Fix java file extension filter in FileFilterTest

The file extension filter in the FileFilterTest class was corrected to specifically filter for files ending in ".java". Previously, the filter would look for any file ending in ".", which was not accurate or intended.

src/test/java/lambdas/FileFilterTest.java

@@ -73,7 +73,7 @@ void listJavaSourceFiles_fileNameFilter() {
 
     @Test
     void listJavaSourceFiles_fileFilter() {
-        File[] javaFiles = root.listFiles(file -> file.getName().endsWith("."));
+        File[] javaFiles = root.listFiles(file -> file.getName().endsWith(".java"));
         assert javaFiles != null;
         assertEquals(8, javaFiles.length);
     }