Difference between Proxy, Decorator, Adaptor, and Bridge Patterns ?

Proxy, Decorator, Adapter, and Bridge are all variations on "wrapping" a class. But their uses are different.

• Proxy could be used when you want to lazy-instantiate an object, or hide the fact that you're calling a remote service, or control access to the object.
  See proxy pattern with details.
• Decorator is also called "Smart Proxy." This is used when you want to add functionality to an object, but not by extending that object's type. This allows you to do so at runtime.
• Adapter / Wrapper is used when you have an abstract interface, and you want to map that interface to another object which has similar functional role, but a different interface.
  In particular Decorator looks very close to Adapter but still there is basic difference:
  

  	Adapter / Wrapper	Decorator
  Composes "origin" class	True	True
  Modifies original interface	True	False
  Modifies behavior of interface	False	True
  Proxies method calls	True	True

   
  See Adapter pattern and Decorator Pattern with examples.
• Bridge is very similar to Adapter, but we call it Bridge when you define both the abstract interface and the underlying implementation. I.e. you're not adapting to some legacy or third-party code, you're the designer of all the code but you need to be able to swap out different implementations.
  See bridge pattern with example.
• Facade is a higher-level (read: simpler) interface to a subsystem of one or more classes. Think of Facade as a sort of container for other objects, as opposed to simply a wrapper. So you don't have to worry about so many things, just call the facade to do all the stuff.
  See Facade pattern with example.

Interface And Abstraction – Spiritual Explanation

“What is the difference between interface and abstract class?”, a typical interview question . Answer is so obvious, and we can list at least 3 differences. Here is an attempt to correlate Object Oriented world with spiritual world, and explain abstraction and interface.

“Soul is an abstract notion realized by concrete living being”. So as per OO world, if you got “Soul” as abstract class, you got to have a concrete class “Life”. Soul can not operate without Life.

Just like I said object is an entity with an objective to live, abstract class can live through concrete class. Inheritance is the only mechanism for an abstract class to live. Spiritually, Soul operates through life. Life is an object with a process thread running in it.

In Spiritual world, it is said that “Soul carries the karma (good or bad actions) from previous life, which has influences in the new life”. If you consider this statement, karma is the action log recorded into the abstract class.

Body is an interface to life (and hence interface to Soul) and real world. so IBody can be implemented by body of human, bird or any living creature. Once the objective is complete in this real world, destructor is called on life object. However actions are kept recorded (like log files) in the abstract class through out this new life.

In every new life actions recorded in abstract class is not revealed, as it is private declaration . Only the ultimate object builder (God)  has access to it. Object builder reads this private variable, and makes the decision of choosing a new interface for Soul. Dispose method is programmed to be called in a timer function. When timer event fires, dispose is called, and the Life object is disposed terminating the process thread.

After thread is terminated and Life is disposed, Object builder reads the record log from Soul class. Based on the karma recorded in the log, a new object with the suitable interface (body) is created, and Life is induced into it with a process thread. And a timer is started in the Life class, which when fires Thread Abort is called.

Shallow Copy and Deep Copy

There are two ways to make a copy of an object called shallow copy and deep copy.
Here is what they mean:

Shallow Copy
Shallow copy is a bit-wise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the references are copied. Thus, if the object you are copying contains references to yet other objects, a shallow copy refers to the same subobjects.

Deep Copy
Deep copy is a complete duplicate copy of an object. If an object has references to other objects, complete new copies of those objects are also made. A deep copy generates a copy not only of the primitive values of the original object, but copies of all subobjects as well, all the way to the bottom. If you need a true, complete copy of the original object, then you will need to implement a full deep copy for the object.

Shallow Copy

Deep Copy

You may like : 
Design patterns using shallow copy and deep copy - Prototype pattern
Shallow and deep copy using clone

MVC Pattern Basics

Definition
The Model-View-Controller (MVC) architectural pattern is used in software engineering to allow for the separation of three common features in GUI applications:

• the data access (typically via a database)
• the business logic (how the data will be used)
• user interaction (how the data and actions will be visually presented)

MVC pattern differenciates all the 3 aspects of application in 3 difft tiers.

Modal
It incorporates following point

•  Holds the business logic of the application.
•  holds data sent between different. tiers like JSP to servlet and handler classes.
• One of the goals of the model is to give a presentation which does not directly refer to any specific DBMS. 

View

• This represents the visible user interface, so handles presentation tier of the application.
• it knows enough about the data to create a coherent presentation, but does not actually do the work of presenting the data. Instead, it provides the ability to manipulate data outside the View through event handlers defined within. 
• handles presentation logic, client side validation.
• e.g. HTML, JSP

Controller

• handles the flow of the application
• Joins the Model with the View and is the heart of the business logic.
• It is the source of activity when an event occurs and defines the event handling actions which access data (from the Model) and are presented to the user (in the View).
• e.g. Servlets, Action Class in Struts, ActionServlet is using struts-config.xml as front controller to handle each request and redirect it to appropriate destination.

Advantages

•  Increases modularity of the application.
•  Increases Maintainability of the application, i.e.  the data presentation can be changed without any notion of how the data is obtained and conversely, the data access can be changed without any knowledge of how it is to be presented. 

Example
to be added soon

Feelings today

Java: Rounding off to 2 decimal places

Seeking the simplest way to round off a float value to 2 decimal places, i found these:

Method 1:
x = (double)int((x+0.005)*100.0)/100.0;

Method 2:
x = Math.round(x*100.0) / 100.0;

Method 3:
DecimalFormat df2 = new DecimalFormat( "#,###,###,##0.00" );
double dd = 100.2397;
double dd2dec = new Double(df2.format(dd)).doubleValue();

Method 4:
f = (float) (Math.round(n*100.0f)/100.0f);

Method 5:
double r = 5.1234;
System.out.println(r); // r is 5.1234
int decimalPlaces = 2;
BigDecimal bd = new BigDecimal(r);
bd = bd.setScale(decimalPlaces, BigDecimal.ROUND_HALF_UP); // setScale is immutable
r = bd.doubleValue();
System.out.println(r); // r is 5.12

[source: www.thescripts.com, accuracy unchecked]

How I did it:
float percentage = score.floatValue()/(qapairs.length*10)*100; //my float value
percentage = Float.valueOf((new DecimalFormat("###.00").format(percentage)));