What are design patterns?
Design patterns are documented tried and tested solutions for recurring problems in a given context. So basically
you have a problem context and the proposed solution for the same. Design patterns existed in some or other form
right from the inception stage of software development. Let’s say if you want to implement a sorting algorithm
the first thing comes to mind is bubble sort. So the problem is sorting and solution is bubble sort. Same holds true
for design patterns.
(I) Which are the three main categories of design patterns?
There are three basic classifications of patterns Creational, Structural, and Behavioral patterns.
Creational Patterns
• Abstract Factory:- Creates an instance of several families of classes
• Builder: - Separates object construction from its representation
• Factory Method:- Creates an instance of several derived classes
• Prototype:- A fully initialized instance to be copied or cloned
• Singleton:- A class in which only a single instance can exist
Note: - The best way to remember Creational pattern is by ABFPS (Abraham Became First President of
States).
Structural Patterns
• Adapter:-Match interfaces of different classes.
• Bridge:-Separates an object’s abstraction from its implementation.
• Composite:-A tree structure of simple and composite objects.
• Decorator:-Add responsibilities to objects dynamically.
• Façade:-A single class that represents an entire subsystem.
• Flyweight:-A fine-grained instance used for efficient sharing.
• Proxy:-An object representing another object.
Note : To remember structural pattern best is (ABCDFFP)
Behavioral Patterns
• Mediator:-Defines simplified communication between classes.
• Memento:-Capture and restore an object's internal state.
• Interpreter:- A way to include language elements in a program.
• Iterator:-Sequentially access the elements of a collection.
• Chain of Resp: - A way of passing a request between a chain of objects.
• Command:-Encapsulate a command request as an object.
• State:-Alter an object's behavior when its state changes.
• Strategy:-Encapsulates an algorithm inside a class.
• Observer: - A way of notifying change to a number of classes.
• Template Method:-Defer the exact steps of an algorithm to a subclass.
• Visitor:-Defines a new operation to a class without change.
Note: - Just remember Music....... 2 MICS On TV (MMIICCSSOTV).
Note :- In the further section we will be covering all the above design patterns in a more detail manner.
(A) Can you explain factory pattern?
· Factory pattern is one of the types of creational patterns. You can make out from the name factory itself it’s
meant to construct and create something. In software architecture world factory pattern is meant to
centralize creation of objects. Below is a code snippet of a client which has different types of invoices.
These invoices are created depending on the invoice type specified by the client. There are two issues with
the code below:-
· First we have lots of ‘new’ keyword scattered in the client. In other ways the client is loaded with lot of
object creational activities which can make the client logic very complicated.
Second issue is that the client needs to be aware of all types of invoices. So if we are adding one more
invoice class type called as ‘InvoiceWithFooter’ we need to reference the new class in the client and
recompile the client also.
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Figure: - Different types of invoice
Taking these issues as our base we will now look in to how factory pattern can help us solve the same. Below
figure ‘Factory Pattern’ shows two concrete classes ‘ClsInvoiceWithHeader’ and ‘ClsInvoiceWithOutHeader’.
The first issue was that these classes are in direct contact with client which leads to lot of ‘new’ keyword
scattered in the client code. This is removed by introducing a new class ‘ClsFactoryInvoice’ which does all the
creation of objects.
The second issue was that the client code is aware of both the concrete classes i.e. ‘ClsInvoiceWithHeader’ and
‘ClsInvoiceWithOutHeader’. This leads to recompiling of the client code when we add new invoice types. For
instance if we add ‘ClsInvoiceWithFooter’ client code needs to be changed and recompiled accordingly. To
remove this issue we have introduced a common interface ‘IInvoice’. Both the concrete classes
‘ClsInvoiceWithHeader’ and ‘ClsInvoiceWithOutHeader’ inherit and implement the ‘IInvoice’ interface.
The client references only the ‘IInvoice’ interface which results in zero connection between client and the
concrete classes ( ‘ClsInvoiceWithHeader’ and ‘ClsInvoiceWithOutHeader’). So now if we add new concrete
invoice class we do not need to change any thing at the client side.
