(B) What are design patterns?
(A) Can you explain factory pattern?
(I) Can you explain abstract factory pattern?
(I)Can you explain builder pattern?
(I) Can you explain prototype pattern?
(A) Can you explain shallow copy and deep copy in prototype patterns?
(B) Can you explain singleton pattern?
(A) Can you explain command patterns?
Other Interview question PDF's
Hi friends , Please do not think you get an architecture position by reading interview
questions. But yes there should be some kind of reference which will help you
quickly revise what are the definition. Just by reading these answers you get
to a position where you are aware of the fundamentals. But if you have not really
worked you will surely fail with scenario based questions. So use this as a
quick revision rather than a shot cut.
To give you a practical understanding i have put all these design patterns in a video
format and uploaded on http://www.questpond.com/FreeDesign1.htm . You can visit http://www.questpond.com and download the complete architecture interview questions PDF which covers SOA
, UML , Design patterns , Togaf , OOPs etc.
I am trying to get familiar with the egghead cafe editor so i am uploading questions
one by one. Please cope up with me.
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
• 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).
• 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)
• 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.
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.
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
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.
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
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’.
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
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.
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.
Figure: - Client, interface and abstract factory
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).
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.
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.
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.
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
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.
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
Figure :- BYREf
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.
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.
Figure: - Deep cloning in action
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.
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.
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, 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.
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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