Design Patterns Interview Questions

Design Patterns Interview Questions




1) What is a software design pattern?
A design pattern is a solution to a general software problem within a particular context.
• Context : A recurring set of situations where the pattern applies.
• Problem : A system of forces (goals and constraints) that occur repeatedly in this
context.
• Solution : A description of communicating objects and classes (collaboration) that
can be applied to resolve those forces.
Design patterns capture solutions that have evolved over time as developers strive for
greater flexibility in their software. Whereas class libraries are reusable source code, and
components are reusable packaged objects, patterns are generic, reusable design
descriptions that are customized to solve a specific problem. The study of design patterns
provides a common vocabulary for communication and documentation, and it provides a
framework for evolution and improvement of existing patterns.



2) Why is the study of patterns important?
As initial software designs are implemented and deployed, programmers often discover
improvements which make the designs more adaptable to change. Design patterns capture
solutions that have evolved over time as developers strive for greater flexibility in their
software, and they document the solutions in a way which facilitates their reuse in other,
possibly unrelated systems. Design patterns allow us to reuse the knowledge of experienced
software designers.
Moreover, the study of design patterns provides a common vocabulary for communication
and documentation, and it provides a framework for evolution and improvement of existing
patterns. As an analogy, consider that during a discussion among programmers, the words
“stack” and “tree” can be used freely without explanation. Software developers understand
fundamental data structures such as a “stack” because these data structures are welldocumented in textbooks and are taught in computer science courses. The study of design
patterns will have a similar (but more profound) effect by allowing designers to say
“composite pattern” or “observer pattern” in a particular design context, without having to
describe all classes, relationships, and collaborations which make up the pattern. Patterns
raise the level of abstraction when discussing and documenting software designs.
3) How do I document a design pattern?
A pattern description must address the following major points:
• Pattern Name and Classification : A short, meaningful name for the pattern,
usually only one or two words. Names provide a vocabulary for patterns, and they
have implied semantics – choose names carefully. Following the GoF book, we can
also group patterns into higher level classifications such as creational, structural,
and behavioral patterns.
• Problem : A general description of the problem context and the goals and
constraints that occur repeatedly in that context. A concrete motivational scenario
can be used to help describe the problem. The problem description should provide
guidance to assist others in recognizing situations where the pattern can be applied.• Solution : The classes and/or objects that participate in the design pattern, their
structure (e.g., in terms of a UML class diagram), their responsibilities, and their
collaborations. The solution provides an abstract description that can be applied in
many different situations. Sample Code in an object-oriented language can be used
to illustrate a concrete realization of the pattern.
• Consequences : A discussion of the results and tradeoffs of applying the pattern.
Variations and language-dependent alternatives should also be addressed.
• Known Uses : Examples of the pattern in real systems. Look for applications of the
pattern in language libraries and frameworks, published system descriptions, text
books, etc. Not every good solution represents a pattern. A general rule of thumb is
that a candidate pattern (also called a “proto-pattern”) should be discovered in a
minimum of three existing systems before it can rightfully be called a pattern.
The following quote by Robert Martin highlights the importance of providing pattern
descriptions: “The revolutionary concept of the GoF book is not the fact that there are
patterns; it is the way in which those patterns are documented. ... Prior to the GoF book,
the only good way to learn patterns was to discover them in design documentation, or
(more probably) code.”
4) Where can I learn more about design patterns?
The best place to start is the seminal work by Erich Gamma, Richard Helm, Ralph Johnson,
and John Vlissides (collectively known as the “Gang of Four” or simply “GoF”) entitled
Design Patterns: Elements of Reusable Object-Oriented Software (Addison-Wesley, 1995).
Warning: This book is not light reading. From the Preface: “Don't worry if you don't
understand this book completely on the first reading. We didn't understand it all on the first
writing.”
It is, however, a book which wears well over time, and it is definitely worth the effort
required to work through it.
Beyond the GoF book, consider the list of references in the Design Patterns section of this
bibliography on object technology, plus the following web links:
• Design Patterns Home Page
• Huston Design Patterns
• Brad Appleton's Software Patterns Links
• Cetus Patterns Links
5) What is an example of a design pattern?
