with Image you can use:
java.awt.image.ReplicateScaleFilter
or java.awt.image.AreaAveragingScaleFilter

with BufferedImage use
java.awt.image.AffineTransformOp

I recently had to do something similar to what you want to do. Your code
should look like very similar to this:

===========================================================
     BufferedImage bufferedImage = null;
     float scaleFactor = 0.5f;

     try {
               BufferedImage unalteredImage = reader.read(0);
               AffineTransformOp op = new
AffineTransformOp(AffineTransform.getScaleInstance(scaleFactor,
scaleFactor), AffineTransformOp.TYPE_NEAREST_NEIGHBOR);
                   BufferedImage scaledImage = op.filter(unalteredImage,
null);
                   bufferedImage = scaledImage;
           } catch (IOException io_excp) { //error handling }

===========================================================

Using 'AffineTransform.getScaleInstance(scaleFactor, scaleFactor)' ensures
that the scaled image has the same aspect ratio as the original image.

You may want to experiment with different values for the second parameter in
the AffineTransformOp instantiation; there are other possibilities besides
AffineTransformOp.TYPE_NEAREST_NEIGHBOR but I'm not particularly clear from
the API how they differ from one another in effect. It might be worthwhile
for you to try a few different values and see which ones give the
fastest/clearest results.

Rhino