Can Multiple Top-Level Classes Exist in a Single Java File, and What Are the Implications?

Java: Multiple Class Declarations in One File

While it's possible to define multiple top-level classes within a single Java file, adhering to certain conventions is essential. One such convention is that only one class may be declared as public, as specified by the Java Language Specification (JLS).

Terminologies

There is no specific term to describe this technique of defining multiple classes in a single file. Unlike inner, nested, or anonymous classes, these top-level classes have a different scope and relationship with their containing file.

Compilation Unit Restriction

The JLS states that the system may enforce the restriction that these non-public secondary classes cannot be "referred to by code in other compilation units of the package." This means that they cannot be treated as package-private.

Implementation

In practice, Java compilers like javac do not strictly enforce this restriction. However, they do have a limitation that makes it highly undesirable to refer to a top-level class from another file unless it shares the same name as the file it resides in.

For example, consider two Java files:

• Foo.java:

public class Foo {
    // ...
}

• Bar.java:

public class Bar {
    // ...
    Baz baz; // Compilation error if compiled separately
}

class Baz {
    // ...
}

If Foo.java references Baz but not Bar, attempting to compile Foo.java independently will result in a compilation error.

Reasoning

This limitation exists because javac cannot automatically determine which source file to search when a class is referenced from another file. To resolve this issue, the compiler requires that any top-level class referenced by an external class must either have the same name as the containing file or be explicitly declared within a class of the same name (e.g., in the example above, if Foo.java also contained a class Bar).

Consequences and Best Practices

While it is technically possible to utilize multiple top-level classes in a single file, it is generally considered poor practice due to the compiler limitations described above. To maintain a reliable build process, it is recommended to adhere to the convention of placing one top-level class per file and clearly declaring its visibility (public or package-private).

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