Simplify relativization of paths in JDKFileTreeWatch

Author: sungshik

src/main/java/engineering/swat/watch/WatchEvent.java

@@ -106,11 +106,12 @@ public Path calculateFullPath() {
     }
 
     /**
-     * @return The file name of the full path of this event, but without
+     * @return The file name of the full path of this event, or {@code null} if
+     * it has zero elements (cf. {@link Path#getFileName()}), but without
      * calculating the full path. This method is equivalent to, but more
-     * efficient than, <code>calculateFullPath().getFileName()</code>.
+     * efficient than, {@code calculateFullPath().getFileName()}.
      */
-    public Path getFileName() {
+    public @Nullable Path getFileName() {
         return relativePath.getParent() == null ? rootPath.getFileName() : relativePath.getFileName();
     }
 

src/main/java/engineering/swat/watch/impl/jdk/JDKFileTreeWatch.java

@@ -68,8 +68,9 @@ public JDKFileTreeWatch(Path rootPath, Path relativePathParent, Executor exec,
             // `rootPath` (instead of `path`, as is the default behavior)
             @Override
             public WatchEvent relativize(WatchEvent event) {
+                var fileName = event.getFileName();
                 return new WatchEvent(event.getKind(), rootPath,
-                    relativePathParent.resolve(event.getFileName()));
+                    fileName == null ? relativePathParent : relativePathParent.resolve(fileName));
             }
 
             // Override to ensure that this watch translates JDK events using