Question

I have this code to validate a java.io.Fileparameter which should not be null, should be accessible, should be a file and not a directory, etc.:

private static final String EXCEPTION_FILE_CAN_NOT_BE_READ =
    "The file %s does not seem to readable.";
private static final String EXCEPTION_PATH_DOES_NOT_EXIST =
    "The path %s does not seem to exist.";
private static final String EXCEPTION_PATH_IS_NOT_A_FILE =
    "The path %s does not seem to correspond to a file.";
private static final String EXCEPTION_PATH_REFERENCE_IS_NULL =
    "The supplied java.io.File path reference can not be null.";

public static Banana fromConfigurationFile(
    File configurationFile) {
  if (configurationFile == null) {
    String nullPointerExceptionMessage =
        String.format(EXCEPTION_PATH_REFERENCE_IS_NULL, configurationFile);
    throw new NullPointerException();
  }
  if (!configurationFile.exists()) {
    String illegalArgumentExceptionMessage =
        String.format(EXCEPTION_PATH_DOES_NOT_EXIST,
            configurationFile.getAbsolutePath());
    throw new IllegalArgumentException(illegalArgumentExceptionMessage);
  }
  if (!configurationFile.isFile()) {
    String illegalArgumentExceptionMessage =
        String.format(EXCEPTION_PATH_IS_NOT_A_FILE,
            configurationFile.getAbsolutePath());
    throw new IllegalArgumentException(illegalArgumentExceptionMessage);
  }
  if (!configurationFile.canRead()) {
    String illegalArgumentExceptionMessage =
        String.format(EXCEPTION_FILE_CAN_NOT_BE_READ,
            configurationFile.getAbsolutePath());
    throw new IllegalArgumentException(illegalArgumentExceptionMessage);
  }
  // ... more tests, like "isEncoding(X)", "isBanana(ripe)", ...
}

Looks like a lot of boilerplate for something I could be “pinching” from somewhere. Especially because these are not all the checks that I need, there is more (e.g. the file is a text file and has the right encoding, …). It seems reasonable to me that there would be a simpler way to do it than this. Perhaps a FileSpecs object to construct through a Builder and to pass to a verifyFileSpecs static helper?

Question: am I doing it wrong or is there code I could reuse?

Answer to the FAQ for post validity:

Shows I made some research beforehand: I looked at the Java 6 SDK, that’s where I got the different methods from, looked at JDK 7 and Files.isReadable, looked at Apache Commons IO, …

Shows that this question is unique: I am specifically asking if there is code that I can reuse, I am not asking “how do I check if a path corresponds to a file and not a directory?”, all of which has already an answer on SO

Why this could be useful to others: teams don’t like boilerplate code like that submitted for code review, checked-in and versioned, potentially maintained (unit tests, etc.) So, borrowing the code from a reputable source would be very helpful, in my opinion.

Answer 1

Yes, I would say above code is not DRY (Don't Repeat Yourself).

Consider using Validate from Apache Commons.

public static Banana fromConfigurationFile(File configurationFile) {
  Validate.notNull(configurationFile, String.format(EXCEPTION_PATH_REFERENCE_IS_NULL, configurationFile));
  Validate.isTrue(configurationFile.exists(), String.format(EXCEPTION_PATH_DOES_NOT_EXIST, configurationFile.getAbsolutePath()));
  Validate.isTrue(configurationFile.isFile()), String.format(EXCEPTION_PATH_IS_NOT_A_FILE, configurationFile.getAbsolutePath()));
  // and more validation...

}