Can Multiple Top-Level Java Classes in One File Interact with External Code?

Multiple Class Declarations in Java: A Detailed Explanation

In Java, multiple top-level classes can coexist within a single file, with only one of them being designated as public. This technique, lacking a specific name analogous to inner, nested, or anonymous classes, presents two key questions:

1. Are these secondary classes restricted from interacting with code outside their compilation unit?

The Java Language Specification (JLS) allows for but does not enforce a restriction prohibiting secondary classes from being referenced by code in other compilation units within the same package. This implies that they cannot be treated as package-private.

2. Do different Java implementations handle this restriction differently?

Contrary to the JLS, the Java compiler javac actively enforces a limitation that effectively discourages referencing top-level classes from external files unless they share the same name as the file they reside in.

Detailed Explanation

This limitation manifests when a class in File A attempts to access a class in File B, where File B also contains another class with the same name as File B. However, if class in File A also accesses the other class in File B with the same name, the compilation error disappears.

To illustrate:

FileA.java:

public class FileA {

    private FileB fileB;

}

FileB.java:

public class FileB {

    private class FileBClass {

    }

}

Compiling FileA.java alone will result in an error:

FileA.java:2: cannot find symbol
symbol  : class FileBClass
location: class FileA

To resolve this, either FileA.java must also reference FileB.java, or FileB.java must be compiled beforehand.

Conclusion

While not explicitly prohibited, javac's enforced limitation discourages referring to top-level classes from external files unless they share the same name as the file they reside in, making this practice impractical in most cases.

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