I have some java code that I want to share with some classmates however I do not want to share the source with them.
What is the standard method to provide someone with a Java executable that they can use but they cannot see the source.
Not sure if this is relevant but the code that I will be giving them will be run on android.
Package only the class files into a .jar file and don't give them the source.
Edit: Android has its own way of packaging applications, I would suggest using it.
Android doesn't use the Java runtime. I only uses Java as a source language, it runs a custom runtime with a custom bytecode format. You can decompile the .dex byte code, but not to Java source code, at least not at the time of this post.
It depends on what you mean by "they can use".
If you just want them to be able to run your application on their phones (or emulators) you can give them an .apk file, which is the standard format for Android Applications. The easiest way to produce an .apk is to use the Android Development Tools (ADT) plugin for Eclipse. Otherwise you'll have to use the aapt command.
If you want to allow them to add your classes to their applications you'll have to give them a .jar file containing your compiled .class files. However, it is possible to turn .class files back into .java files using a decompiler so you if you really don't want them to see your code you may be stuck.
There are tools - known as obfuscators - which make the decompiling process harder, but personally I wouldn't bother with them here as they also make debugging harder too.
It won't be possible for anyone to turn your .apk back into source. Android uses a different Virtual Machine to standard Java and so has .dex files with Dalvik bytecode rather than .class files with JVM bytecode, and currently there are no tools that turn .dex files back in to .java source files.