Following the lead of the “Gang of Four” (GoF), design pattern descriptions usually contain
multiple sections including
• Intent
• Motivation
• Applicability
• Structure
• Participants
• Collaborations
• Consequences
• Implementation
• Sample Code
• Known Uses
• Related Patterns
A complete discussion of even a small pattern is beyond the scope of a simple FAQ entry,
but it is possible to get the idea by examining an abbreviated discussion of one of thesimplest and most easily understood patterns. Consider the Singleton pattern, whose intent
reads as follows:
Intent: Ensure that a class has one instance, and provide a global point of access to it.
Almost every programmer has encountered this problem and formulated an approach for
solving it in a general way – some solutions are better than others. The solution offered by
the GoF would look something like the following when coded in Java.
public class Singleton
{
private static Singleton instance = null;
public static Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}
protected Singleton() { ... }
// possibly another constructor form
public void someMethod() { ... }
//... other methods
}
The programmer would access the single instance of this class by writing something similar
to
Singleton.getInstance().someMethod()
or similar to
Singleton s = Singleton.getInstance();
s.method1();
...
s.method2();
...
For a more complete discussion of the Singleton pattern, see the chapter “Singleton” in the
book Design Patterns: Elements of Reusable Object-Oriented Software by the “Gang of
Four” (Addison-Wesley, 1995), or the chapter “Singleton” in the book Patterns in Java,
Volume 1 by Mark Grand (John Wiley & Sons, 1998). For information about variations on
the Singleton Pattern, see the chapter entitled “To Kill a Singleton” in the book Pattern
Hatching: Design Patterns Applied by John Vlissides or the article “Implementing the
Singleton Pattern in Java” by Rod Waldhoff.6) Calendar is an abstract class. The getInstance() method
tries to instantiate GregorianCalendar() i.e., parent
instantiating a derived class. This looks Non-OO? Ex:
Calendar a=Calendar.getInstance(); Can somebody explain
why is it so?
The Calender class is an abstact class, true, however,the point you missed is that the
getInstance() returns the " Calendar using the default timezone and locale. " , in your case,
the GregorianCalender a class that IS a Calender (a Suzuki IS a car, a 747 IS a plane..the
standard OO terminology. So what you get is a class that does some specialized work based
on the default locale. Other methods
public static synchronized Calendar getInstance(TimeZone zone,Locale aLocale)
public static synchronized Calendar getInstance(TimeZone zone)
return Calenders for specific timezones and locales. The closest parallel is possibly the
Factory Method design pattern.
7) What major patterns do the Java APIs utilize?
Design patterns are used and supported extensively throughout the Java APIs. Here are
some examples:
• The Model-View-Controller design pattern is used extensively throughout the Swing
API.
• The getInstance() method in java.util.Calendar is an example of a simple form of
the Factory Method design pattern.
• The classes java.lang.System and java.sql.DriverManager are examples of the
Singleton pattern, although they are not implemented using the approach
recommended in the GoF book but with static methods.
• The Prototype pattern is supported in Java through the clone() method defined in
class Object and the use of java.lang.Cloneable interface to grant permission for
cloning.
• The Java Swing classes support the Command pattern by providing an Action
interface and an AbstractAction class.
• The Java 1.1 event model is based on the observer pattern. In addition, the
interface java.util.Observable and the class java.util.Observer provide support for
this pattern.
• The Adapter pattern is used extensively by the adapter classes in java.awt.event.
• The Proxy pattern is used extensively in the implementation of Java's Remote
Method Invocation (RMI) and Interface Definition Language (IDL) features.
• The structure of Component and Container classes in java.awt provide a good
example of the Composite pattern.
• The Bridge pattern can be found in the separation of the components in java.awt
(e.g., Button and List), and their counterparts in java.awt.peer.8) How can I make sure at most one instance of my class
is ever created?
This is an instance where the Singleton design pattern would be used. You need to make
the constructor private (so nobody can create an instance) and provide a static method to
get the sole instance, where the first time the instance is retrieved it is created:
public class Mine {
private static Mine singleton;
private Mine() {
}
public static synchronized Mine getInstance() {
if (singleton == null) {
singleton = new Mine();
}
return singleton;
}
// other stuff...
}
9) When would I use the delegation pattern instead of
inheritence to extend a class's behavior?
Both delegation and inheritance are important concepts in object-oriented software design,
but not everyone would label them as patterns. In particular, the seminal book on design
patterns by the “Gang of Four” contains a discussion of inheritance and delegation, but the
authors do not treat these topics as specific patterns. It is reasonable to think of them as
design concepts which are more general than specific design patterns.
