Can Multiple Java Classes Exist in One File, and What Are the Implications?

Multiple Class Declarations in One Java File

In Java, it is possible to define multiple classes within the same source file. However, this practice has several caveats and limitations.

Lack of a Specific Name for the Technique

Despite being a somewhat common practice, this technique does not have a specific or official name, unlike other class categorizations such as inner, nested, or anonymous classes.

Compilation Limitations

While the Java Language Specification (JLS) states that the system may enforce the restriction on referring to non-public classes in other compilation units, this limitation is not consistently implemented across Java compilers.

Javac's Limitations

Specifically, Javac may issue an error when compiling a class that refers to a top-level class from another file if neither class is the namesake of the file they reside in. This limitation is due to the compiler's inability to determine which source file to reference for the non-public class.

Avoidance of the Issue

To avoid this limitation, developers typically adhere to the convention of placing a single top-level class in each source file. This practice also allows for greater flexibility in changing the access level of classes without affecting the compilation process.

Newer Javac Feature

Newer versions of Javac also provide a warning when encountering auxiliary classes that are referenced from outside their source file, further encouraging the use of a single top-level class per file.

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