what does "start a class" mean? if you mean "invoke the main method" like the JVM would, simply call the main method like any other method:
// class 1
public class DoStuff {
//methods, whatever
public static void main(String[] args) {
// stuff
// things
}
}
...
// class 2
public class DoOtherStuff {
public void invokeAnotherClassesMain() {
DoStuff.main(null);
}
}
but if you're needing to do that in order to get things done, you've probably got design issues