Abstract Factory Definition Provides one level of interface higher than the factory pattern. It is used to return one o f several factories. Where to use & benefits yCreates families of related or dependent o bjects like Kit. yProvides a class library of products, exposing interface not implementation. yNeeds to isolate concrete classes f rom their super classes. yA system needs independent of how its products are created, composed, and represented. yTry to enforce a constraint. yAn alternative to Facade to hide platform-specifi c classes yEasily extensible to a system or a family yRelated patterns include oFactory method, which is often implemented with an a bstract factory. oSingleton, which is often implemented with an a bstract factory. oPrototype , which is o ften implemented with an abstract factory. oFacade, which is often used with an abstract factory by providing an interface forcreating implementing class. Example Suppose you need to wri te a p rogram to show data in two different places. Let's say from a local or a remote database. You need to make a connection to a database before working on the data. In this case, you have two choices, local or remote. You may use abstract factory design pattern to design the interface in the foll owing wa y: classDataInfo {} interface Local { DataInfo[] loadDB(String filename); }
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Provides one level of interface higher than the factory pattern. It is used to return one of severalfactories.
Where to use & benefits
y Creates families of related or dependent objects like Kit.y Provides a class library of products, exposing interface not implementation.
y Needs to isolate concrete classes from their super classes.y A
system needs independent of how its products are created, composed, and represented.y Try to enforce a constraint.y An alternative to Facade to hide platform-specific classes
y Easily extensible to a system or a familyy R elated patterns include
o Factory method, which is often implemented with an abstract factory.o Singleton, which is often implemented with an abstract factory.
o Prototype, which is often implemented with an abstract factory.o Facade, which is often used with an abstract factory by providing an interface for
creating implementing class.
Example
Suppose you need to write a program to show data in two different places. Let's say from a localor a remote database. You need to make a connection to a database before working on the data.
In this case, you have two choices, local or remote. You may use abstract factory design patternto design the interface in the following way:
Construct a complex object from simple objects step by step.
Where to use & benefits
y Make a complex object by specifying only its type and content. The built object is
shielded from the details of its construction.y Want to decouple the process of building a complex object from the parts that make up
the object.y
Isolate code for construction and representation.y Give you finer control over the construction process.y R elated patterns include
o A bstract Factory, which focuses on the layer over the factory pattern (may besimple or complex), whereas a builder pattern focuses on building a complex
object from other simple objects.o Composite, which is often used to build a complex object.
Example
To build a house, we will take several steps:
y build foundation,
y build frame,y build exterior,
y build interior.
Let's use an abstract class HouseBuilder to define these 4 steps. Any subclass of HouseBuilder will follow these 4 steps to build house (that is to say to implement these 4 methods in the
subclass). Then we use aWork Shop class to force the order of these 4 steps (that is to say thatwe have to build interior after having finished first three steps). The TestBuilder class is used to
test the coordination of these classes and to check the building process.
To fine tune the above example, every do method can be designed as a class. Similar functional
class can be designed once and used by other classes. e.g. Window, Door, Kitchen, etc.
Another example, such as writing a Pizza program. Every gradient can be designed as a class.One pizza at least consists of several gradients. Different pizza has different gradients. A builder
Provides an abstraction or an interface and lets subclass or implementing classes decide whichclass or method should be instantiated or called, based on the conditions or parameters given.
Where to use & benefits
y Connect parallel class hierarchies.y A class wants its subclasses to specify the object.
y A class cannot anticipate its subclasses, which must be created.y A
family of objects needs to be separated by using shared interface.y The code needs to deal with interface, not implemented classes.y Hide concrete classes from the client.
y Factory methods can be parameterized.y The returned object may be either abstract or concrete object.
y Providing hooks for subclasses is more flexible than creating objects directly.y Follow naming conventions to help other developers to recognize the code structure.
y R elated patterns includeo A bstract Factory , which is a layer higher than a factory method.
o Template method, which defines a skeleton of an algorithm to defer some steps tosubclasses or avoid subclasses
o Prototype, which creates a new object by copying an instance, so it reducessubclasses.
o Singleton, which makes a returned factory method unique.