Inheritance is a relationship between two classes where one class, called a subclass in this
context, inherits the attributes and operations of another class, called its superclass.
Inheritance can be a powerful design/reuse technique, especially when it is used in the
context of the Liskov Substitution Principle. (The article by Robert Martin at
http://www.objectmentor.com/publications/lsp.pdf provides an excellent explanation of the
ideas behind Barbara Liskov’s original paper on using inheritance correctly.) The primary
advantages of inheritance are
1. it is directly supported by object-oriented languages, and
2. it provides the context for polymorphism in strongly-typed object-oriented
languages such as C++ and Java.
But since the inheritance relationship is defined at compile-time, a class can’t change its
superclass dynamically during program execution. Moreover, modifications to a superclass
automatically propagate to the subclass, providing a two-edged sword for software
maintenance and reuse. In summary, inheritance creates a strong, static coupling between
a superclass and its subclasses.
Delegation can be viewed as a relationship between objects where one object forwards
certain method calls to another object, called its delegate. Delegation can also a powerful
design/reuse technique. The primary advantage of delegation is run-time flexibility – the
delegate can easily be changed at run-time. But unlike inheritance, delegation is not directly
supported by most popular object-oriented languages, and it doesn’t facilitate dynamic
polymorphism.
As a simple example, consider the relationship between a Rectangle class and a Window
class. With inheritance, a Window class would inherit its rectangular properties from classRectangle. With delegation, a Window object would maintain a reference or pointer to a
Rectangle object, and calls to rectangle-like methods of the Window object would be
delegated to corresponding methods of the Rectangle object.
Now let’s consider a slightly more complex example. Suppose employees can classified
based on how they are paid; e.g., hourly or salaried. Using inheritance, we might design
three classes: an Employee class which encapsulates the functionality common to all
employees, and two subclasses HourlyEmployee and SalariedEmployee which encapsulates
pay-specific details. While this design might be suitable for some applications, we would
encounter problems in a scenario where a person changes, say from hourly to salaried. The
class of an object and the inheritance relationship are both static, and objects can’t change
their class easily (but see the State pattern for tips on how to fake it).
A more flexible design would involve delegation – an Employee object could delegate payrelated method calls to an object whose responsibilities focused solely on how the employee
is paid. In fact, we might still use inheritance here in a slightly different manner by creating
an abstract class (or interface) called PayClassification with two subclasses
HourlyPayClassification and SalariedPayClassification which implement classification-specific
computations. Using delegation as shown, it would be much easier to change the pay
classification of an existing Employee object.
This second example illustrates an important point: In implementing delegation, we often
want the capability to replace the delegate with another object, possibly of a different class.
Therefore delegation will often use inheritance and polymorphism, with classes of potential
delegates being subclasses of an abstract class which encapsulates general delegate
responsibilities.
One final point. Sometimes, the choice between delegation and inheritance is driven by
external factors such as programming language support for multiple inheritance or design
constraints requiring polymorphism. Consider threads in Java. You can associate a class
with a thread in one of two ways: either by extending (inheriting) directly from class
Thread, or by implementing the Runnable interface and then delegating to a Thread object.
Often the approach taken is based on the restriction in Java that a class can only extend
one class (i.e., Java does not support multiple inheritance). If the class you want to
associate with a thread already extends some other class in the design, then you would
have to use delegation; otherwise, extending class Thread would usually be the simpler
approach.
10) Which patterns were used by Sun in designing the
Enterprise JavaBeans model?
Many design patterns were used in EJB, and some of them are clearly identifiable by their
naming convention. Here are several:
1. Factory Method: Define a interface for creating classes, let a subclass (or a helper
class) decide which class to instantiate.
This is used in EJB creation model. EJBHome defines an interface for creating the
EJBObject implementations. They are actually created by a generated container
class. See InitialContextFactory interface that returns an InitialContext based on a
properties hashtable.
2. Singleton: Ensure a class has only one instance, and provide a global point of
access to it.
There are many such classes. One example is javax.naming.NamingManager
3. Abstract Factory: Provide an interface for creating families of relegated or
dependent objects without specifying their concrete classes.
We have interfaces called InitialContext, InitialContextFactory. InitialContextFactory
has methods to get InitialContext.4. Builder: Separate the construction of a complex factory from its representation so
that the same construction process can create different representations.
