Summary

Change java.nio.file.Path::getExtension to return a non-null string representing the file extension of the path's file name with the period separator character included.

Problem

The present incarnation of Path::getExtension can return null in cases where no extension is found. This creates scenarios where NullPointerExceptions can become a problem. Not including the extension separator character in the extension also makes other path manipulations such as removing or replacing the extension somewhat more complex.

Solution

Define the the path's file name's file extension to be that portion of the string representation of the file name after and including the last period character as opposed to the portion of the file name string after the last period. A returned empty string "" instead of null now signifies that no extension was found, and a string containing only a single period character "." instead of solely an empty string "" now signifies an empty extension.

Specification

--- a/src/java.base/share/classes/java/nio/file/Path.java
+++ b/src/java.base/share/classes/java/nio/file/Path.java
@@ -257,53 +257,47 @@ public static Path of(URI uri) {
      * determined by the position of a period character ('.', U+002E FULL STOP)
      * within the file name string. If the file name element is {@code null},
      * or if the file name string does not contain a period character, or if
-     * the only period in the file name string is its first character, then
-     * the extension is {@code null}. Otherwise, the extension is the substring
-     * after the last period in the file name string. If this last period is
-     * also the last character in the file name string, then the  extension is
-     * {@linkplain String#isEmpty empty}.
+     * the only period in the file name string is its first character,  then
+     * the extension is {@linkplain String#isEmpty empty}. Otherwise, the
+     * extension is the substring starting with the last period in the file
+     * name string. If this last period is also the last character in the file
+     * name string, then the extension contains only a single period character.
+     *
+     * @apiNote
+     * Unless no extension is found, in which case an empty string is returned,
+     * the returned extension includes the period separator character. For
+     * example, the extension which would be returned for a path with file name
+     * "photograph.jpg" is ".jpg".
      *
      * @implSpec
      * The default implementation is equivalent for this path to:
      * <pre>{@code
      * int lastPeriod = fileName.lastIndexOf('.');
-     * if (lastPeriod <= 0)
-     *     return null;
-     * return (lastPeriod == fileName.length() - 1)
-     *     ? ""
-     *     : fileName.substring(lastPeriod + 1);
+     * return lastPeriod <= 0 ? return "" : fileName.substring(lastPeriod);
      * }</pre>
      *
-     * @return  the file name extension of this path, which might be the
-     *          empty string, or {@code null} if no extension is found
+     * @return  the non-{@code null} file extension of this path's file name,
+     *          possibly only a single period character, or an empty string if
+     *          no extension is found
      *
      * @since 20
      */
     default String getExtension() {}