The static field should be accessed in a static way

问题

I have different Exception category Enum as below

public enum GSBBCacheCategory  {
    SEARCH(9001),
    UPDATE_PERSECURITY(9002),
    CROSS_REFERENCING_PERSECURITY(9003),
    METADATA_SEARCH(9004),
    REMOVEALL(9005),
    UPDATE_BACKOFFICE(9002);

    private int exceptionCode;

    GSBBCacheCategory(int exceptionCode)
    {
        this.exceptionCode = exceptionCode;
    }

    public int getExceptionCode()
    {
        return exceptionCode;
    }
} 

public enum GSBBEncryptionCategory  {
.
.
.
}

I want to provide one place to access these Enum in client code. Presently I achieved this as below

public class GSBBExceptionCodes
{
 public static GSBBDecryptionCategory decryptionCategory;
 public static GSBBCacheCategory cacheCategory;
}

Now to access exception code I have do something like below

public static void main(String[] args) {
     System.out.println(GSBBExceptionCodes.decryptionCategory.ERRORCODE_DECRYPTION_FAILURE);
     System.out.println(GSBBExceptionCodes.cacheCategory.UPDATE_PERSECURITY);
}

Which says “The static field GSBBDecryptionCategory.ERRORCODE_DECRYPTION_FAILURE should be accessed in a static way”

Is it possible to achieve above without any warning?

回答1:

There are two ways to reference a static member (either a field or a method). One is WhateverClass.theField, and the other is someInstance.theField where someInstance has a compile-time type of WhateverClass. The former is much clearer, and so your IDE is helpfully telling you to use it instead of the latter.

The reason it's better is that referencing a static member by an instance makes it look like the method has something to do with that instance, which it doesn't. Here's a real-life example:

Thread myThread = getMyThread();
myThread.start();
myThread.sleep(5000);

At first blush, it looks like you're asking myThread to sleep for 5 seconds (5000 milliseconds), but that's not what you're doing at all. You're asking the current thread to sleep, because that last line is exactly the same as invoking Thread.sleep(5000). That second example is much more clearly a static method.

Or, here's another example. Let's say your static fields were mutable.

public class Foo {
    public static int value = 1;
}

(This public static mutable field is a bad idea for other reasons, but simplifies the example). Now let's say you do:

Foo one = new Foo();
Foo two = new Foo();
one.value = 2;
two.value = 3;
System.out.println(one.value);
System.out.println(two.value);

Kinda looks like that should print "2" and then "3", right? But no -- it'll print "3", "3" because both assignments to .value are in fact to the same, static field. It's just an optical illusion that the one or two instances have anything to do with anything.

Imho, the ability to reference static members from instances is a misfeature. But it's there, so you should avoid it. Which is what the compiler is trying to suggest you do.

回答2:

Try this :

public static void main(String[] args) {
     System.out.println(GSBBDecryptionCategory.ERRORCODE_DECRYPTION_FAILURE);
     System.out.println(GSBBCacheCategory.UPDATE_PERSECURITY);
}

You are now accessing the field in a static way which should remove the warning.

回答3:

It sounds like instead of having these as public static fields, they should be inner classes:

public class GSBBExceptionCodes {
  public enum GSBBCacheCategory {
    ...
  }
}

来源：https://stackoverflow.com/questions/18214493/the-static-field-should-be-accessed-in-a-static-way

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