Quick Java question about private static final key

问题:

I'm declaring a field:

private static final String filename = "filename.txt";

First, does the order of private static final matter? If not, is there a standard accepted sequence or convention?

Second, the filename in my application is fixed. Is this the best was to store its value?

回答1:

I use Checkstyle with Eclipse, which results in a warning if the declaration is in a different order to the one you've specified, citing the Java Language Specification (JLS). For example,

private final static String filename = "filename.txt";

results in

'static' modifier out of order with the JLS suggestions.

They have this page which lists the order they expect, though following the links on that page through to the JLS I can't see anything to back up their assertion of a suggested order.

Having said that, the order they suggest seems to correspond to the order in most of the code I've seen, so it seems as good a convention as any to adopt.

回答2:

1. No. But that is the sequence I usually see used.

2. It's a reasonable choice, but some would prefer a configuration file, either Properties or another file format (e.g. XML). That way, you can change the filename without recompiling.

回答3:

It's common in Java to give constants (static final values) an all-uppercase name, so I would write:

private static final String FILENAME = "filename.txt";

See also Code Conventions for the Java Programming Language. (Those are Sun's code conventions that the majority of Java programmers use).

回答4:

The most accepted order of these keywords is private static final. Also you can remember the order of these keywords using PSF pattern that:

P => private / public / protected
S => static / abstract / ...
F => final

回答5:

1. The order doesn't matter, but you can always play around with it - there's only 6 possibilities to test.

2. I'm not aware of any convention, though I put the visibility modifier first (public/private/protected) so you can eyeball it and it lines up.

3. If it's fixed then you can do that, but I always think something is a constant only to discover later (during testing, for example) that I want to pass it in. An argument on the command line or a properties file works for that case, and is a minimum of effort to set up.

回答6:

see: http://docs.oracle.com/javase/specs/jls/se5.0/html/classes.html#8.3.1

8.3.1 Field Modifiers

FieldModifiers:
  FieldModifier
  FieldModifiers FieldModifier

FieldModifier: one of
  Annotation public protected private
  static final transient volatile

...

If two or more (distinct) field modifiers appear in a field declaration, it is customary, though not required, that they appear in the order consistent with that shown above in the production for FieldModifier.

回答7:

To complete the nice answer by @Hobo above by a current link

8.1.1. Class Modifiers

A class declaration may include class modifiers.

     ClassModifier:
         (one of) 
         Annotation public protected private 
         abstract static final strictfp

[...]

If two or more (distinct) class modifiers appear in a class declaration, then it is customary, though not required, that they appear in the order consistent with that shown above in the production for ClassModifier.