Examples
To illustrate such concept, let's use a simple example. To paint a picture, you may need severalsteps. A shape is an interface. Several implementing classes may be designed in the following
You may use several constructors with different parameters to instantiate the object you want. It
is another way to design with Factory pattern. For example,
class Painting {
...
Painting(Point a, Point b) {
new Line(a, b); //draw a line
}
Painting(Point a, int w, int h) {
new Square(a, w, h); //draw a square
}
Painting(Point a, int r){
new Circle(a, r); //draw a circle
}
...
}
You may use several methods to finish the drawing jobs. It is so-called factory method pattern.for example,
class Painting {
...
Painting(Point a, Point b) {
draw(a, b); //draw a line
}
Painting(Point a, int w, int h) {
draw(a, w, h); //draw a square
}
Painting(Point a, int r){
draw(a, r); //draw a circle
}
...
}
The above draw() methods are overloaded.
Here is a popular example of Factory design pattern. For example, you have several database
storages located in several places. The program working on the database is the same. The user may choose local mode or remote mode. The condition is the choice by the user. You may
design your program with Factory pattern.When the local mode is set, you may instantiate anobject to work on the local database. If the remote mode is set, you may instantiate an objectwhich may have more job to do like remote connection, downloading, etc.
interfaceDatabaseService {
publicDataInfogetDataInfo() throws Exception;
publicFieldInfogetFieldInfo() throws Exception;
public void write(FieldInfo fi) throws Exception;
public void modify(FieldInfo fi) throws Exception;
public void write(FieldInfo fi) throws Exception {...};
public void modify(FieldInfo fi) throws Exception {...};
public void delete(FieldInfo fi) throws Exception {...};
//....
}
classDataManager{
Data data = null;
...
if (local) {
data = new Data(localFile);
...
}if (remote){
data = new Data(connectRemote, databaseFile);
...
}
data.write(someInfo);
data.modify(someInfo);
....
}
To illustrate how to use factory design pattern with class level implementation, here is a real
world example. A company has a website to display testing result from a plain text file.
R ecently, the company purchased a new machine which produces a binary data file, another newmachine on the way, it is possible that one will produce different data file. How to write a system
to deal with such change. The website just needs data to display. Your job is to provide thespecified data format for the website.
Here comes a solution. Use an interface type to converge the different data file format. Thefollowing is a skeleton of implementation.
The prototype is typically used to clone an object, i.e. to make a copy of an object. When anobject is complicated or time consuming to be created , you may take prototype pattern to make
such object cloneable. Assume the Complex class is a complicated, you need to implementCloneable interface and override the clone method(protected Object clone()).
class Complex implements Cloneable {
int[] nums = {1,2,3,4,5};
public Object clone() {
try {
returnsuper.clone();
}catch(CloneNotSupportedExceptioncnse) {
System.out.println(cnse.getMessage());
return null;
}
}
int[] getNums() {
returnnums;
}
}
class Test {static Complex c1 = new Complex();
static Complex makeCopy() {
return (Complex)c1.clone();
}
public static void main(String[] args) {
Complex c1 = makeCopy();
int[] mycopy = c1.getNums();
for(int i = 0; i <mycopy.length; i++)
System.out.print(mycopy[i]);
}
}
C:\ Command Prompt
C:\> java Test12345
Cloning is a shallow copy of the original object. If the cloned object is changed, the original
object will be changed accordingly. See the following alteration.
One instance of a class or one value accessible globally in an application.
Where to use & benefits
y Ensure unique instance by defining class final to prevent cloning.
y May be extensible by the subclass by defining subclass final.y Make a method or a variable public or/and static.
y Access to the instance by the way you provided.y W
ell control the instantiation of a class.y Define one value shared by all instances by making it static.y R elated patterns include
o A bstract factory, which is often used to return unique objects.o Builder , which is used to construct a complex object, whereas a singleton is used
to create a globally accessible object.o Prototype, which is used to copy an object, or create an object from its prototype,
whereas a singleton is used to ensure that only one prototype is guaranteed.
