[java] transparency in sprites??

ok im working on sprites for a java applet, and i want to put them in the applet now.. how should i handle the transparency masking or hwatever? should i just use a single image and fill the background with a color like purple, then tell java to use purple as the transparency? can it do that? or do i need to create like a mask which is basically the same image with all the positive space filled with black, and negative space as white? thanks