This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
There are no reflection calls to get a list of all packages, or a list of all classes in a package, because both of these are potentially unlimited lists. If this is a development environment and you're looking at class files laid out on disk, just use java.io.File to look at them. If the classes are in JAR files, then use the java.util.jar.JarFile class directly.