Factory Design Patterns

Factory Design Patterns And its Example
Intent:
Create objects without exposing the instantiation logic to the clients.

• Client uses the Factory to create the product  and then use it through common interface
• Factory instantiates the concrete product

Applicability:

• To control/manage the number of instances

                  e.g Connection pooling.

• To conditionally instantiate objects

                 e.g. instantiate ProductA for customers with rating “A”

• To hide the concrete class details from the client by using a common interface for the newly created objects

                e.g. EnumSet hides RegualrEnumSet and  JumboEnumSet classes from the clients 

Example applications:

• A drawing application need to create multiple types of shapes

•  Java API examples:

                 – java.util.EnumSet
                 – java.util.Collections
                 – javax.sql.DataSource
                 – java.sql.DriverManager

Implementation Issues:

         Parameterized factories :

             Avoid if/else blocks as it violates OCP.

             Use reflection API to create respective concrete classes.

                                                                      Structure

                                                                           

Example Code:

package edu.mit.cs;

import java.io.FileInputStream;
import java.io.InputStream;
import java.util.Properties;

import javax.imageio.ImageReader;

public class ImageReaderFactory
{
    private static final String GIF = "gif";
    private static final String JPEG = "jpg";
    private static Properties prop = null;
   
    public static ImageReader getImageReader(InputStream is)
    {
        ImageReader reader = null;
       
        if (prop == null) {
          prop = new Properties();
          try {
            prop.load(new FileInputStream("D:\\test.prop"));
          } catch(Exception ex) {ex.printStackTrace();}
        }
       
        String imageType = determineImageType(is);
        String className = prop.getProperty(imageType);
       
        if (className != null) {
          try {
            reader = (ImageReader)Class.forName(className).newInstance();
          } catch (Exception ex) {ex.printStackTrace();}
        }
        return reader;
    }
   
    private static String determineImageType(InputStream is) {
      return JPEG;
    }
   
    public static void main(String[] args) {
      ImageReader reader = ImageReaderFactory.getImageReader(null);
      System.out.println(reader);
    }
}


package dp.factory;

public class JpegReader implements ImageReader
{
    public DecodedImage getDecodedImage()
    {
      DecodedImage decodedImage = null;
        // ...
        return decodedImage;
    }
}

package dp.factory;

public interface ImageReader
{
    public DecodedImage getDecodedImage();
}


package dp.factory;

public class GifReader implements ImageReader
{
    public DecodedImage getDecodedImage()
    {
        DecodedImage decodedImage = null;
        // ...
        return decodedImage;
    }
}

package dp.factory;

public class DecodedImage {

}