private void fillEmptyFields() {
            if (fileNameGenerator == null) {
                fileNameGenerator = DefaultsFactory.createFileNameGenerator();
            }
            if (backupStrategy == null) {
                backupStrategy = DefaultsFactory.createBackupStrategy();
            }
            if (cleanStrategy == null) {
                cleanStrategy = DefaultsFactory.createCleanStrategy();
            }
            if (flattener == null) {
                flattener = DefaultsFactory.createFlattener2();
            }
        }
Build configured {@link FilePrinter} object.

@return the built configured {@link FilePrinter} object
android/android_framework/base/src/main/java/com/elvishew/xlog/printer/file/FilePrinter.java