Question

The error I get is exactly the error specified in:

http://java.syntaxerrors.info/index.php?title=Own_file

(class must be defined in its own file.)

but they don’t give a solution there how to solve it, other than just having a file per public class.

Thank you, eclipse, for making me do this, but this is not mandatory in Java. Is there a way to get rid of this error?

Answer 1

Yes, it is mandatory in Java. Each public class has to be in a separate file named exactly the same way as the class.

See this question about it. The Java language specification writes that this is not 100% mandatory for compilers, but they usually do that. And since it is a good thing, and is noted in the spec, all compilers do it.

When packages are stored in a file system (§7.2.1), the host system may choose to enforce the restriction that it is a compile-time error if a type is not found in a file under a name composed of the type name plus an extension (such as .java or .jav)

If you want to have multiple classes in the same file, that’s a different story. You can do it in two ways:

• declare them as package-private classes with class Foo after the body of the main class. You can have any number of non-public classes in the same file
• declare them as static inner classes: public static class InnerFoo inside the main class body. That way they will be visible to other classes by FooClass.InnerFoo