NoSuchMethod:main error

[Sami Devine]

I found the following eg in K & B SCJP 6

class Zoo {
public String coolMethod() {
return "Wow baby";
}
}
class Moo {
public void useAZoo() {
Zoo z = new Zoo();
// If the preceding line compiles Moo has access
// to the Zoo class
// But... does it have access to the coolMethod()?
System.out.println("A Zoo says, " + z.coolMethod());
// The preceding line works because Moo can access the
// public method
}
}

I have saved the file as Zoo.java. After javac Zoo.java both Zoo.class and Moo.class are created.
But for java Moo and java Zoo, I get "Exception in thread "main":NoSuchMethodError:main"
Does this indicate that I need to use the main method in Moo?
When is the main method not needed.
Thanks

[Christophe Verré]

The main method is needed if you want to execute your program. The Moo does not have it, so you can't do "java Moo". In fact, none of your classes have a main method, so you're not expected to execute any of them. Just to compile them.