Can Multiple Top-Level Classes Exist in a Single Java File?

Multiple Class Declarations in a Single Java File

In Java, it is possible to define multiple top-level classes in one file, but with the restriction that only one of these classes can be public (as per JLS §7.6).

Is there a specific term for this technique?

While there is not a formal name for this practice analogous to "inner," "nested," or "anonymous" classes, it can be referred to as top-level multiple class declaration.

Are secondary classes restricted from being referenced in other compilation units?

The JLS does indeed state that the Java Virtual Machine (JVM) may enforce a restriction that these secondary classes cannot be "referred to by code in other compilation units of the package." However, this limitation is not consistently enforced across Java implementations.

Explanation of Java's Implementation Behavior

While some Java implementations may not actively prohibit this cross-file referencing, they have limitations that effectively discourage it. For instance, if a public class named Foo refers to another top-level class named Bar from a different file, the compilation of Foo may fail with a similar error: "cannot find symbol: class Baz". This error arises because the compiler does not know which source file to search for the referenced class Bar.

To avoid these issues, it is advisable to limit each source file to only one top-level class, unless the secondary classes need to access the public class for some reason.

The above is the detailed content of Can Multiple Top-Level Classes Exist in a Single Java File?. For more information, please follow other related articles on the PHP Chinese website!