READ
This
section presents each pattern in its category:
· Creational
· Structural
· Behavioral
Creational: Factory
Method Pattern
Description: Define an interface for
creating an object, but let the
subclasses decide which class to instantiate. The Factory method lets a
class defer instantiation to subclasses.
Components: Creator, Concrete Creator,
Product
· Creator: abstract definition the factory methods
used to build the product
· Concrete Creator: implementation of the creator
· Product: final object being built
Factory method pattern that
creates car engines based on your budget.
Listing 1. Engine.java
public interface Engine
{
public int getSpeed();
public void accelerate();
public void decelerate();
}
Listing 2. StandardEngine.java
public class StandardEngine implements Engine {
Listing 3. TurboEngine.java
public class TurboEngine implements Engine {
Listing 4. EngineFactory.java
public class EngineFactory
{
public
static Engine getEngine( int budget )
{
if(
budget >= 10000 )
{
return new
TurboEngine();
}
return new
StandardEngine();
}
}
Example 2 :
interface Currency {
String getSymbol();
}
class Rupee implements Currency {
@Override
public String getSymbol() {
return "Rs";
}
}
class CurrencyFactory {
public static Currency createCurrency (String country) {
if (country. equalsIgnoreCase ("India")){
return new Rupee();
}else if(country. equalsIgnoreCase ("Singapore")){
return new SGDDollar();
}else if(country. equalsIgnoreCase ("US")){
return new USDollar();
}
throw new IllegalArgumentException("No such currency");
}
}
Creational: Singleton
Pattern
Description: Creates a single instance of a class for the entire JVM to share,
which is good for stateless business objects or classes that represent specific
pieces of physical hardware for which there is a one-to-one mapping of software
objects to physical devices.
Components: Singleton object
· Singleton object: there is one instance of each
singleton object per JVM. Sometimes
this object is initialized in a static code block and only accessible through a
getInstance() method. The constructor for a singleton is usually private to
stop anyone from creating any non-controlled instances of the class.
·
Singleton pattern restricts
the instantiation of a class and ensures that only one instance of the class
exists in the java virtual machine. The singleton class must provide a
global access point to get the instance of the class. Singleton pattern is used
for logging,
drivers objects, caching and thread
pool.
·
Eager initialization
package com.journaldev.singleton;
public class EagerInitializedSingleton {
private static final EagerInitializedSingleton instance = new
EagerInitializedSingleton();
//private constructor to avoid client applications to use
constructor
private EagerInitializedSingleton(){}
public static EagerInitializedSingleton getInstance(){
return instance;
}
}
Lazy Initialization
package com.journaldev.singleton;
public class LazyInitializedSingleton {
private static LazyInitializedSingleton instance;
private LazyInitializedSingleton(){}
public static LazyInitializedSingleton
getInstance(){
if(instance == null){
instance = new
LazyInitializedSingleton();
}
return instance;
}
}
Thread Safe Singleton
Below code snippet provides the double checked
locking implementation.
public static ThreadSafeSingleton getInstanceUsingDoubleLocking(){
if(instance == null){
synchronized (ThreadSafeSingleton.class) {
if(instance == null){
instance = new ThreadSafeSingleton();
}
}
}
return instance;
}
Enum Singleton
To
overcome this situation with Reflection, Joshua Bloch suggests the use
of Enum to implement Singleton design pattern as Java ensures that any enum
value is instantiated only once in a Java program. Since Java Enum values are globally accessible, so is the
singleton. The drawback is that the enum type is somewhat inflexible; for
example, it does not allow lazy initialization.
package com.journaldev.singleton;
public enum EnumSingleton {
INSTANCE;
public static void doSomething(){
//do something
}
}
Builder
pattern
The intention of the builder pattern is to find a solution to the telescoping
constructor anti-pattern[citation
needed]. The telescoping constructor anti-pattern occurs when the increase of object constructor parameter
combination leads to an exponential list of constructors. Instead of using
numerous constructors, the builder
pattern uses another object, a builder, that receives each initialization
parameter step by step and then returns the resulting constructed object at
once.
public class User {
02
private final String firstName; // required
03
private final String lastName; // required
04
private final int age; // optional
05
private final String phone; // optional
06
private final String address; // optional
07
08
private User(UserBuilder
builder) {
09
this.firstName = builder.firstName;
10
this.lastName = builder.lastName;
11
this.age = builder.age;
12
this.phone = builder.phone;
13
this.address = builder.address;
14
}
15
16
public String getFirstName() {
17
return firstName;
18
}
19
20
public String getLastName() {
21
return lastName;
22
}
23
24
public int getAge() {
25
return age;
26
}
27
28
public String getPhone() {
29
return phone;
30
}
31
32
public String getAddress() {
33
return address;
34
}
35
36
public static class UserBuilder {
37
private final String firstName;
38
private final String lastName;
39
private int age;
40
private String phone;
41
private String address;
42
43
public UserBuilder(String firstName,
String lastName) {
44
this.firstName = firstName;
45
this.lastName = lastName;
46
}
47
48
public UserBuilder age(int age) {
49
this.age = age;
50
return this;
51
}
52
53
public UserBuilder phone(String phone)
{
54
this.phone = phone;
55
return this;
56
}
57
58
public UserBuilder address(String
address) {
59
this.address = address;
60
return this;
61
}
62
63
public User build() {
64
return new User(this);
65
}
66
67
}
68
}
1
|
public User getUser() {
|
2
|
return new
|
3
|
User.UserBuilder('Jhon', 'Doe')
|
4
|
.age(30)
|
5
|
.phone('1234567')
|
6
|
.address('Fake address
1234')
|
7
|
.build();
|
8
|
}
|
Design Patterns - Composite Pattern
Composite pattern is used where
we need to treat a group of objects in similar way as a single object. Composite
pattern composes objects in term of a tree structure to represent part as well
as whole hierarchy. This type of design pattern comes under structural pattern
as this pattern creates a tree structure of group of objects.
