Decorator

• Provide an alternative to subclassing.
• Add new function to an object without affecting other objects.
• Make a responsibility easily added and removed dynamically.
• More flexibility than static inheritance.
• Transparent to the object.
• Related patterns include
  • Adapter pattern, which provides a different interface to the object it adapts, whereas a decorator changes an object's responsibilities,
  • Proxy pattern, which controls access to the object, whereas a decorator focuses on adding new functions to an object,
  • Composite pattern, which aggregates an object, whereas a decorator adds additional responsibilities to an object, and
  • Strategy pattern, which changes the guts of an object, whereas a decorator changes the skin of an object.
  • Facade pattern, which provides a way of hiding a complex class, whereas a decorator adds function by wrapping a class.

Example

A JScrollPane object can be used to decorate a JTextArea object or a JEditorPane object. A window can be decorated with different borders like BevelBorder, CompoundBorder, EtchedBorder TitledBorder etc. These border classes working as decorators are provided in Java API.

Decorator pattern can be used in a non-visual fashion. For example, BufferedInputStream, DataInputStream, and CheckedInputStream are decorating objects of FilterInputStream class. These decorators are standard Java API classes.

To illustrate a simple decorator pattern in non-visual manner, we design a class that prints a number. We create a decorator class that adds a text to the Number object to indicate that such number is a random number. Of course we can subclass the Number class to achieve the same goal. But the decorator pattern provides us an alternative way.

import java.util.Random;
class Number {
   public void print() {
       System.out.println(new Random().nextInt());
   }
}

class Decorator {
    public Decorator() {
        System.out.print("Random number: ");//add a description to the number printed
        new Number().print();
    }
}

class SubNumber extends Number{
    public SubNumber() {
       super();
       System.out.print("Random number: ");
       print();
    }
}

class Test {
    public static void main(String[] args) {
        new Decorator();
        new SubNumber();
    }
}

 C:\ Command Prompt

C:\> java Test Random number: 145265744 Random number: 145265755

Recap: The Number class just produces a random number. We use Decorator and SubNumber classes to attach or add a description to the random number which is produced by Number class. This is how Decorator pattern works.

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