Java: static abstract (again) - best practice how to work around

annotations could be fine for your purpose.

@FileProperties(desc="data file")
public class DataFile extends XFile { ... }

FileProperties props = DataFile.class.getAnnotation(FileProperties.class);
String desc = props.desc(); 

Accessing the info still requires reflection, however it's a little better than using static field/method.

Java compiler does not enforce that all subclasses are annotated as such. You can add your logic to the compiler (using annotation processing) but that's too complicated. It's ok to check it at runtime.

Update:

This is also possible:

@FileInfoClass ( DataFileInfo.class )
@public class DataFile

This sounds like a great time to pull out the Fundamental Theorem of Software Engineering:

Any problem can be solved by adding another layer of indirection.

The problem you have right here is that a file carries around multiple pieces of information - what the type of the file is, a description of the file, the file contents, etc. I'd suggest splitting this into two classes - one class representing a concrete file on disk and its contents, and a second that is an abstract description of some file type. This would allow you to treat the file type class polymorphically. For example:

public interface FileType {
     String getExtension();
     String getDescription();

     /* ... etc. ... */
}

Now, you can make subclasses for each of the file types you use:

public class TextFileType implements FileType {
     public String getExtension() {
         return ".txt";
     }
     public String getDescription() {
         return "A plain ol' text file.";
     }
     /* ... */
}

You can then have some large repository of these sorts of objects, which would allow you to query their properties without having an open file of that type. You could also associate a type with each actual file you use by just having it store a FileType reference.

The question is not clear enough to provide an objective answer. Since I cannot give you a fish, this answer is more on the lines of "Teach you to fish" :)

When faced with design issues like these, where you think "duh..now sure why such a simple thing is so hard" more often than not, you are either designing it just incorrectly, or you are overcomplicating things. If I am empathizing correctly, your design issue seems like a "common requirement" yet the language is not allowing for any elegant solutions.

• Trace back your design steps/decisions
• question all the "obvious" and "of course" you are basing your design on (you are using quite a few above)
• see if things can be simplified (don't take any of the OO concepts to their logical extreme. Make compromises based on ROI)

...and you will most likely arrive at an acceptable answer.

If you still don't, post back the classes and interfaces you think you want (with compile errors since language is not allowing certain things), and maybe we can help you tune your design.

To restate the problem: you want your per-file-type classes to have statically available information on the type (e.g., name and description).

We can easily get part-way there: create a separate class for your type info, and have a static instance of this (appropriately instantiated) in each per-file-type class.

package myFileAPI;

public class TypeInfo { 
    public final String name;
    public final String description;

    public TypeInfo(String name, String description) {
        this.name = name;
        this.description = description;
    }
}

and, say:

package myFileAPI;

public class TextFile {
    public static final TypeInfo typeInfo
                   = new TypeInfo("Text", "Contains text.");
}

Then you can do stuff like:

System.out.println(TextFile.typeInfo.name);

(Of course, you could also use getters in TypeInfo to encapsulate the underlying strings.)

However, as you said, what we really want is to enforce the existence of a particular signature static method in all your per-file-type classes at compile time, but the 'obvious' design path leads to requiring an abstract static method in a common superclass which isn't allowed.

We can enforce this at run-time though, which may be good enough to ensure it is coded correctly. We introduce a File superclass:

package myFileAPI;

public abstract class File {

    public static TypeInfo getTypeInfo() {
        throw new IllegalStateException(
                    "Type info hasn't been set up in the subclass");
    }

}

If TextFile now extends File, we will get this exception when calling TextFile.getTypeInfo() at runtime, unless TextFile has a same-signature method.

This is quite subtle: code with TextFile.getTypeInfo() in still compiles, even when there is no such method in TextFile. Even though static methods are bound at compile time, the compiler can still look through the class hierarchy to determine the compile-time static call target.

So, we need code like:

package myFileAPI;

public class TextFile extends File {

    private static final TypeInfo typeInfo
                      = new TypeInfo("Text", "Contains text.");

    // Shadow the superclass static method
    public static TypeInfo getTypeInfo() {
        return typeInfo;
    }

}

Note that we are still shadowing the superclass method, and so File.getTypeInfo() can still be 'meaninglessly' called.