Building graphical user interfaces - Objects First with Java: A Practical Introduction Using BlueJ - page 400

Code 11.13
continued
Adding a collection
of filters

public ImageViewer()
{

filters = createFilters();
...

}

private List<Filter> createFilters()
{

List<Filter> filterList = new ArrayList<Filter>();
filterList.add( new DarkerFilter( "Darker" ));
filterList.add( new LighterFilter( "Lighter" ));
filterList.add( new ThresholdFilter( "Threshold" ));

return filterList;

}

// Other methods omitted.
}

Once we have this structure in place, we can make the last two necessary changes:

■ We change the code that creates the filter menu items so that it iterates over the filter collec-
tion. For every filter, it creates a menu item and uses the filter's getName method to deter-
mine the item's label.

■ Having done this, we can write a generic applyFilter method that receives a filter as a
parameter and applies this filter to the current image.
The imageviewer2-0 project includes a complete implementation of these changes.

Exercise 11.35 Open the imageviewer2-0 project. Study the code for the new method to
create and apply filters in class ImageViewer . Pay special attention to the makeMenuBar
and applyFilter methods. Explain in detail how the creation of the filter menu items and
their activation works. Draw an object diagram. Note, in particular, that the filter variable in
makeMenuBar has been declared as final , as discussed in Section 11.4.8. Make sure that
you understand why this is necessary.

Exercise 11.36 What needs to be changed to add a new filter to your image viewer?

Exercise 11.37 Challenge exercise You might have observed that the apply methods of
all of the Filter subclasses have a very similar structure: iterate over the whole image and
change the value of each pixel independently of surrounding pixels. It should be possible to iso-
late this duplication in much the same way as we did in creating the Filter class.

Create a method in the Filter class that iterates over the image and applies a filter-specific
transformation to each individual pixel. Replace the bodies of the apply methods in the three
Filter subclasses with a call to this method, passing the image and an object that can apply
the appropriate transformation.