Why does java not allow multiple public classes in a java file ?

Why should a public class be declared/kept in a java file with the same name as the class ?

Just because that's how the designers of the Java language designed it. They probably thought that would make it easier to find the source of a specific class - if you know the class name, you know for what file to look. There's no technical reason why this would be necessary.

Not even public class, if you define a public interface or a enum in java, you should give them a dedicated file with the same name of the interface or enum.

Please search; this question comes up quite frequently.

The Oak specification indicated it was "necessary for efficient package importation."

Ref: Java History 101: Once Upon an Oak.

marc weber wrote:The Oak specification indicated it was "necessary for efficient package importation."

Ref: Java History 101: Once Upon an Oak.

Thanks for the link. I visited it and saw this : "The compiler would have to make an additional pass through all the compilation units (.java files) to figure out which classes were where, which would make the compilation even slower. "

The meaning of "...additional pass through all..." is not clear. Can you tell me what it means ?

If a particular class is not found in the current file, the compiler would have to open and parse every .java file in the directory to find it.