 ----------- x -- (cross multiply) x + 15 2 3(x + 6) = 2(x + 15) 3x + 18 = 2x + 30 (-2x from both) x + 18 = 30 (-18 from both) x = 12 Test: 12 + 6 18 2 -------------- = --- = --- 12 + 9 + 6 27 3
Pawel
2. (x) + (x + 2) = 60 2x + 2 = 60 2x = 58 x = 29 29, 30, & 31
Pawel
ok
Ifeanyi
on number 2 question How did you got 2x +2
Ifeanyi
combine like terms. x + x + 2 is same as 2x + 2
Pawel
x*x=2
felecia
2+2x=
felecia
Mark and Don are planning to sell each of their marble collections at a garage sale. If Don has 1 more than 3 times the number of marbles Mark has, how many does each boy have to sell if the total number of marbles is 113?
mariel Reply
Mark = x,. Don = 3x + 1 x + 3x + 1 = 113 4x = 112, x = 28 Mark = 28, Don = 85, 28 + 85 = 113
Pawel
how do I set up the problem?
Harshika Reply
what is a solution set?
Harshika
find the subring of gaussian integers?
Rofiqul
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Shirley Reply
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Abdullahi
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Mark
I need quadratic equation link to Alpa Beta
Abdullahi Reply
find the value of 2x=32
Felix Reply
divide by 2 on each side of the equal sign to solve for x
corri
X=16
Michael
Want to review on complex number 1.What are complex number 2.How to solve complex number problems.
Beyan
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Mark
use the y -intercept and slope to sketch the graph of the equation y=6x
Only Reply
how do we prove the quadratic formular
Seidu Reply
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Darius
hello, if you have a question about Algebra 2. I may be able to help. I am an Algebra 2 Teacher
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Opoku
what is math number
Tric Reply
4
Trista
x-2y+3z=-3 2x-y+z=7 -x+3y-z=6
Sidiki Reply
can you teacch how to solve that🙏
Mark
Solve for the first variable in one of the equations, then substitute the result into the other equation. Point For: (6111,4111,−411)(6111,4111,-411) Equation Form: x=6111,y=4111,z=−411x=6111,y=4111,z=-411
Brenna
(61/11,41/11,−4/11)
Brenna
x=61/11 y=41/11 z=−4/11 x=61/11 y=41/11 z=-4/11
Brenna
Need help solving this problem (2/7)^-2
Simone Reply
x+2y-z=7
Sidiki
what is the coefficient of -4×
Mehri Reply
-1
Shedrak
A soccer field is a rectangle 130 meters wide and 110 meters long. The coach asks players to run from one corner to the other corner diagonally across. What is that distance, to the nearest tenths place.
Kimberly Reply
Jeannette has $5 and $10 bills in her wallet. The number of fives is three more than six times the number of tens. Let t represent the number of tens. Write an expression for the number of fives.
August Reply
What is the expressiin for seven less than four times the number of nickels
Leonardo Reply
How do i figure this problem out.
how do you translate this in Algebraic Expressions
linda Reply
why surface tension is zero at critical temperature
Shanjida
I think if critical temperature denote high temperature then a liquid stats boils that time the water stats to evaporate so some moles of h2o to up and due to high temp the bonding break they have low density so it can be a reason
s.
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
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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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