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The example I'm going to use consists of a class called
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ImageReaderFactory which provides the factory method which is this one;
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createImageReader. As you can see the method takes an InputStream
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as input and returns an object of type ImageReader, and it's
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static so that we can invoke it even if we
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don't have an instance of the ImageReaderFactory. So what does the
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method do? Well the method first invokes, getImageType, passing the
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InputStream as a parameter and this method figures out the type
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of the image that is stored in this Inputstream and it's
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an integer. Then, based on the value of this integer, the
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method does one of several things. If the image type is
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a GIF, it will invoke the constructor for GifReader passing the
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stream as a parameter. And what will happen is that the
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GIF reader will read a GIF from the stream, create a
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corresponding object and return it. So in this case, the ImageReader
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object return will be the object representing a GIF as appropriate.
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Similarly, if the image type is JPEG, then the method will
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invoke the constructor for JPEG Reader and in this case, this constructor
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will read from the stream a JPEG, create a corresponding object
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and return it. And so on for different types of images. So
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why is this a situation in which it is appropriate to
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use the factory method pattern? One, because it corresponds exactly to the
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cases that we saw before, of applicability. This is a case
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in which we don't know the type of the object that we
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need to create until we run the code, because it depends
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on the value of the InputStream. It depends on the content
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of the InputStream. So, until we read the InputStream, we cannot
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figure out whether we need to create a GIF, a JPEG or
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some other type of image. So in this case, we want to
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do, we want to simply delegate to this classes the creation of
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the object, once we know what type of object needs to
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be created. So perfect example of application of a factory method pattern.
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