Question

Guys, I’ve come across such legal behaviour:

File B.java:

final class C {}
final class D {}

File A.java:

class B {}
public class A {}

Questions:

1. When class X is required to be placed into its own X.java file? Does class visibility/final matter here?
2. Is there any official spec on this class/java relation?

Thanks a lot.

Answer 1

The de-facto standard in most implementations is that a source file can only contain one top-level public type definition. The name of the source file must be the name of that type.

A source file can contain nested types, but by definition they’re not a top-level public type.

This is recommended in, but not required by, the Java Language Specification.

JLS 7.6 Top Level Type Declarations

When packages are stored in a file system, 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 either of the following is true:

• The type is referred to by code in other compilation units of the package in which the type is declared.
• The type is declared public (and therefore is potentially accessible from code in other packages).

This restriction implies that there must be at most one such type per compilation unit. This restriction makes it easy for a compiler for the Java programming language or an implementation of the Java virtual machine to find a named class within a package; for example, the source code for a public type wet.sprocket.Toad would be found in a file Toad.java in the directory wet/sprocket, and the corresponding object code would be found in the file Toad.class in the same directory.

Note that final has nothing to do with accessibility, so it’s not a relevant issue in this matter.

Related questions

• What should be the name of a Java source that contains more than one class?

See also

• Java Tutorials/Packages/Managing Source and Class Files
• Joseph D. Darcy’s blog – Nested, Inner, Member, and Top-Level Classes