<< Chapter < Page
  Design patterns   Page 1 / 1
Chapter >> Page >
The State Design Pattern models objects that changes state, i.e. change behavior as a result of what has happened to them. This is also called "dynamic reclassification".

Objects are often discussed in terms of having a "state" that describes their exact conditions in a given time, based upon the values of their properties. The particular values of the properties affect the object's behavior. For instance, one can say that the exact behavior of an object's getColor() method is different if the "color" property of the given object is set to "blue" instead of "red" because getColor() returns a different value in the two situations.

Furthermore, the object may make decisions at run time as to exactly what to do dependent upon the values its properties possess. For instance, if the sky is blue (sky.setColor(Color.blue)) , then the sun should be visible.

public boolean sunIsVisible() { if(getColor()==Color.blue) {return true; }else { return false;} }

One issue with the above solution is that it is a hard-coded logic solution, not an architected solution. The sky does not intrinsically behave a certain way if it is blue, but rather it should figure out what to do in that situation.

Wouldn't it be better if the sky intrinsically acted properly if it were blue? One could imagine two objects: a SkyBlue and a SkyNonBlue . The SkyBlue class' sunIsVisible() method would always return true while the SkyNonBlue version would always return false.

What one needs now is the ability for a sky object to dynamically (i.e. at run time) change its class to/from SkyBlue and SkyNonBlue . What we'd like to accomplish is called " dynamic reclassification ".

We've seen code that does change its specific behavior depending on what particular strategy was installed. So, the setColor() method could install a strategy that would always return true if its sunIsVisible() method were to be called.

But does the user of the Sky class care about the stratregy?

Of course not. The user only cares that it does its job.

The state design pattern is a fully encapsulated, self-modifying strategy design pattern.

UML Class Diagram of the State Design Pattern

One design pattern that is used very often in conjunction with the state pattern is the Null Object Pattern .

Notice these things about the pattern:

  1. Any methods whose behaviors depend on the state of the object are simply delegated on in to the state, and handled there. Thus you will see the same methods in the context as in the states. Since the states are separate objects from the context, all the properties of the context need to have accessor methods that are at least package visible.
  2. The " Context " object needs to add a " set " accessor method so the states can modify which state is the active state. This method would be package visible so as to encapsulate the behavior away from the sight of the user.

Questions & Answers

can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
Commplementary angles
Idrissa Reply
im all ears I need to learn
right! what he said ⤴⤴⤴
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
kkk nice
Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
im not good at math so would this help me
Rachael Reply
I'm not good at math so would you help me
what is the problem that i will help you to self with?
how do you translate this in Algebraic Expressions
linda Reply
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
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Design patterns. OpenStax CNX. Jun 04, 2009 Download for free at http://cnx.org/content/col10678/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Design patterns' conversation and receive update notifications?