This pattern creates a class
that contains group of its own objects. This class provides ways to modify its
group of same objects.
import java.util.ArrayList;
import java.util.List;
public class Employee {
private String name;
private String dept;
private int salary;
private List<Employee> subordinates;
// constructor
public Employee(String name,String dept, int sal) {
this.name = name;
this.dept = dept;
this.salary = sal;
subordinates = new ArrayList<Employee>();
}
public void add(Employee e) {
subordinates.add(e);
}
public void remove(Employee e) {
subordinates.remove(e);
}
public List<Employee>
getSubordinates(){
return subordinates;
}
public String toString(){
return ("Employee :[
Name : " + name + ", dept : " + dept + ", salary
:" + salary+" ]");
}
}
public class CompositePatternDemo {
public static void main(String[] args) {
Employee CEO = new Employee("John","CEO", 30000);
Employee
headSales = new Employee("Robert","Head Sales", 20000);
Employee
headMarketing = new Employee("Michel","Head Marketing", 20000);
Employee clerk1
= new Employee("Laura","Marketing", 10000);
Employee clerk2
= new Employee("Bob","Marketing", 10000);
Employee salesExecutive1 = new Employee("Richard","Sales", 10000);
Employee
salesExecutive2 = new Employee("Rob","Sales", 10000);
CEO.add(headSales);
CEO.add(headMarketing);
headSales.add(salesExecutive1);
headSales.add(salesExecutive2);
headMarketing.add(clerk1);
headMarketing.add(clerk2);
//print all employees of the organization
System.out.println(CEO);
for (Employee headEmployee :
CEO.getSubordinates()) {
System.out.println(headEmployee);
for (Employee employee :
headEmployee.getSubordinates()) {
System.out.println(employee);
}
}
}
}
Design Patterns - Facade Pattern
Facade pattern hides
the complexities of the system and provides an interface to the client using
which the client can access the system. This type of design pattern comes
under structural pattern as this pattern adds an interface to existing system
to hide its complexities.
Step 1
Create an interface.
Shape.java
public interface Shape { void draw(); }
Step 2
Create concrete classes implementing the same interface.
Rectangle.java
public class Rectangle implements Shape { @Override public void draw() { System.out.println("Rectangle::draw()"); } }
Square.java
public class Square implements Shape { @Override public void draw() { System.out.println("Square::draw()"); } }
Circle.java
public class Circle implements Shape { @Override public void draw() { System.out.println("Circle::draw()"); } }
Step 3
Create a facade class.
ShapeMaker.java
public class ShapeMaker {
private Shape circle;
private Shape rectangle;
private Shape square;
public ShapeMaker() {
circle = new Circle();
rectangle = new Rectangle();
square = new Square();
}
public void drawCircle(){
circle.draw();
}
public void drawRectangle(){
rectangle.draw(); }
public void drawSquare(){ square.draw(); }
}
Step 4
Use the facade to draw various types of shapes.
FacadePatternDemo.java
public class FacadePatternDemo {
public static void main(String[] args) {
ShapeMaker shapeMaker = new ShapeMaker();
shapeMaker.drawCircle();
shapeMaker.drawRectangle();
shapeMaker.drawSquare(); } }
Step 5
Verify the output.
Circle::draw() Rectangle::draw() Square::draw()
Design Patterns - Proxy Pattern
In proxy pattern, a class
represents functionality of another class. This type of design pattern
comes under structural pattern.
In proxy pattern, we create object having original object to
interface its functionality to outer world.
Implementation
We are going to create an Image interface and concrete classes
implementing the Image interface. ProxyImage is a a proxy class to reduce memory
footprint of RealImage object loading.
ProxyPatternDemo, our demo class, will use ProxyImage to get an Imageobject to load and display
as it needs.
Step 1
Create an interface.
Image.java
public interface Image { void display(); }
Step 2
Create concrete classes implementing the same interface.
RealImage.java
public class RealImage implements Image { private String fileName; public RealImage(String fileName){ this.fileName = fileName; loadFromDisk(fileName); } @Override public void display() { System.out.println("Displaying " + fileName); } private void loadFromDisk(String fileName){ System.out.println("Loading " + fileName); } }
ProxyImage.java
public class ProxyImage implements Image{ private RealImage realImage; private String fileName; public ProxyImage(String fileName){ this.fileName = fileName; } @Override public void display() { if(realImage == null){ realImage = new RealImage(fileName); } realImage.display(); } }
Step 3
Use the ProxyImage to get object of RealImage class when required.
ProxyPatternDemo.java
public class ProxyPatternDemo { public static void main(String[] args) { Image image = new ProxyImage("test_10mb.jpg"); //image will be loaded from disk image.display(); System.out.println(""); //image will not be loaded from disk image.display(); } }
Step 4
Verify the output.
Loading test_10mb.jpg Displaying test_10mb.jpg Displaying test_10mb.jpg
Design Patterns - Observer Pattern
1.
interest rate change (subject) …update
news paper, bank, credit union and etc (observer)
2. New
Auto policy (subject)…update house policy department and house security package
(observer)
2.
Observer pattern is used when there is one-to-many relationship
between objects such as if one object is
modified, its depenedent objects are to be notified automatically. Observer
pattern falls under behavioral pattern category.
Implementation
Observer pattern uses three actor classes. Subject, Observer and
Client. Subject is an object having methods to attach and detach observers to a
client object. We have created an abstract class Observer and a concrete classSubject that is extending class Observer.
ObserverPatternDemo, our demo class, will use Subject and concrete class object to show
observer pattern in action.
Step 1
Create Subject class.
Subject.java
import java.util.ArrayList; import java.util.List; public class Subject { private List<Observer> observers = new ArrayList<Observer>(); private int state; public int getState() { return state; } public void setState(int state) { this.state = state; notifyAllObservers(); } public void attach(Observer observer){ observers.add(observer); } public void notifyAllObservers(){ for (Observer observer : observers) { observer.update(); } } }
Step 2
Create Observer class.