In one line the creation of objects is taken care by ‘ClsFactoryInvoice’ and the client disconnection from the
concrete classes is taken care by ‘IInvoice’ interface.
Figure: - Factory pattern
Below are the code snippets of how actually factory pattern can be implemented in C#. In order to avoid
recompiling the client we have introduced the invoice interface ‘IInvoice’. Both the concrete classes
‘ClsInvoiceWithOutHeaders’ and ‘ClsInvoiceWithHeader’ inherit and implement the ‘IInvoice’ interface.
Figure :- Interface and concrete classes
We have also introduced an extra class ‘ClsFactoryInvoice’ with a function ‘getInvoice()’ which will generate
objects of both the invoices depending on ‘intInvoiceType’ value. In short we have centralized the logic of object
creation in the ‘ClsFactoryInvoice’. The client calls the ‘getInvoice’ function to generate the invoice classes. One
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of the most important points to be noted is that client only refers to ‘IInvoice’ type and the factory class
‘ClsFactoryInvoice’ also gives the same type of reference. This helps the client to be complete detached from the
concrete classes, so now when we add new classes and invoice types we do not need to recompile the client.
Figure: - Factory class which generates objects
Note :- The above example is given in C# . Even if you are from some other technology you can still map the
concept accordingly. You can get source code from the CD in ‘FactoryPattern’ folder.
(I) Can you explain abstract factory pattern?
Abstract factory expands on the basic factory pattern. Abstract factory helps us to unite similar factory pattern
classes in to one unified interface. So basically all the common factory patterns now inherit from a common
abstract factory class which unifies them in a common class. All other things related to factory pattern remain
same as discussed in the previous question.
A factory class helps us to centralize the creation of classes and types. Abstract factory helps us to bring
uniformity between related factory patterns which leads more simplified interface for the client.
Figure: - Abstract factory unifies related factory patterns
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Now that we know the basic lets try to understand the details of how abstract factory patterns are actually
implemented. As said previously we have the factory pattern classes (factory1 and factory2) tied up to a common
abstract factory (AbstractFactory Interface) via inheritance. Factory classes stand on the top of concrete classes
which are again derived from common interface. For instance in figure ‘Implementation of abstract factory’ both
the concrete classes ‘product1’ and ‘product2’ inherits from one interface i.e. ‘common’. The client who wants to
use the concrete class will only interact with the abstract factory and the common interface from which the
concrete classes inherit.
Figure: - Implementation of abstract factory
Now let’s have a look at how we can practically implement abstract factory in actual code. We have scenario
where we have UI creational activities for textboxes and buttons through their own centralized factory classes
‘ClsFactoryButton’ and ‘ClsFactoryText’. Both these classes inherit from common interface ‘InterfaceRender’.
Both the factories ‘ClsFactoryButton’ and ‘ClsFactoryText’ inherits from the common factory
‘ClsAbstractFactory’. Figure ‘Example for AbstractFactory’ shows how these classes are arranged and the client
code for the same. One of the important points to be noted about the client code is that it does not interact with the
concrete classes. For object creation it uses the abstract factory ( ClsAbstractFactory ) and for calling the concrete
class implementation it calls the methods via the interface ‘InterfaceRender’. So the ‘ClsAbstractFactory’ class
provides a common interface for both factories ‘ClsFactoryButton’ and ‘ClsFactoryText’.
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Figure: - Example for abstract factory
Note: - We have provided a code sample in C# in the ‘AbstractFactory’ folder. People who are from different
technology can compare easily the implementation in their own language.
We will just run through the sample code for abstract factory. Below code snippet ‘Abstract factory and factory
code snippet’ shows how the factory pattern classes inherit from abstract factory.
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Figure: - Abstract factory and factory code snippet
Figure ‘Common Interface for concrete classes’ how the concrete classes inherits from a common interface
‘InterFaceRender’ which enforces the method ‘render’ in all the concrete classes.
Figure: - Common interface for concrete classes
The final thing is the client code which uses the interface ‘InterfaceRender’ and abstract factory
‘ClsAbstractFactory’ to call and create the objects. One of the important points about the code is that it is
completely isolated from the concrete classes. Due to this any changes in concrete classes like adding and
removing concrete classes does not need client level changes.