Example
One file system, one window manager, one printer spooler, one Test engine, one Input/Outputsocket and etc.
To design a Singleton class, you may need to make the class final like java.Math, which is not
allowed to subclass, or make a variable or method public and/or static, or make all constructors private to prevent the compiler from creating a default one.
For example, to make a unique remote connection,
final class RemoteConnection {
private Connect con;
private static RemoteConnectionrc = new RemoteConnection(connection);
privateRemoteConnection(Connect c) {
con = c;
....
}
public static RemoteConnectiongetRemoteConnection() {
Ans. Provides a solution for pre-processing and post-processing a request. It allows us todeclaratively apply filters for intercepting requests and responses. For ex. Servlet filters.
Q10. What is Front Controller pattern?
Ans. It manages and handles requests through a centralized code. This could either be through aservlet or a JSP (through a Java Bean). This Controller takes over the common processing which
happens on the presentation tier. The front controller manages content retrieval, security, viewmanagement and retrieval.
Q11. What is View Helper pattern?
Ans. There generally are two parts to any application ± the presentation and the business logics.
The ³View´ is responsible for the output-view formatting whereas ³Helper´ component is
responsible for the business logic. Helper components do content retrieval, validation andadaptation. Helper components generally use Business delegate pattern to access businessclasses.
Q12. What is Composite View pattern?
Ans. This pattern is used for creating aggregate presentations (views) from atomic sub-components. This architecture enables says piecing together of elementary view components
which makes the presentation flexible by allowing personalization and customization.
Q13. What is Service to Worker pattern?
Ans. This is used in larger applications wherein one class is used to process the requests while
the other is used to process the view part. This differentiation is done for maintainability.
Q14. What is Dispatcher View pattern?
Ans. This is similar to Service toWorker pattern except that it is used for smaller applications. Inthis one class is used for both request and view processing.
Q15. What is Business Delegate pattern?
Ans. T
his pattern is used to reduce the coupling between the presentation and business-logic tier.It provides a proxy to the façade from where one could call the business classes or DAO class.
This pattern can be used with Service Locator pattern for improving performance.
Q16. What is Value Object (VO) pattern?
Ans. Value Object is a serializable object which would contain lot of atomic values. These are
normal java classes which may have different constructors (to fill in the value of different data)
and getter methods to get access to these data. VOs are used as a course grained call which gets
lots of data in one go (this reduces remote overhead). The VO is made serializable for it to be
transferred between different tiers within a single remote method invocation.
What is Session Façade pattern?
Ans. This pattern hides the complexity of business components and centralizes the workflow. It provides course-grained interfaces to the clients which reduces the remote method overhead.
This pattern fits well with declarative transactions and security management.
Q18. What is Value Object Assembler pattern?
Ans. This pattern allows for composing a Value Object from different sources which could be
EJBs, DAOs or Java objects.
Q19. What is Value List Handler pattern?
Ans. This pattern provides a sound solution for query execution and results processing.
Q20. What is Service Locator pattern?
Ans. It provides a solution for looking-up, creating and locating services and encapsulating their complexity. It provides a single point of control and it also improves performance.
Q21. What is Data Access Object pattern?
Ans. It provides a flexible and transparent access to the data, abstracts the data sources and hides
the complexity of Data persistence layer. This pattern provides for loose coupling between business and data persistence layer.
Q22. What is EJB Command pattern?
Ans. Session Façade and EJB Command patterns are competitor patterns. It wraps business logicin command beans, decouples the client and business logic tier, and reduces the number of
remote method invocations.
Q23. What is Version Number pattern?
Ans. This pattern is used for transaction and persistence and provides a solution for maintainingconsistency and protects against concurrency. Every time a data is fetched from the database, itcomes out with a version number which is saved in the database. Once any update is requested
on the same row of the database, this version is checked. If the version is same, the update isallowed else not.
Q24. What all patterns are used to improve performance and scalability of the application?