Observer.java
public abstract class Observer { protected Subject subject; public abstract void update(); }
Step 3
Create concrete observer classes
BinaryObserver.java
public class BinaryObserver extends Observer{ public BinaryObserver(Subject subject){ this.subject = subject; this.subject.attach(this); } @Override public void update() { System.out.println( "Binary String: " + Integer.toBinaryString( subject.getState() ) ); } }
OctalObserver.java
public class OctalObserver extends Observer{ public OctalObserver(Subject subject){ this.subject = subject; this.subject.attach(this); } @Override public void update() { System.out.println( "Octal String: " + Integer.toOctalString( subject.getState() ) ); } }
HexaObserver.java
public class HexaObserver extends Observer{ public HexaObserver(Subject subject){ this.subject = subject; this.subject.attach(this); } @Override public void update() { System.out.println( "Hex String: " + Integer.toHexString( subject.getState() ).toUpperCase() ); } }
Step 4
Use Subject and concrete observer objects.
ObserverPatternDemo.java
public class ObserverPatternDemo { public static void main(String[] args) { Subject subject = new Subject(); new HexaObserver(subject); new OctalObserver(subject); new BinaryObserver(subject); System.out.println("First state change: 15"); subject.setState(15); System.out.println("Second state change: 10"); subject.setState(10); } }
Step 5
Verify the output.
First state change: 15 Hex String: F Octal String: 17 Binary String: 1111 Second state change: 10 Hex String: A Octal String: 12 Binary String: 1010
Design Patterns - Strategy Pattern
In Strategy pattern, a class behavior or its algorithm can be
changed at run time. This type of design pattern comes under behavior pattern.
In Strategy pattern, we create objects which represent various
strategies and a context object whose behavior varies as per its strategy
object. The strategy object changes the executing algorithm of the context
object.
Implementation
We are going to create a Strategy interface defining an action and
concrete strategy classes implementing the Strategy interface. Context is a class which uses a Strategy.
StrategyPatternDemo, our demo class, will use Context and strategy objects to demonstrate
change in Context behaviour based on strategy it deploys or uses.
Step 1
Create an interface.
Strategy.java
public interface Strategy { public int doOperation(int num1, int num2); }
Step 2
Create concrete classes implementing the same interface.
OperationAdd.java
public class OperationAdd implements Strategy{ @Override public int doOperation(int num1, int num2) { return num1 + num2; } }
OperationSubstract.java
public class OperationSubstract implements Strategy{ @Override public int doOperation(int num1, int num2) { return num1 - num2; } }
OperationMultiply.java
public class OperationMultiply implements Strategy{ @Override public int doOperation(int num1, int num2) { return num1 * num2; } }
Step 3
Create Context Class.
Context.java
public class Context {
private Strategy strategy;
public Context(Strategy strategy){
this.strategy = strategy; }
public int executeStrategy(int num1, int num2){
return strategy.doOperation(num1, num2); } }
Step 4
Use the Context to see change in behaviour when it
changes its Strategy.
StrategyPatternDemo.java
public class StrategyPatternDemo {
public static void main(String[] args) {
Context context = new Context(new OperationAdd()); System.out.println("10 + 5 = " + context.executeStrategy(10, 5));
context = new Context(new OperationSubstract()); System.out.println("10 - 5 = " + context.executeStrategy(10, 5));
context = new Context(new OperationMultiply()); System.out.println("10 * 5 = " + context.executeStrategy(10, 5));
} }
Step 5
Verify the output.
10 + 5 = 15 10 - 5 = 5 10 * 5 = 50
Top 10 Java design pattern interview question
Answer
Java
Reflection makes it possible to
inspect classes, interfaces, fields and methods at runtime, without knowing the
names of the classes, methods etc. at compile time. It is also possible to
instantiate new objects, invoke methods and get/set field values using
reflection.
Design pattern
interview question in Java
design pattern interview question are integral part of any good list of core Java interview questions. Java is a popular Object oriented programming language and have lots of design pattern and design principles, contributed by many developers and open source framework. As a Java programmer its expected from you to know OOPS concept likeAbstraction, Encapsulation and polymorphism, What is design pattern in Java, Some popular Java design pattern and most importantly when to use those design pattern in Java application. purpose of asking design pattern interview question in Java is to check whether Java programmer are familiar to those essential design patterns or not. Design patterns in Java interviews are as important as multi-threading, collection and programming questions. If you are senior or experienced Java programmer than expect more complex and tough design pattern in Java interview e.g. Chain of responsibility design pattern and solving real time software design questions.
What is Observer design pattern in Java? When do you use Observer pattern in Java?
This is one of the most common Java design pattern interview question. Observer pattern is based upon notification, there are two kinds of object Subject and Observer. Whenever there is change on subject's state observer will receive notification. See What is Observer design pattern in Java with real life example for more details.
design pattern interview question are integral part of any good list of core Java interview questions. Java is a popular Object oriented programming language and have lots of design pattern and design principles, contributed by many developers and open source framework. As a Java programmer its expected from you to know OOPS concept likeAbstraction, Encapsulation and polymorphism, What is design pattern in Java, Some popular Java design pattern and most importantly when to use those design pattern in Java application. purpose of asking design pattern interview question in Java is to check whether Java programmer are familiar to those essential design patterns or not. Design patterns in Java interviews are as important as multi-threading, collection and programming questions. If you are senior or experienced Java programmer than expect more complex and tough design pattern in Java interview e.g. Chain of responsibility design pattern and solving real time software design questions.
What is Observer design pattern in Java? When do you use Observer pattern in Java?
This is one of the most common Java design pattern interview question. Observer pattern is based upon notification, there are two kinds of object Subject and Observer. Whenever there is change on subject's state observer will receive notification. See What is Observer design pattern in Java with real life example for more details.
What is decorator pattern in Java? Can you give an example of Decorator pattern?
Decorator pattern is another popular java design pattern question which is common because of its heavy usage injava.io package. BufferedReader and BufferedWriter are good example of decorator pattern in Java. SeeHow to use Decorator pattern in Java fore more details.
What is Singleton pattern in Java?
Singleton pattern in Java is a pattern which allows only one instance of Singleton class available in whole application. java.lang.Runtime is good example of Singleton pattern in Java. There are lot's of follow up questions on Singleton pattern see 10 Java singleton interview question answers for those followups
Volatile :
Can you write thread-safe Singleton in Java?
