Skip to main content

Design Pattern : Observer Design Pattern


Type : Behavioral Design Pattern

Summary

In Observer Design Pattern, an Object called subject maintains one to many relationship with Observers, so that once the state of the Subject changes it can automatically inform Observers registered with the subject.

Advantage

An Object(Subject) can notify other Objects(Observer) without being tightly coupled with Observer and depend on Observer Class.

Disadvantage

Due to the indirect communication between Subject and Observer's it can lead to performance issues.


Important Points

Multiple Subscribers for a Subject.
Subjects know its Subscribers.
Generally the Subject triggers the method that updates Observers.
New Observers can be added to the System with minimal changes.

Details

Following are different participants in Observer Pattern

Subject(Interface)

› Keeps track of its observers

› Provides an interface for attaching and detaching Observer objects

Observer(Interface)

› Defines an interface for update notification

ConcreteSubject

› The object being observed

› Sends a notification to its observers when its state changes

ConcreteObserver

› The observing object

› Stores state that should stay consistent with the subject's

› Implements the Observer update interface to process any changes in Subject
Simple Example

Let's think about a simple Mobile APP which is all about Questions-Answers Application. When the User selects a particular answer, based on whether the answer is right or wrong we have to update many UI components. For instance if correct we have to update the Points, and also we have to update the counts for correct/wrong answers etc.



Class Diagram















Comments

Popular posts from this blog

Converting Java Map to String

Java Collections framework, String manipulation etc is something that we often encounter in Development process.
For processing collections (like checking null/empty, Intersection, Disjunction) We do have some of the very use full libraries.

Some of the Collection related libraries are Apche Commons Collections and Google  Collections(Guava).

Problem Use Case

This article explains how to convert a Java Map to String(and vice versa) using different libraries and technique.

One way is to use StringBuilder(Or String) and loop though the Map and build the String by applying some sort of separator ( for key:value and entry). Here we have to take care of the null value etc.

Without Any Library
If we want to convert the map to a String with key value separator and also individual entry seperator in the resulting String, we have to write code for that. For a simple Map, we have to iterate though the map, take care of the null values etc. Following is a sample to get String built out from Map C…

Masking Credit Card number in Java

Sometimes we need to mask crucial information like Credit Card Numbers, CVV numbers etc before storing  or logging the information. This example mask Credit Card Number (Except last 4 Digit) from a Text which contains information along with Credit Card Number.

The following example demonstrates how we can easily mask the credit card with Matcher and Pattern Classes. This Sample Code uses Matcher and Pattern.
Pattern Used in this sample is not optimized for Credit Card Numbers, this pattern will get any numerical numbers in the String Content.  Based on the Credit Card Type a more efficient and Strict RegEx can be used to mask the Credit Card.
/**Mask the Credit card number but last four digit value **/   Pattern PATTERN = Pattern.compile("[0-9]+"); String message = content; Matcher matcher = PATTERN.matcher(message); String maskingChar = "*"; StringBuilder finalMask = new StringBuilder(maskingChar); while …

Invoking EJB deployed on a remote machine

Invoking EJB deployed on a remote machineIn case we are calling remote ejb( ejb deployed on remote machines),The JNDI lookup might lookup like,Properties env = new Properties();env.put(Context.INITIAL_CONTEXT_FACTORY, "org.jnp.interfaces.NamingContextFactory");env.put(Context.PROVIDER_URL, "XX.XXX.XX.XX:1099");env.put("java.naming.factory.url.pkgs", "org.jboss.naming:org.jnp.interfaces"); Context ctx = new InitialContext(env);If we are calling local ejb then we can simply create InitialContext without any parameters.Like,Context ctx = new InitialContext();