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Oct 17, 2019 in Design Patterns
Q: Give example of decorator design pattern in Java ?

1 Answer

Oct 17, 2019
These were some of the design pattern questions I have seen in most of interviews, there are many more specially in software design which is important in google interviews and various other companies like Amazon, Microsoft etc. Please share if you have faced any interesting design questions which is worth sharing.

Decorator Design Pattern

Intent

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Client-specified embellishment of a core object by recursively wrapping it.

Wrapping a gift, putting it in a box, and wrapping the box.

Problem

You want to add behavior or state to individual objects at run-time. Inheritance is not feasible because it is static and applies to an entire class.

Discussion

Suppose you are working on a user interface toolkit and you wish to support adding borders and scroll bars to windows. You could define an inheritance hierarchy like ...

Decorator scheme

But the Decorator pattern suggests giving the client the ability to specify whatever combination of "features" is desired.

Widget* aWidget = new BorderDecorator(

  new HorizontalScrollBarDecorator(

    new VerticalScrollBarDecorator(

      new Window( 80, 24 ))));

aWidget->draw();

This flexibility can be achieved with the following design

Decorator scheme

Another example of cascading (or chaining) features together to produce a custom object might look like ...

Stream* aStream = new CompressingStream(

  new ASCII7Stream(

    new FileStream("fileName.dat")));

aStream->putString( "Hello world" );

The solution to this class of problems involves encapsulating the original object inside an abstract wrapper interface. Both the decorator objects and the core object inherit from this abstract interface. The interface uses recursive composition to allow an unlimited number of decorator "layers" to be added to each core object.

Note that this pattern allows responsibilities to be added to an object, not methods to an object's interface. The interface presented to the client must remain constant as successive layers are specified.

Also note that the core object's identity has now been "hidden" inside of a decorator object. Trying to access the core object directly is now a problem.
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