There are multiple ways to write thread-safe singleton in Java e.g by writing singleton using double checked locking, by using static Singleton instance initialized during class loading. By the way using Java enum to create thread-safe singleton is most simple way. See Why Enum singleton is better in Java for more details.
What is Factory pattern in Java? What is advantage of using static factory method to create object?
Factory pattern in Java is a creation Java design pattern and favorite on many Java interviews. Factory pattern used to create object by providing static factory methods. There are many advantage of providing factory methods e.g. caching immutable objects, easy to introduce new objects etc. See What is Factory pattern in Java and benefits for more details.
What is difference between Factory and Abstract factory in Java
see here of this Java interview question.
What is Builder design pattern in Java? When do you use Builder pattern ?
Builder pattern in Java is another creational design pattern in Java and often asked in Java interviews because of its specific use when you need to build an object which requires multiple properties some optional and some mandatory. SeeWhen to use Builder pattern in Java for more details
Can you write code to implement producer consumer design pattern in Java?
Producer consumer design pattern is a concurrency design pattern in Java which can be implemented using multiple way. if you are working in Java 5 then its better to use Concurrency util to implement producer consumer pattern instead of plain old wait and notify in Java. Here is a good example of implementing producer consumer problem using BlockingQueue in Java.
What is difference between Abstraction and Encapsulation in Java?
I have already covered answer of this Java interview question in my previous post as Difference between encapsulation and abstraction in Java. See there to answer this question.
This was my list of 10 popular design pattern interview question in Java. I have not included MVC (Model View Controller) design pattern because that is more specific to J2EE and Servlet JSP interview, but if you are going for any Java interview which demands experience in J2EE than you must prepare MVC design pattern. That's all on Java design pattern interview question and answers. Please let us know if you have any other interesting question on Java design pattern.
Full DP explanation.
Adapter ... wrap a legacy object that provides
an incompatible interface with an object that supports the desired interface
its used so that two unrelated interfaces
can work together. The object that joins these unrelated interface is called an Adapter.
mobile charger as an
adapter because mobile battery needs 3 volts to charge but the normal socket
produces either 120V (US) or 240V (India)
Bank account type –
savings account, current account,
Facade pattern hides the complexities of the system and provides an interface to
the client using which the client can access the system. This type of design
pattern comes under structural pattern as this pattern adds an interface to
exiting system to hide its complexities.
This pattern involves a single
class which provides simplified methods which are required by client and
delegates calls to existing system classes methods.
We're
going to create a Shape interface and concrete classes
implementing the Shape interface. A facade class ShapeMaker is defined as a next step.
Proxy:
A
Proxy can also be defined as a surrogate. In the real work a cheque or credit card is a proxy for what
is in our bank account. It can be used in place of cash, which is
what is needed, and provides a means of accessing that cash when required. And
that's exactly what the Proxy pattern does - controls and manage access to the object they are
"protecting".
BEHAVIOUR
Strategy
Strategy pattern is one of the behavioral design pattern. Strategy
pattern is used when we have multiple algorithms for
a specific task and client decides the actual implementation to be used at
runtime.
payment strategies –
using Credit Card or using PayPal.
Observer :
Whenever there is change on
subject's state observer will receive notification
1.Credit
Card/bank Account transaction –Txt Message – phone, email
2. New
customer (Comcast/ Auto insurance)---- send notification to sales/marketing
team to check if the customer is interested for cable(channels) / home
insurance.
CREATIONAL:
Factory
Method :
we create object without exposing the
creation logic to the client and refer to newly created object using a common
interface.
Shape interface and concrete classes implementing the Shape interface
Factory Method ... define "createInstance"
placeholder in the base class, each derived class calls the "new"
operator and returns an instance of itself
Bank account type – savings
account, current account,
factory
design pattern in java, you will notice that we have a single Factory class
that returns the different sub-classes based on the input provided and factory
class uses if-else or switch statement to achieve this.
Abstract
Factory- Good
Full DP explanation.
Front Controller :
The front controller design pattern is used
to provide a centralized request handling mechanism so that all requests will
be handled by a single handler. This handler can do the authentication/
authorization/ logging or tracking of request and then pass the requests to
corresponding handlers
To make it simple, let
us say that we can use constructor based dependency injection for mandatory
dependencies and setter based injection for optional dependencies. It is a
thumb of rule!!
Lets say for example.
If you want to
instantiate a class you always do it with its constructor. So if you are using
constructor based injection, the only way to instantiate the class is through
that constructor. If you pass the dependency through constructor it becomes
evident that it is a mandatory dependency.
On the other hand, if
you have a setter method in a POJO class, you may or may not set value for your
class variable using that setter method. It is completely based on your need.
i.e. it is optional. So if you pass the dependency through setter method of a
class it implicitly means that it is an optional dependency. Hope you are
clear!!
10 interview question on Singleton Pattern
in Java
What
is Singleton class? Have you used Singleton before?
Singleton is a class which has only one instance in whole application and provides a
getInstance()
method to access the singleton instance. There are many classes in JDK which is
implemented using Singleton pattern like java.lang.Runtime which provides
getRuntime()
method to get access of it and used to get free memory and total memory in Java.Singleton is a class which has only one instance in whole application and provides a
1) Which classes are candidates of Singleton? Which kind of class do you make Singleton in Java?
Here they will check whether candidate has enough experience on usage of singleton or not. Does he is familiar of advantage/disadvantage or alternatives available for singleton in Java or not.
Answer:
Any class which you want to be available to whole application and whole
only one instance is viable is candidate of becoming Singleton. One
example of this is Runtime
class , since on whole java application only one runtime
environment can be possible making Runtime Singleton is
right decision.
2) Can you write code for
getInstance() method of a Singleton class in Java?
Answer:
Until asked don’t write code using double checked locking as it is more complex
and chances of errors are more but if you have deep knowledge of double checked
locking, volatile
variable and lazy loading than this is your chance to shine. I have shared code examples
of writing singleton classes using enum, using static factory and with double
checked locking in my recent post Why
Enum Singletons are better in Java, please see there.