InitialContextFactoryBuilder can create a InitialContextFactory.
5. Adapter: Convert the interface of a class into another interface clients expect.
In the EJB implementation model, we implement an EJB in a class that extends
SessionBean or a EntityBean. We don't directly implement the EJBObject/home
interfaces. EJB container generates a class that adapts the EJBObject interface by
forwarding the calls to the enterprise bean class and provides declarative
transaction, persistence support.
6. Proxy: Provide a surrogate for other object to control access to it.
We have remote RMI-CORBA proxies for the EJB's.
7. Memento: Without violating encapsulation, capture and externalize an object's
internal state so that the object can be restored to this state later.
Certainly this pattern is used in activating/passivating the enterprise beans by the
container/server.
11) What is an analysis pattern?
An analysis pattern is a software pattern not related to a language or implementation
problem, but to a business domain, such as accounting or health care. For example, in
health care, the patient visit activity would be subject to a number of patterns.
There is a good overview from an OOPSLA '96 presentation at
http://www.jeffsutherland.org/oopsla96/fowler.html
A good text would be: Martin Fowler's Martin Analysis Patterns : Reusable Object Models,
ISBN: 0201895420, published by Addison-Wesley.
In summary, analysis patterns are useful for discovering and capturing business processes.
12) What are the differences between analysis patterns
and design patterns?
Analysis pattern are for domain architecture, and design pattern are for implementation
mechanism for some aspect of the domain architecture. In brief, analysis pattern are more
high level and more (end-user) functional oriented.
13) How does "Extreme Programming" (XP) fit with
patterns?
Extreme Programming has a large emphasis on the concept of refactoring: Writing code
once and only once.
Patterns, particularly the structural patterns mentioned by the Gang of Four, can give good
pointers about how to acheive that goal.
(XP states when and where factoring should happen, patterns can show you how.)14) What is the disadvantage of using the Singleton
pattern? It is enticing to use this pattern for all the classes
as it makes it easy to get the reference of the singleton
object.
The intent of the Singleton pattern is to ensure a class has only one instance and to provide
a global point of access to it. True, the second part about providing a global point of access
is enticing, but the primary disadvantage of using the Singleton pattern for all classes is
related to the first part of the intent; i.e., that it allows only one instance of the class. For
most classes in an application, you will need to create multiple instances. What purpose
would a Customer class serve if you could create only one Customer object?
15) How do you write a Thread-Safe Singleton?
I have written plenty of non-thread-safe Singletons but it wasn't until recently when I
tracked it down that I realized that thread-safety could be a big problem.
The problem is that in the typical Singleton implementation (at least the ones I've seen)
there is the ability to create multiple versions of the single instance...I know, "But How?".
Well, in the getInstance() call the instance is checked for null, and then immediately
constructed if it is null, and then the instance is returned.
The problem is that the thread (Ta) making the call could swap-out immediately after
checking for a null. A subsequent thread (Tb) could then make a call to get the instance and
construct an instance of the Singleton. When the original thread (Ta) is then swapped back
in, it would construct and return a completely separate object. BAD KITTY!
The following code snippet shows an example of a thread-safe Singleton.
package com.jgk.patterns.singleton;
public class JGKSingleton {
/* Here is the instance of the Singleton */
private static JGKSingleton instance_;
/* Need the following object to synchronize */
/* a block */
private static Object syncObject_;
/* Prevent direct access to the constructor
private JGKSingleton() {
super();
}
public static JGKSingleton getInstance() {
/* in a non-thread-safe version of a Singleton */
/* the following line could be executed, and the */
/* thread could be immediately swapped out */
if (instance_ == null) {
synchronized(syncObject_) {
if (instance_ == null) {
instance_ = new JGKSingleton();
}
}
}
return instance_;}
}
NOTE: The 2nd check for if (instance_ == null) is needed to avoid making another unnecessary
construct.
Don't let this byte you! ;-)
16) What is the Reactor pattern?
The new book "Pattern-oriented Software Architecture Volume 2" ISBN 0471606952 has a
chapter on the Reactor pattern. It falls under the general category of "Event Handling
Patterns". To quote the leading bit of the chapter,
"The Reactor architectural pattern allows event-driven applications to demultiplex and
dispatch service requests that are delivered to an application from one or more clients"
It is used in a synchronous manner, so that if the callback you delegate the event to takes a
while to complete you will run into problems with scalability.
