■ You cannot have more than one public class per file.
■ The only five allowable class keywords are: public, abstract, final, extends, and implements.
■ Classes cannot be protected, private, native, synchronized, or final and abstract together. (But inner classes can be private.)
■ A class must be made abstract if it contains just one or more abstract methods.
■ Classes can be made abstract any time you wish – even when not necessary due to Java rules.
■ Classes that are abstract can contain non-abstract methods.
■ A class
statement, along with an interface
and a constructor, are the only three
Java statements that can appear with absolutely no modifiers.
Methods can’t do so, as they need a return type or void.
Variables can't, as they need a type.
■ Note that you don't need a class statement to get a good compile. The JVM will compile a completely empty source file without error.