Decorator pattern with Simple Example

Decorator

---

Definition

Attach additional responsibilities or functions to an object dynamically or statically. Also known as Wrapper.

Where to use & benefits

• 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.

Example

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 Test {

    public static void main(String[] args) {

        new Decorator();

        }

}

java Test

Random number: 145265744