17) What are Process Patterns?
Basically process patterns define a collection of best practices, techniques, methods for
developing object-oriented software.
A good reference site is by Scott Ambler. He also has two books on the topic plus a white
paper on the subject you can download.
http://www.ambysoft.com/processPatternsPage.html.
18) How and where did the concept of design patterns get
started?
Work on patterns has been influenced by the works of Christopher Alexander who published
on topics related to urban planning and building architecture in the late 1970s. The history
of patterns for software design began in the late 1980s and reached an important milestone
with the publishing of the first book fully dedicated to this subject by the "Gang of Four",
Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, Design Patterns - Elements
of Reusable Object-Oriented Software). In conjunction, the emergence of object-oriented
software development fostered the work on other topics related to design patterns such as
application frameworks, analysis patterns, language idioms, and so on.
19) Where can I find good examples of the Prototype
pattern?
The prototype pattern is actually quite simple to implement in Java.
Recall that the idea of prototype is that you are passed an object and use that object as a
template to create a new object. Because you might not know the implementation details of
the object, you cannot create a new instance of the object and copy all of its data. (Some of
the data may not be accessible via methods). So you ask the object itself to give you a copy
of itself.
Java provides a simple interface named Cloneable that provides an implementation of the
Prototype pattern. If you have an object that is Cloneable, you can call its clone() method to
create a new instance of the object with the same values.The trick here is that you must override the clone() method to increase its visibility, and just
call super.clone(). This is because the implementation of clone(), defined in
java.lang.object, is protected. For example:
public class CopyMe implements Cloneable {
public Object clone() {
return super.clone();
}
}
Note that Cloneable is an empty interface! It merely acts as a tag to state that you really
want instance of the class to be cloned. If you don't implement Cloneable, the super.clone()
implementation will throw a CloneNotSupportedException.
The Object implementation of clone() performs a shallow copy of the object in question.
That is, it copies the values of the fields in the object, but not any actual objects that may
be pointed to. In other words, the new object will point to the same objects the old object
pointed to.
As an example of using the cloning:
CopyMe thing = new Copyme();
CopyMe anotherThing = (Copyme)thing.clone();
This example is pretty trivial, but the real power comes when you don't know what you're
actually cloning.
For example, suppose you define an interface that represents a customer:
public interface Customer extends Cloneable {
public Object clone(); // require making it public!
public String getName();
public void setName(String name);
...
}
You might have several different implementations of this interface, possibly storing data in a
file, database, or using EJB Entity beans. If a shallow copy of the data is sufficient to
represent a copy of the object, Java's clone() method works great.
You might have a method that needs to make a copy of the data to store it in a Hashtable,
for example:
public void storeCustomer(Customer customer) {
Customer copy = (Customer)customer.clone();
dataStore.put(copy);
}
Note that this method knows nothing about what type of customer we're getting. This
pattern will work for any actual type of Customer, no matter how the data is stored. For
example:
FileBasedCustomer c1 = new FileBasedCustomer(...);
RDBMSBasedCustomer c2 = new RDBMSBasedCustomer(...);
EJBBasedCustomer c3 = new EJBBasedCustomer(...);
manager.storeCustomer(c1);manager.storeCustomer(c2);
manager.storeCustomer(c3);
20) What are Anti-Patterns?
There isn't really a "clean-cut" definition out there just yet, unlike Design Patterns.
Basically, as Design Patterns (and more particularly, Process Patterns) try to codify a
standard vocabulary for working solutions to problems that reappear frequently, AntiPatterns represent an effort to define and classify reoccuring non-solutions, i.e., things that
lots of projects do that fail to yield a solution, or actually prevent a project from working or
being finished.
The most basic example I can think of is "The Hammer", inspired by the old addage, "If
your only tool is a hammer, everything looks like a nail" (or the variation, "If your only tool
is a hammer, everything looks like your left thumb." Hammer describes a regular,
reoccuring problem in inexperienced engineers (particularly those who've only used one
language for the bulk of their carreer so far), that of trying to force-feed all problems into
the solutions they already know.
http://www.antipatterns.com/ has more information.
21) What patterns are particularly useful in building
networked applications?
I suggest starting with Pattern-Oriented Software Architecture: Patterns for Concurrent and
Networked Objects (POSA2). POSA2 specifically brings together 17 interrelated patterns
addressing Service Access and Configuration, Event Handling, Synchronization, and
Concurrency.
