Java J2ee Planet

The Interpreter Pattern defines a grammatical representation for a language and an interpreter to interpret the grammar. The best example you can get for this is Java itself which is an interpreted language. It converts the code written in English to a byte code format so as to make possible for all the operating systems to understand it. This quality of it makes it platform independent. The development of languages can be done when you find different cases but, somewhat similar, it is advantageous to use a simple language which can be interpreted by the system and can deal with all these cases at the same time. To make this interpreter clearer, let’s take an example. The “musical notes” is an “Interpreted Language”. The musicians read the notes, interpret them according to “Sa, Re, Ga, Ma…” or “Do, Re, Me… ” etc and play the instruments, what we get in output is musical sound waves. Think of a program which can take the Sa, Re, Ga, Ma etc and produce the sounds for the frequenc...

The mediator pattern deals with the complexity which comes in the coding when number of classes increase. I will explain this. When we begin with development, we have a few classes and these classes interact with each other producing results. Now, consider slowly, the logic becomes more complex and functionality increases. Then what happens? We add more classes and they still interact with each other but it gets really difficult to maintain this code now. Mediator pattern takes care of this problem. It makes the code more maintainable. It promotes loose-coupling of classes such that only one class (Mediator) has the knowledge of all the classes, rest of the classes have their responsibilities and they only interact with the Mediator. A very common example can be airplanes interacting with the control tower and not among themselves. The control tower knows exactly, where each of the airplanes is, and guides them whereas the airplanes have their own responsibilities of landing and take...

This is another of the data-driven pattern. The client invokes a particular module using a command. The client passes a request, this request gets propagated as a command. The command request maps to particular modules. According to the command, a module is invoked. This pattern is different from the Chain of Responsibility in a way that, in the earlier one, the request passes through each of the classes before finding an object that can take the responsibility. The command pattern however finds the particular object according to the command and invokes only that one. It’s like there is a server having a lot of services to be given, and on Demand (or on command), it caters to that service for that particular client. A classic example of this is a restaurant. A customer goes to restaurant and orders the food according to his/her choice. The waiter/ waitress takes the order (command, in this case) and hands it to the cook in the kitchen. The cook can make several types of food...

  The Iterator pattern is one, which allows you to navigate through a collection of data using a common interface without knowing about the underlying implementation. Iterator should be implemented as an interface. This allows the user to implement it anyway its easier for him/her to return data. We use iterators quite frequently in everyday life. For example, remote control of TV. Any remote control we use, either at home/hotel or at a friend's place, we just pick up the TV remote control and start pressing Up and Down or Forward and Back keys to iterate through the channels. What sort of interface can Iterator be in case of Remote Controls? /** * Iterator interface has method declarations for iterating through * TV channels. All remote controls implement Iterator. */ public interface Iterator {   public Channel nextChannel(int currentChannel); public Channel prevChannel(int currentChannel); }// End of interface The channel iterator is common for all t...

The Iterator pattern is one, which allows you to navigate through a collection of data using a common interface without knowing about the underlying implementation. Iterator should be implemented as an interface. This allows the user to implement it anyway its easier for him/her to return data. We use iterators quite frequently in everyday life. For example, remote control of TV. Any remote control we use, either at home/hotel or at a friend’s place, we just pick up the TV remote control and start pressing Up and Down or Forward and Back keys to iterate through the channels. What sort of interface can Iterator be in case of Remote Controls? /** * Iterator interface has method declarations for iterating through * TV channels. All remote controls implement Iterator. */ public interface Iterator {   public Channel nextChannel(int currentChannel); public Channel prevChannel(int currentChannel); }// End of int...

The chain of responsibility pattern is based on the same principle as written above. It decouples the sender of the request to the receiver. The only link between sender and the receiver is the request which is sent. Based on the request data sent, the receiver is picked. This is called “data-driven”. In most of the behavioral patterns, the data-driven concepts are used to have a loose coupling. The responsibility of handling the request data is given to any of the members of the “chain”. If the first link of the chain cannot handle the responsibility, it passes the request data to the next level in the chain, i.e. to the next link. For readers, familiar with concepts of Java, this is similar to what happens in an Exception Hierarchy. Suppose the code written throws an ArrayIndexOutOfBoundsException. Now, this exception is because of some bug in coding and so, it gets caught at the correct level. Suppose, we have an app...