3)
Is it better to make whole getInstance()
method synchronized or just critical section is enough? Which one you will
prefer?
This is really nice question and I mostly
asked to just quickly
check whether candidate is aware of performance trade
off of unnecessary locking or not. Since locking only make sense when we need
to create instance and rest of the time its just read only access so locking of
critical section is always better option.
read more about synchronization on How
Synchronization works in JavaAnswer: This is again related to
double checked locking pattern, well synchronization is costly and when you
apply this on whole method than call to getInstance() will be synchronized and contented. Since synchronization is only needed during initialization on singleton instance, to prevent creating another instance of Singleton, It’s better to only synchronize critical section and not whole method. Singleton pattern is also closely related to factory design pattern where getInstance() serves as static factory method.
4) What is lazy and early loading of Singleton and how will you implement it?
This is another great Singleton interview question in terms of understanding of concept of loading and cost associated with class loading in Java. Many of which I have interviewed not really familiar with this but its good to know concept.
Answer:
As there are many ways to implement Singleton like using double checked
locking or Singleton class with static
final
instance initialized during class loading. Former is called lazy loading because Singleton instance
is created only when client calls getInstance() method while later is called
early loading because Singleton instance is created when class is loaded into
memory.
5) Example of Singleton in standard Java Development Kit?
This is open question to all, please
share
which classes are Singleton in JDK. Answer to this question is java.lang.Runtime5) Example of Singleton in standard Java Development Kit?
This is open question to all, please
Answer: There are many classes in Java Development Kit which is written using singleton pattern, here are few of them:
- Java.lang.Runtime with getRuntime() method
- Java.awt.Toolkit with getDefaultToolkit()
- Java.awt.Desktop with
getDesktop()
6) What is double checked locking in Singleton?
One of the most hyped question on Singleton pattern and really demands complete understanding to get it right because of Java Memory model caveat prior to Java 5. If a guy comes up with a solution of using volatile keyword with Singleton instance and explains it then it really shows it has in depth knowledge of Java memory model and he is constantly updating his Java knowledge.
Answer:
Double checked locking is a technique to prevent creating another instance of
Singleton when call to getInstance() method is made in multi-threading
environment. In Double checked locking pattern as shown in below example,
singleton instance is checked two times before initialization.
public
Singleton getInstance(){
if(_INSTANCE == null){synchronized(Singleton.class){
//double checked locking - because second check of Singleton instance with lock
if(_INSTANCE == null){
_INSTANCE =
new Singleton();
}
}
}
return _INSTANCE;
}
Double
checked locking should only be used when you have requirement for lazy
initialization otherwise use
Enum to implement singleton or simple static final variable.
7) How do you prevent for creating another instance of Singleton using
clone()
method?This type of questions generally comes
some time by asking how to break singleton or when Singleton is not Singleton
in Java.Answer: Preferred way is not to
implement 7) How do you prevent for creating another instance of Singleton using
Clonnable interface as why should one wants to create clone() of Singleto and if you do just throw Exception from clone() method as “Can not create clone of Singleton class”.
8) How do you prevent for creating another instance of Singleton using reflection?
Open to all. In my opinion throwing exception from constructor is an option.
Answer: This is similar to previous interview question. Since constructor of Singleton class is supposed to be private it prevents creating instance of Singleton from outside but Reflection can access private fields and methods, which opens a threat of another instance. This can be avoided by throwing Exception from constructor as “Singleton already initialized”
9)
How do you prevent for creating another instance of Singleton during
serialization?
Another great question which requires
knowledge of Serialization
in Java and how to use it for persisting Singleton classes. This is open to
you all but in my opinion use of readResolve()
method can sort this out for you.Answer: You can prevent this by using readResolve() method, since during serialization readObject() is used to create instance and it return new instance every time but by using readResolve you can replace it with original Singleton instance. I have shared code on how to do it in my post Enum as Singleton in Java. This is also one of the reason I have said that use Enum to create Singleton because serialization of enum is taken care by JVM and it provides guaranted of that.
10) When is Singleton not a Singleton in Java?
There is a very good article present in Sun's Java site which discusses various scenarios when a Singleton is not really remains Singleton and multiple instance of Singleton is possible. Here is the link of that article http://java.sun.com/developer/technicalArticles/Programming/singletons/
Apart from these questions on Singleton pattern, some of my reader contributer few more questions, which I included here. Thank you guys for your contribution.
11) Why you should avoid the
singleton anti-pattern at all and replace it with DI?
Answer:
Singleton Dependency Injection: every class that needs access to a singleton
gets the object through its constructors or with a DI-container.
Singleton
Anti-Pattern: with more and more classes calling getInstance the code gets
more and more tightly coupled, monolithic, not testable and hard to change and
hard to reuse because of not configurable, hidden dependencies. Also, there
would be no need for this clumsy double checked locking if you call getInstance
less often (i.e. once).
12)
How many ways you can write Singleton Class in Java?
Answer: I know atleast four ways to implement Singleton pattern in Java
1) Singleton by synchronizing
getInstance() method2) Singleton with public static final field initialized during class loading.Answer: I know atleast four ways to implement Singleton pattern in Java
1) Singleton by synchronizing
3) Singleton generated by static nested class, also referred as Singleton holder pattern.
4) From Java 5 on-wards using Enums
13) How to write thread-safe Singleton in Java?
Answer:
Thread-Saffe Singleton usually refers to write thread
safe code which creates one and only one instance of Singleton if called by
multiple thread at same time. There are many ways to achieve this like by using
double checked locking technique as shown above and by using Enum
or Singleton initialized by classloader.
At
last few more questions for your practice, contributed by Mansi:
14)
Singleton vs Static Class?
15) When to choose Singleton over Static Class?
16) Can you replace Singleton with Static Class in Java?
17) Difference between Singleton and Static Class in java?
18) Advantage of Singleton over Static Class?
15) When to choose Singleton over Static Class?
16) Can you replace Singleton with Static Class in Java?
17) Difference between Singleton and Static Class in java?
18) Advantage of Singleton over Static Class?