The patterns and many of the examples in POSA2 come primarily from the design and
implementation of the ACE framework.
22) Are there any good Java-specific patterns books
available?
The Java-specific patterns books are:
• Java Design Patterns: A Tutorial
• Patterns in Java, Volume 1
• Patterns in Java, Volume 2
• Concurrent Programming in Java , Second Edition: Design Principles and Patterns
• SanFrancisco Design Patterns
As far as the good part of the question.... Doug Lea's Concurrent Programming book is
probably the best of the bunch. However, its patterns are specific to concurrent
programming. Many people don't like the quality of Mark Grand's two Patterns in Java
books. [They are rated 3 and 2 stars at Amazon, respectively]. The first printing of the
Cooper tutorial book was riddled with errors. If you get that, be sure to get at least the
second printing. [Look on last line of page before TOC.] The SanFrancisco book is definitely
good, but I'm not sure how good if you aren't using SF.23) What are Collaboration Patterns?
Collaboration Patterns are repeatable techniques used by teams of people to help them
work together (collaborate). Ellen Gottesdiener of EBG Consulting has created these
patterns in order to help facilitate good teamwork. These patterns really have nothing to do
with object-oriented development or Java, besides the fact that they can help with
requirements gathering or CRC design sessions. In a nutshell, Collaboration Patterns are
techniques to help make meetings useful.
24) Is it correct from a design point of view to make an
object both an Observer and Observable at the same time?
Yes, and this can be the preferred pattern in some cases.
For example, suppose you were writing a supply chain management system for a retail
chain. Each store object in your system generates item-sold events; when the chain
generates enough of these for a particular product, a buyer object generates a purchase
order for more of the product. However, the buyer object has no particular interest in
individual item sold events. Instead, the buyer (Observer) registers to receive out-of-stock
events from the warehouse (Observable); the warehouse, as Observer, registers with the
individual stores (Observables) to receive item-sold events. Thus, the warehouse is both
Observer and Observable. (Please note that this is a synthetic example, and probably not
the way to organize a supply chain.)
Another reason to use one or more central Observer-Observable object in between ultimate
Observers and Observables is to fully decouple the two. In some cases, Observer and
Observable may exist on different machines, and may rely on the central ObserverObservable to hide this complication.
A good source for more details is the Publisher-Subscriber section of Buschmann et al.,
Pattern-Oriented Software Architecture: A System of Patterns.
25) How can I maintain a single instance of an object in an
applet?
In start(), instead of always creating a new object, return the existing one if it exists or
create a new one if it doesn't.
26) What is the best way to generate a universally unique
object ID? Do I need to use an external resource like a file
or database, or can I do it all in memory?
I need to generate unique id's that will be used for node 'ID' attribute values within XML
documents. This id must be unique system-wide. The generator must be available to a
number of servlets that add various node structures to my XML docs as a service. What is
the best way to tackle this? The 'possible' ways I can see:
• Keep the maximum ID value in a flat-file where the service would read it upon startup and increment it. Upon shutdown or failure, it would write the latest max id to
the file.
• Calculate the max id by searching the XML itself. This will be tougher since XML
requires an alpha-numeric value (not strictly numeric).• Use a database (MySQL) with a two-field table where one field is the incremental
counter.
I just have this feeling that none of the above are the most efficient ways of doing this.
Regards, -Andy]
There is an additional way to do that that doesn't rely on an external file (or database) like
the one you have presentred. If has been presented in the EJB Design Patterns book,
written by Floyd Marinescu, and available in a pdf format for free from the given link.
The suggested solution is based on the UUID for EJB pattern, that comes out from this
question:
How can universally unique primary keys can be generated in menory without requiring a
database or a singleton?
Without enetring in the specifics (you can fully check out the pattern by reading the
appropriate chapter), the solution is to generate a 32 digit key, encoded in hexadecimal
composed as follows:
1. Unique down to the millisecond. Digits 1-8 are are the hex encoded lower 32 bits of
the System.currentTimeMillis() call.
2. Unique across a cluster. Digits 9-16 are the encoded representation of the 32 bit
integer of the underlying IP address.
3. Unique down to the object in a JVM. Digits 17-24 are the hex representation of the
call to System.identityHashCode(), which is guaranteed to return distinct integers for
distinct objects within a JVM.