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Figure: - Client, interface and abstract factory
(I)Can you explain builder pattern?
Builder falls under the type of creational pattern category. Builder pattern helps us to separate the construction of
a complex object from its representation so that the same construction process can create different
representations. Builder pattern is useful when the construction of the object is very complex. The main objective
is to separate the construction of objects and their representations. If we are able to separate the construction and
representation, we can then get many representations from the same construction.
Figure: - Builder concept
To understand what we mean by construction and representation lets take the example of the below ‘Tea
preparation’ sequence.
You can see from the figure ‘Tea preparation’ from the same preparation steps we can get three representation of
tea’s (i.e. Tea with out sugar, tea with sugar / milk and tea with out milk).
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Figure: - Tea preparation
Now let’s take a real time example in software world to see how builder can separate the complex creation and its
representation. Consider we have application where we need the same report to be displayed in either ‘PDF’ or
‘EXCEL’ format. Figure ‘Request a report’ shows the series of steps to achieve the same. Depending on report
type a new report is created, report type is set, headers and footers of the report are set and finally we get the
report for display.
Figure: - Request a report
Now let’s take a different view of the problem as shown in figure ‘Different View’. The same flow defined in
‘Request a report’ is now analyzed in representations and common construction. The construction process is same
for both the types of reports but they result in different representations.
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Figure: - Different View
We will take the same report problem and try to solve the same using builder patterns. There are three main parts
when you want to implement builder patterns.
• Builder: - Builder is responsible for defining the construction process for individual parts. Builder has those
individual processes to initialize and configure the product.
• Director: - Director takes those individual processes from the builder and defines the sequence to build the
product.
• Product: - Product is the final object which is produced from the builder and director coordination.
First let’s have a look at the builder class hierarchy. We have a abstract class called as ‘ReportBuilder’ from
which custom builders like ‘ReportPDF’ builder and ‘ReportEXCEL’ builder will be built.
Figure: - Builder class hierarchy
Figure ‘Builder classes in actual code’ shows the methods of the classes. To generate report we need to first
Create a new report, set the report type (to EXCEL or PDF) , set report headers , set the report footers and finally
get the report. We have defined two custom builders one for ‘PDF’ (ReportPDF) and other for ‘EXCEL’
(ReportExcel). These two custom builders define there own process according to the report type.
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Figure: - Builder classes in actual code
Now let’s understand how director will work. Class ‘clsDirector’ takes the builder and calls the individual method
process in a sequential manner. So director is like a driver who takes all the individual processes and calls them in
sequential manner to generate the final product, which is the report in this case. Figure ‘Director in action’ shows
how the method ‘MakeReport’ calls the individual process to generate the report product by PDF or EXCEL.
Figure: - Director in action
The third component in the builder is the product which is nothing but the report class in this case.
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Figure: - The report class
Now let’s take a top view of the builder project. Figure ‘Client,builder,director and product’ shows how they
work to achieve the builder pattern. Client creates the object of the director class and passes the appropriate
builder to initialize the product. Depending on the builder the product is initialized/created and finally sent to the
client.
Figure: - Client, builder, director and product
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The output is something like this. We can see two report types displayed with their headers according to the
builder.
Figure: - Final output of builder
Note :- In CD we have provided the above code in C# in ‘BuilderPattern’ folder.
Can you explain prototype pattern?
Prototype pattern falls in the section of creational pattern. It gives us a way to create new objects from the
existing instance of the object. In one sentence we clone the existing object with its data. By cloning any changes
to the cloned object does not affect the original object value. If you are thinking by just setting objects we can get
a clone then you have mistaken it. By setting one object to other object we set the reference of object BYREF. So
changing the new object also changed the original object. To understand the BYREF fundamental more clearly
consider the figure ‘BYREF’ below. Following is the sequence of the below code:-
• In the first step we have created the first object i.e. obj1 from class1.
• In the second step we have created the second object i.e. obj2 from class1.
• In the third step we set the values of the old object i.e. obj1 to ‘old value’.