Read more: http://javarevisited.blogspot.com/2011/03/10-interview-questions-on-singleton.html#ixzz2C0TFk1iN
Observer design
pattern in Java is a fundamental core Java pattern where Observe watch for any change in state or property of
Subject. For Example Company updates all its shareholders for any
decision they make here Company is Subject and Shareholders are Observers, any
change in policy of company and Company notifies all its Shareholders or
Observer. This was simple real world explanation of Observer pattern. In this
article we will in detail what is Observer Design pattern, what is benefit of Observer design Pattern, Example or Observer pattern in Java and few other points.
Just like Decorator design Pattern andFactory Pattern in Java, Observer pattern is
also used in JDK.
Observer design Pattern Java Code Example
What is Observer design Pattern?
Observer design pattern in Java is very important pattern and as name suggest it’s used to
observe things. Suppose you want to notify for change in a particular object
than you observer that object and changes are notified to you. Object which is being observed
is refereed as Subject and classes which observe subject are called
Observer. This is beautiful pattern and used heavily along with Model View Controller Design pattern
where change in model is propagated to view so that it
can render it with modified information. Observer pattern is also a very popular Java interview questions and mostly asked on Senior or mid senior level.
Problem which is solved by Observer Pattern:
If we have requirement that if
particular object change its state and on depending upon
This changes some or group of
objects automatically change their state we need to implement observer pattern
it will reduce coupling between objects.
In real world if try to find
example see when we subscribe for New Phone connection whenever customer is registered with that company all other departments are
notified accordingly and then depending upon the state the do their jobs like do the verification of their address then if customer state is verified then dispatch the welcome kit etc.
How Observer Design Pattern is implemented in Java;
For implementation of this pattern
java makes our task very easy, developer need not to do so much for
implementing this pattern .In java.util package we can find interfaces ,classes and methods for implementing this pattern.
Public Interface
Observer:
Any class who implements this interface must be notified when subject
or observable object change its status.
Update (Observable Ob, Object
arg): This method is called when subject is changed.
Class Observable:
It’s a subject to whom observer
wants to observe.
Some Important Method:
addObserver(Observer o):add Observers in the set of observers for this subject or
observalbel object.
deleteObserver(Observer o): delete Observers in the set of observers .
hasChanged():check if object has changed.
clearChanged():this method will indicate that subject has no changes or all the
observers has been notified when changes is made.
notifyObservers(): notify all the observers if object has changed .
Read more:
What is decorator design pattern in Java?
·
Decorator
design pattern is used to enhance the
functionality of a particular object at run-time or dynamically.
·
At the
same time other instance of same class
will not be affected by this so individual object gets the new behavior.
·
Basically we wrap the original object through
decorator object.
·
Decorator
design pattern is based on abstract classes and we derive concrete
implementation from that classes,
·
It’s a
structural design pattern and most widely used.
When to use Decorator pattern in Java
·
When sub
classing is become impractical and we need large number of different
possibilities to make independent object or we can say we have number of
combination for an object.
·
Secondly
when we want to add functionality to individual object not to all object at
run-time we use decorator design pattern.
Example – Currency class
Based on the
locale we can provide/add functionality
Advantage of Decorator design Pattern in Java
In brief we see what
the main advantages of using decorator design patterns are.
1. Decorator Pattern is
flexible than inheritance because inheritance add responsibilities at compile
time and it will add at run-time.
2. Decorator pattern
enhance or modify the object functionality
Disadvantage
Main disadvantage of
using Decorator Pattern in Java is that the code maintenance can be a problem
as it provides a lot of similar kind of small objects (each decorator).
if anyone wants to add some functionality to individual object or change
the state of particular object at run time it is possible only by decorator pattern
Factory Method Design Pattern
05/11/2009
A factory method pattern is a
creational pattern. It is used to instantiate an object from one among a set of
classes based on a logic.
Assume that you have a set of classes
which extends a common super class or interface. Now you will create a concrete
class with a method which accepts one or more arguments. This method is our
factory method. What it does is, based on the arguments passed factory method
does logical operations and decides on which sub class to instantiate. This
factory method will have the super class as its return type. So that, you can
program for the interface and not for the implementation. This is all about
factory method design pattern.
Sample factory method design pattern
implementation in Java API
For a reference of how the factory
method design pattern is implemented in Java, you can have a look at
SAXParserFactory. It is a factory class which can be used to intantiate SAX
based parsers to pares XML. The method newInstance is the factory method which instantiates the sax parsers based on
some predefined logic.
Block diagram for The Design Pattern
Sample Java Source Code for Factory
Method Design Pattern
Based on comments received from
users, I try to keep my sample java source code as simple as possible for a
novice to understand.
Base class:
package
com.javapapers.sample.designpattern.factorymethod;//super
class that serves as type to be instantiated for factory method patternpublic
interface Pet { public
String speak();} |
First subclass:
package
com.javapapers.sample.designpattern.factorymethod;//sub
class 1 that might get instantiated by a factory method patternpublic
class Dog implements Pet { public
String speak() { return
"Bark bark..."; }} |
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Second subclass:
package
com.javapapers.sample.designpattern.factorymethod;//sub
class 2 that might get instantiated by a factory method patternpublic
class Duck implements Pet { public
String speak() { return
"Quack quack..."; }} |
Factory class:
package
com.javapapers.sample.designpattern.factorymethod;//Factory
method pattern implementation that instantiates objects based on logicpublic
class PetFactory { public
Pet getPet(String petType) { Pet
pet = null; //
based on logic factory instantiates an object if
("bark".equals(petType)) pet
= new Dog(); else
if ("quack".equals(petType)) pet
= new Duck(); return
pet; }} |
Using the factory method to
instantiate
package
com.javapapers.sample.designpattern.factorymethod;//using
the factory method patternpublic
class SampleFactoryMethod { public
static void main(String args[]){ //creating
the factory PetFactory
petFactory = new PetFactory(); //factory
instantiates an object Pet
pet = petFactory.getPet("bark"); //you
don't know which object factory created System.out.println(pet.speak()); }} |
Output of the above sample program for Factory Method
Pattern
Bark bark |
Factory Design pattern is based on Encapsulation object oriented concept. Factory design pattern is used to create objects or Class in Java and it provides loose coupling and high cohesion. Factory pattern encapsulate object creation logic which makes it easy to change it later when you change how object gets created or you can even introduce new object with just change in one class.
examples of factory method design pattern from JDK is :
valueOf() method which returns object created by factory equivalent to value of parameter passed.
getInstance() method which creates
instance of Singleton class.
newInstance() method which is used to
create and return new instance from factory method every time called
When to use Factory design pattern in Java
- Static
Factory methods are common in frameworks where library code needs to create objects of types which
may be sub classed by applications using the
framework.