4. Unique within an object within a millisecond. Finally digits 25-32 represent a
random 32 bit integer generated on every method call using the cryptographically
strong java.security.SecureRandom class.
27) Is there some kind of Design pattern which would
make it possible to use the Same code base in EJB and non
EJB context?
A good suggestion would be using Delegation
class PieceOfCode {
public Object myMethod() {}
}
class EJBImpl ... {
PieceOfCode poc = new PieceOfCode();
public Object myMethod() {
return poc.myMethod();
}
}
This should not be a violation of EJB specs, since EJBs can use simple java classes for their
use. Think about Dependant Objects and so on.
28) What is session facade?
Session facade is one design pattern that is often used while developing enterprise
applications.
It is implemented as a higher level component (i.e.: Session EJB), and it contains all the
iteractions between low level components (i.e.: Entity EJB). It then provides a singleinterface for the functionality of an application or part of it, and it decouples lower level
components simplifying the design.
Think of a bank situation, where you have someone that would like to transfer money from
one account to another.
In this type of scenario, the client has to check that the user is authorized, get the status of
the two accounts, check that there are enough money on the first one, and then call the
transfer. The entire transfer has to be done in a single transaction otherwise is something
goes south, the situation has to be restored.
As you can see, multiple server-side objects need to be accessed and possibly modified.
Multiple fine-grained invocations of Entity (or even Session) Beans add the overhead of
network calls, even multiple transaction. In other words, the risk is to have a solution that
has a high network overhead, high coupling, poor reusability and mantainability.
The best solution is then to wrap all the calls inside a Session Bean, so the clients will have
a single point to access (that is the session bean) that will take care of handling all the rest.
Obviously you need to be very careful when writing Session Facades, to avoid the abusing
of it (often called "God-Bean").
For a detailed description of this pattern, check this page: Core J2EE Patterns - Session
Facade or get Floyd Marinescu's EJB Design Patterns, in PDF format.
29) How is JDO different from VO ?
JDO is a persistence technology that competes against entity beans in enterprise application
development. It allows you to create POJOs (plain old java objects) and persist them to the
database - letting JDO take care of the storage.
Value objects, on the other hand, represent an abstract design pattern used in conjuction
with entity beans, jdbc, and possibly even JDO to overcome commonly found isolation and
transactional problems in enterprise apps. Value objects alone do not allow you to persist
objects - they are simple data holders used to transfer data from the database to the client
and back to the database.
Side note: I know that many books out there still refer to these data holders as value
objects but the correct term is DTO: data transfer objects. Value objects refer to objects
that hold a value. A good example of this java.lang.Integer object which holds an int.
30) How can I implement the MVC design pattern using
JSP?
The MVC (Model View Controller) design pattern is a pattern/architecture that can be used
by GUI's. It seperates the application's data, user interface and control logic into three
separate entities. This ensures the system will be more maintainable in the future as
changes to one component will not have an affect on the others.
The MVC pattern also conforms with the JSP Model 2 architecture.
The MVC pattern can be easily implemented in web applications using JSTL core, JSP,
Servlets and JavaBeans.
JSTL makes it easy for HTML designers to use dynamic data in their pages without having to
learn in depth java. The tags are in a recognizable HTML like format meaning a smaller
learning curve than taking on the JSP specification after taking on the Java specification.
JSTL also makes it easy for web developers who are developing all aspects of the application
in helping you keep the content separate from the display by keeping your JSP clean from
any Java code. Since JSTL and its EL (expression Language) are really only suited for
accessing data, it forces you to use a MVC pattern so long as you keep all JSP and Java
syntax/code out of the JSP pages.A common scenario might look like this, a user sends a request to a server. The request is
handled by a Servlet (the controller) which will initialize any JavaBeans (the model) required
to fulfill the user's request. The Servlet (the controller) will then forward the request, which
contains the JavaBeans (the model), to a JSP (the view) page which contains only HTML and
JSTL syntax. The JSTL will format the data into a human readable format before being sent
back to the user. Thus completing the full MVC process.
JSTL also has a tag library for XML processing, this could be used in an MVC environment as
you could replace the JavaBeans with an XML document to describe your data, so long as
the XML is still constructed in a controller such as a Servlet before being passed to the JSP
(the view).
JSTL also has a tag library for database access, using this in your JSP (the view) pages
would NOT comply to the MVC pattern as there will be no separation between the model and
the view.