Creational Patterns - Abstract Factory Pattern

* Provides one level of interface higher than the factory pattern It is used to return one of several factories.

* Defines an interface for creating objects, but let subclasses to decide which class to instantiate and Refers to the newly created object through a common interface.

• Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
• A hierarchy that encapsulates: many possible “platforms”, and the construction of a suite of “products”.
• The new operator considered harmful.

Problem

If an application is to be portable, it needs to encapsulate platform dependencies. These “platforms” might include: windowing system, operating system, database, etc. Too often, this encapsulatation is not engineered in advance, and lots of #ifdef case statements with options for all currently supported platforms begin to procreate like rabbits throughout the code.

This pattern is one level of abstraction higher than factory pattern. This means that the abstract factory returns the factory of classes. Like Factory pattern returned one of the several sub-classes, this returns such factory which later will return one of the sub-classes.

Let’s understand this pattern with the help of an example.

Suppose we need to get the specification of various parts of a computer based on which work the computer will be used for.

The different parts of computer are, say Monitor, RAM and Processor. The different types of computers are PC, Workstation and Server.

So, here we have an abstract base class Computer.

package creational.abstractfactory; public abstract class Computer {
	/** * Abstract method, returns the Parts ideal for * Server * @return Parts */ public abstract Parts getRAM(); /** * Abstract method, returns the Parts ideal for * Workstation * @return Parts */ public abstract Parts getProcessor(); /** * Abstract method, returns the Parts ideal for * PC * @return Parts */ public abstract Parts getMonitor();
}// End of class

This class, as you can see, has three methods all returning different parts of computer. They all return a method called Parts. The specification of Parts will be different for different types of computers. Let’s have a look at the class Parts.

package creational.abstractfactory; public class Parts {
	/** * specification of Part of Computer, String */ public String specification; /** * Constructor sets the name of OS * @param specification of Part of Computer */ public Parts(String specification) { this.specification = specification; } /** * Returns the name of the part of Computer * * @return specification of Part of Computer, String */ public String getSpecification() { return specification; }
}// End of class

And now lets go to the sub-classes of Computer. They are PC, Workstation and Server.

package creational.abstractfactory; public class PC extends Computer {
	/** * Method over-ridden from Computer, returns the Parts ideal for * Server * @return Parts */ public Parts getRAM() { return new Parts("512 MB"); } /** * Method over-ridden from Computer, returns the Parts ideal for * Workstation * @return Parts */ public Parts getProcessor() { return new Parts("Celeron"); } /** * Method over-ridden from Computer, returns the Parts ideal for * PC * @return Parts */ public Parts getMonitor() { return new Parts("15 inches"); }
}// End of class

package creational.abstractfactory; public class Workstation extends Computer {
	/** * Method over-ridden from Computer, returns the Parts ideal for * Server * @return Parts */ public Parts getRAM() { return new Parts("1 GB"); } /** * Method over-ridden from Computer, returns the Parts ideal for * Workstation * @return Parts */ public Parts getProcessor() { return new Parts("Intel P 3"); } /** * Method over-ridden from Computer, returns the Parts ideal for * PC * @return Parts */ public Parts getMonitor() { return new Parts("19 inches"); }
}// End of class

package creational.abstractfactory; public class Server extends Computer{
	/** * Method over-ridden from Computer, returns the Parts ideal for * Server * @return Parts */ public Parts getRAM() { return new Parts("4 GB"); } /** * Method over-ridden from Computer, returns the Parts ideal for * Workstation * @return Parts */ public Parts getProcessor() { return new Parts("Intel P 4"); } /** * Method over-ridden from Computer, returns the Parts ideal for * PC * @return Parts */ public Parts getMonitor() { return new Parts("17 inches"); }
}// End of class

Now let’s have a look at the Abstract factory which returns a factory “Computer”. We call the class ComputerType.

package creational.abstractfactory; /** * This is the computer abstract factory which returns one * of the three types of computers. * */ public class ComputerType {
	private Computer comp; public static void main(String[] args) {
		ComputerType type = new ComputerType(); Computer computer = type.getComputer("Server"); System.out.println("Monitor: "+computer.getMonitor().getSpecification()); System.out.println("RAM: "+computer.getRAM().getSpecification()); System.out.println("Processor: "+computer.getProcessor().getSpecification());
	}
		/** * Returns a computer for a type * * @param computerType String, PC / Workstation / Server * @return Computer */
	public Computer getComputer(String computerType) {
		if (computerType.equals("PC")) comp = new PC(); else if(computerType.equals("Workstation")) comp = new Workstation(); else if(computerType.equals("Server")) comp = new Server(); return comp;
	}
}// End of class

Running this class gives the output as this:

Monitor: 17 inches
RAM: 4 GB
Processor: Intel P 4.

When to use Abstract Factory Pattern?
One of the main advantages of Abstract Factory Pattern is that it isolates the concrete classes that are generated. The names of actual implementing classes are not needed to be known at the client side. Because of the isolation, you can change the implementation from one factory to another.