- Some or
all concrete products can be created in multiple ways, or we want to leave
open the option that in the future there may be new ways to create the
concrete product.
- Factory method is used when
Products don't need to know how they are created.
- We can use factory pattern
where we have to create an object of any one of sub-classes depending on
the data provided
Here is complete code example of Factory pattern in Java:
interface Currency {
String getSymbol();
}
//
Concrete Rupee Class code
class Rupee implements Currency {
@Override
public String
getSymbol() {
return "Rs";
}
}
//
Concrete SGD class Code
class SGDDollar implements Currency {
@Override
public String
getSymbol() {
return "SGD";
}
}
Example - Logger – Console in development box or log file/ db in qa/production or
Advantage of Factory method Pattern in Java:
Factory pattern in Java is heavily used everywhere including JDK,
open source library and other frameworks.In following are main advantages of
using Factory pattern in Java:
1) Factory method design pattern decouples the calling class
from the target class, which result in less coupled and highly cohesive code?
E.g.: JDBC is a good example for this pattern; application code
doesn't need to know what database it will be used with, so it doesn't know what
database-specific driver classes it should use. Instead, it uses factory methods to
get Connections, Statements, and other objects to work with. Which gives you
flexibility to change your back-end database without changing your DAO layer in
case you are using ANSI SQL features and not coded on DBMS specific feature?
2) Factory pattern in
Java enables the subclasses to provide extended version of an object, because creating an object
inside factory is more flexible than creating an object directly in the client.
Since client is working on interface level any time you can enhance the
implementation and return from Factory.
3) Another benefit of
using Factory design pattern in Java
is that it encourages consistency in Code since every time object
is created using Factory rather than using different constructor at different
client side.
4) Code written using
Factory design pattern in Java is also easy to debug and troubleshoot because
you have a centralized method for object creation and every client is getting
object from same place.
Some more advantages
of factory method design pattern is:
1. Static
factory method used in factory design pattern enforces use of Interface
than implementation which itself a good practice. for example:
2. Since static
factory method have return type as Interface, it allows you to replace implementation with better performance
version in newer release.
3. Another advantage of static
factory method pattern is that they can cache frequently used object and eliminate duplicate object
creation. Boolean.valueOf() method is good example which caches true and false
boolean value.
5 Factory method
pattern offers alternative
way of creating object.
6. Factory
pattern can also be used to
hide information related to creation of object.
Abstract Factory Design Pattern
11/11/2009
Factory of factories. To keep things
simple you can understand it like, you have a set of ‘related’ factory method design pattern. Then you will put all those set of simple factories inside a
factory pattern. So in turn you need not be aware of the final concrete class
that will be instantiated. You can program for the interface using the top
factory.
There is also a view that abstract
factory is ‘also’ implemented using prototype instead of factory methords
pattern. Beginners for now please don’t yourself with that. Just go with
factory methods pattern.
As there is a word ‘abstract’ in the
pattern name don’t mistake and confuse it with java ‘abstract’ keyword. It is
not related to that. This abstract is from object oriented programming paradim.
Sample abstract factory design
pattern implementation in Java API
XML API implements abstract factory.
There is a class nameSchemaFactory. This acts as a factory and supports implemenation of multiple
schemas using abstract factory design pattern.
Sample Java Source Code for Factory
Method Design Pattern
Following is the interface, that will
be returned as the final end product from the factories.
package
com.javapapers.sample.designpattern.abstractfactory;public
interface Animal { public
void breathe();} |
Following is the interface for which
the factory implementation should be done. Inturn all abstract factory will
return this type.
package
com.javapapers.sample.designpattern.abstractfactory;public
interface AnimalFactory { public
Animal createAnimal();} |
One of the factory from a predefined
set which will instantiate the above interface.
package
com.javapapers.sample.designpattern.abstractfactory;public
class SeaFactory implements AnimalFactory { public
Animal createAnimal() { return
new Shark(); }} |
Second factory from a predefined set
which will instantiate the Animal interface.
package
com.javapapers.sample.designpattern.abstractfactory;public
class LandFactory implements AnimalFactory { public
Animal createAnimal() { return
new Elephant(); }} |
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Implementation of an Animal. This class is grouped with the first abstract factory.
package
com.javapapers.sample.designpattern.abstractfactory;public
class Shark implements Animal { public
void breathe() { System.out.println("I
breathe in water! He he!"); }} |
Implementation of an Animal. This
class is grouped with the second abstract factory.
package
com.javapapers.sample.designpattern.abstractfactory;public
class Elephant implements Animal { public
void breathe() { System.out.println("I
breathe with my lungs. Its easy!"); }} |
Following class consumes the abstract
factory.
package
com.javapapers.sample.designpattern.abstractfactory;public
class Wonderland { public
Wonderland(AnimalFactory factory)
{ Animal
animal = factory.createAnimal(); animal.breathe(); }} |
Testing the abstract factory design
pattern.
package
com.javapapers.sample.designpattern.abstractfactory;public
class SampleAbstractFactory { public
static void main(String args[]){ new
Wonderland(createAnimalFactory("water")); } public
static AnimalFactory createAnimalFactory(String
type){ if("water".equals(type)) return
new SeaFactory(); else return
new LandFactory(); }} |
Output of the above sample program for abstract factory
pattern
I breathe
in water! He he! |
Singleton Design Pattern
02/05/2011
Singleton design pattern is the first design pattern I learned (many years back). In early days when someone asks me,
“do you know any design pattern?” I quickly and promptly answer “I know singleton
design pattern” and the question follows, “do you know anything other than
singleton” and I stand stumped!
