What is the best way to scale images in Java?

I would really recommend giving imgscalr a look.

It is released under an Apache 2 license, hosted on GitHub, been deployed in a handful of web applications already, has a very simple, but pedantically documented API, has code that works around 2 major image bugs in the JDK for you transparently that you'll only ever notice if you suddenly start getting "black" images after a scale operation or horrible-looking results, gives you the best possible looking results available in Java, is available via Maven as well as a ZIP and is just a single class.

Basic use looks like this:

BufferedImage img = ImageIO.read(...); // load image
BufferedImage scaledImg = Scalr.resize(img, 320);

This is the simplest call where the library will make a best-guess at the quality, honor your image proportions, and fit the result within a 320x320 bounding box. NOTE, the bounding box is just the maximum W/H used, since your image proportions are honored, the resulting image would still honor that, say 320x200.

If you want to override the automatic mode and force it to give you the best-looking result and even apply a very mild anti-alias filter to the result so it looks even better (especially good for thumbnails), that call would look like:

BufferedImage img = ImageIO.read(...); // load image
BufferedImage scaledImg = Scalr.resize(img, Method.QUALITY, 
                                       150, 100, Scalr.OP_ANTIALIAS);

These are all just examples, the API is broad and covers everything from super-simple use cases to very specialized. You can even pass in your own BufferedImageOps to be applied to the image (and the library automatically fixes the 6-year BufferedImageOp JDK bug for you!)

There is a lot more to scaling images in Java successfully than the library does for you, for example always keeping the image in one of the best supported RGB or ARGB image types while operating on it. Under the covers the Java2D image processing pipeline falls back to an inferior software pipeline if the image type used for any image operations is poorly supported.

If all that sounded like a lot of headaches, it sort of is... that's why I wrote the library and open-sourced it, so folks could just resize their images and move on with their lives without needing to worry about it.

Hope that helps.

Have a look at the Java Image I/O API to read/write the image. Then use AffineTransform to resize.

Also, here's a complete example using java.awt.Image.