• In the fourth step we set the obj1 to obj2.
• In the fifth step we change the obj2 value.
• Now we display both the values and we have found that both the objects have the new value.
Figure :- BYREf
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The conclusion of the above example is that objects when set to other objects are set BYREF. So changing new
object values also changes the old object value.
There are many instances when we want the new copy object changes should not affect the old object. The
answer to this is prototype patterns.
Lets look how we can achieve the same using C#. In the below figure ‘Prototype in action’ we have the customer
class ‘ClsCustomer’ which needs to be cloned. This can be achieved in C# my using the ‘MemberWiseClone’
method. In JAVA we have the ‘Clone’ method to achieve the same. In the same code we have also shown the
client code. We have created two objects of the customer class ‘obj1’ and ‘obj2’. Any changes to ‘obj2’ will not
affect ‘obj1’ as it’s a complete cloned copy.
Figure: - Prototype in action
Note :- You can get the above sample in the CD in ‘Prototype’ folder. In C# we use the ‘MemberWiseClone’
function while in JAVA we have the ‘Clone’ function to achieve the same.
(A) Can you explain shallow copy and deep copy in prototype patterns?
There are two types of cloning for prototype patterns. One is the shallow cloning which you have just read in the
first question. In shallow copy only that object is cloned, any objects containing in that object is not cloned. For
instance consider the figure ‘Deep cloning in action’ we have a customer class and we have an address class
aggregated inside the customer class. ‘MemberWiseClone’ will only clone the customer class ‘ClsCustomer’ but
not the ‘ClsAddress’ class. So we added the ‘MemberWiseClone’ function in the address class also. Now when
we call the ‘getClone’ function we call the parent cloning function and also the child cloning function, which
leads to cloning of the complete object. When the parent objects are cloned with their containing objects it’s
called as deep cloning and when only the parent is clones its termed as shallow cloning.
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Figure: - Deep cloning in action
(B) Can you explain singleton pattern?
There are situations in a project where we want only one instance of the object to be created and shared between
the clients. No client can create an instance of the object from outside. There is only one instance of the class
which is shared across the clients. Below are the steps to make a singleton pattern:-
1) Define the constructor as private.
2) Define the instances and methods as static.
Below is a code snippet of a singleton in C#. We have defined the constructor as private, defined all the instance
and methods using the static keyword as shown in the below code snippet figure ‘Singleton in action’. The static
keyword ensures that you only one instance of the object is created and you can all the methods of the class with
out creating the object. As we have made the constructor private, we need to call the class directly.
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Figure: - Singleton in action
Note :- In JAVA to create singleton classes we use the STATIC keyword , so its same as in C#. You can get a
sample C# code for singleton in the ‘singleton’ folder.
(A) Can you explain command patterns?
Command pattern allows a request to exist as an object. Ok let’s understand what it means. Consider the figure
‘Menu and Commands’ we have different actions depending on which menu is clicked. So depending on which
menu is clicked we have passed a string which will have the action text in the action string. Depending on the
action string we will execute the action. The bad thing about the code is it has lot of ‘IF’ condition which makes
the coding more cryptic.
Figure: - Menu and Commands
Command pattern moves the above action in to objects. These objects when executed actually execute the
command.
As said previously every command is an object. We first prepare individual classes for every action i.e. exit,
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open, file and print. Al l the above actions are wrapped in to classes like Exit action is wrapped in
‘clsExecuteExit’ , open action is wrapped in ‘clsExecuteOpen’, print action is wrapped in ‘clsExecutePrint’ and
so on. All these classes are inherited from a common interface ‘IExecute’.
Figure: - Objects and Command
Using all the action classes we can now make the invoker. The main work of invoker is to map the action with the
classes which have the action.
So we have added all the actions in one collection i.e. the arraylist. We have exposed a method ‘getCommand’
which takes a string and gives back the abstract object ‘IExecute’. The client code is now neat and clean. All the
‘IF’ conditions are now moved to the ‘clsInvoker’ class.
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Figure: - Invoker and the clean client
Note: - You can find a sample code for C# code in command pattern in ‘Command’ folder.
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