Decorator pattern : 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.

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();

    }

}