A java beginner will know about
singleton design pattern. At least he will think that he knows singleton
pattern. The definition is even easier than Newton’s third law. Then what is
special about the singleton pattern. Is it so simple and straightforward, does
it even deserve an article? Do you believe that you know 100% about singleton
design pattern? If you believe so and you are a beginner read through the end,
there are surprises for you.
There are only two points in the
definition of a singleton design pattern,
1. there should be only one
instance allowed for a class and
2. we should allow global point
of access to that single instance.
GOF says, “Ensure a class has only
one instance, and provide a global point of access to it. [GoF, p127]“.
The key is not the problem and
definition. In singleton pattern, trickier part is implementation and
management of that single instance. Two points looks very simple, is it so
difficult to implement it. Yes it is very difficult to ensure “single instance”
rule, given the flexibility of the APIs and many flexible ways available to
access an instance. Implementation is very specific to the language you are
using. So the security of the single instance is specific to the language used.
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Strategy for Singleton instance
creation
We suppress the constructor and don’t
allow even a single instance for the class. But we declare an attribute for
that same class inside and create instance for that and return it. Factory design pattern can be used to create the singleton instance.
package
com.javapapers.sample.designpattern;public
class Singleton { private
static Singleton singleInstance; private
Singleton() {} public
static Singleton getSingleInstance() { if
(singleInstance == null) { synchronized
(Singleton.class) { if
(singleInstance == null) { singleInstance
= new Singleton(); } } } return
singleInstance; } |
You need to be careful with multiple
threads. If you don’t synchronize the method which is going to return the
instance then, there is a possibility of allowing multiple instances in a
multi-threaded scenario. Do the synchronization at block level considering the
performance issues. In the above example for singleton pattern, you can see
that it is threadsafe.
Early and lazy instantiation in
singleton pattern
The above example code is a sample
for lazy instantiation for singleton design pattern. The single instance will
be created at the time of first call of the getSingleInstance() method. We can
also implement the same singleton design pattern in a simpler way but that
would instantiate the single instance early at the time of loading the class.
Following example code describes how you can instantiate early. It also takes
care of the multithreading scenario.
package
com.javapapers.sample.designpattern;public
class Singleton { private
static Singleton singleInstance = new
Singleton(); private
Singleton() {} public
static Singleton getSingleInstance() { return
singleInstance; }} |
Singleton and Serialization
Using serialization, single
instance contract of the singleton pattern can be violated. You can serialize
and de-serialize and get a new instance of the same singleton class. Using java
api, you can implement the below method and override the instance read from the
stream. So that you can always ensure that you have single instance.
ANY-ACCESS-MODIFIER
Object readResolve() throws ObjectStreamException; |
This article is an attempt to explain
the basics on singleton design pattern. If you want more insight on singleton
refer this technical article “When
is a Singleton not a Singleton?” and its
references.
Usage of Singleton Pattern in Java
API
java.lang.Runtime#getRuntime()
java.awt.Desktop#getDesktop()
.
Both Abstract Factory and Factory design pattern are creational design pattern and use to decouple clients from creating
object they need, But there is a significant difference between Factory and
Abstract Factory design pattern, Factory design pattern produces implementation of Products e.g. Garment Factory produce different kinds of clothes, On
the other hand Abstract Factory design pattern adds another layer of abstraction over Factory Pattern and Abstract Factory
implementation itself e.g. Abstract Factory will allow you to choose a
particular Factory implementation based upon need which will then produce
different kinds of products.
In short
1) Abstract
Factory design pattern creates Factory
2) Factory design pattern creates Products
Difference
between Abstract Factory and Factory design pattern in Java
Let see another example of
Abstract Factory and Factory design pattern in Java from JDK itself to get a
better understanding.
If you have done some XML work in Java e.g. reading
XML files using DOM parser, you may be familiar with DocumentBuilderFactory class which is an example
abstract factory design pattern because it returns a factory called DocumentBuilder which then used to create Document.
//Example of Abstract
Factory and Factory design pattern in Java
DocumentBuilderFactory abstractFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder factory = abstractFactory.newDocumentBuilder();
Document doc = factory.parse(stocks)
DocumentBuilderFactory abstractFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder factory = abstractFactory.newDocumentBuilder();
Document doc = factory.parse(stocks)
In this example DocumentBuilderFactory (Abstract Factory) creates DocumentBuilder (Factory) which creates Documents (Products).
Let's see some more difference between Abstract Factory and
Factory design pattern in Java in point form :
1) One more difference
between Abstract Factory and Factory design pattern is that AbstractFactory pattern uses composition to delegate
responsibility of creating object to another class while Factory design pattern uses inheritance and relies on derived class or
sub class to create object.
2) Abstract Factory may use Factory design pattern for creating objects but they are not
just limited to that they can also use Builder
design pattern to build object by doing series of steps or Prototype pattern to build object by copying or customizing
prototype of that object. It completely depends upon your implementation
whether to use Factory pattern or Builder pattern for creating products.
When to use Abstract Factory and Factory method design pattern in Java
Factory method design pattern are modern way of creating objects. It
offers some notable advantages over new() operator to create Objects e.g.
By using Factory method design pattern client is completely decoupled with
object creation code, which enforces Encapsulation and result is loosely coupled
and highly cohesive system. Any change e.g. a new product from Factory requires almost no change
in existing clients. See When
to use Factory method design pattern in Java for more scenarios. On the
other hand if you need an additional level of abstraction over
your Factory pattern than Abstract
Factory is the right design pattern to use. Abstract Factory allows you to
use different Factory implementation for different purpose. Abstract Factory pattern can be implemented using Factory
method and Singleton
design pattern in Java. One of the best
example of Abstract Factory and Factory pattern in Java is DocumentBuilderFactory and DocumentBuilder javax.xml.parsers package.
That's all on difference
between Abstract Factory and Factory design pattern in Java. In short Abstract Factory design pattern provides abstraction over Factory pattern itself while Factory design pattern provides abstraction over products.
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