Better boolean file filters.

Author: Gary Gregory

src/main/java/org/apache/commons/io/filefilter/FalseFileFilter.java

@@ -86,4 +86,9 @@ public FileVisitResult accept(final Path file, final BasicFileAttributes attribu
         return FileVisitResult.TERMINATE;
     }
 
+    @Override
+    public IOFileFilter not() {
+        return TrueFileFilter.INSTANCE;
+    }
+
 }

src/main/java/org/apache/commons/io/filefilter/IOFileFilter.java

@@ -93,7 +93,7 @@ default AndFileFilter and(final IOFileFilter... fileFilter) {
      * @return a new filter
      * @since 2.9.0
      */
-    default NotFileFilter not() {
+    default IOFileFilter not() {
         return new NotFileFilter(this);
     }
 

src/main/java/org/apache/commons/io/filefilter/TrueFileFilter.java

@@ -86,4 +86,9 @@ public FileVisitResult accept(final Path file, final BasicFileAttributes attribu
         return FileVisitResult.CONTINUE;
     }
 
+    @Override
+    public IOFileFilter not() {
+        return FalseFileFilter.INSTANCE;
+    }
+
 }

src/test/java/org/apache/commons/io/filefilter/FileFilterTestCase.java

@@ -524,6 +524,8 @@ public void testTrue() throws Exception {
         assertFiltering(filter, (Path) null, true);
         //
         assertSame(TrueFileFilter.TRUE, TrueFileFilter.INSTANCE);
+        assertSame(FalseFileFilter.FALSE, TrueFileFilter.INSTANCE.not());
+        assertSame(FalseFileFilter.INSTANCE, TrueFileFilter.INSTANCE.not());
     }
 
     @Test
@@ -536,6 +538,8 @@ public void testFalse() throws Exception {
         assertFiltering(filter, (File) null, false);
         assertFiltering(filter, (Path) null, false);
         assertSame(FalseFileFilter.FALSE, FalseFileFilter.INSTANCE);
+        assertSame(TrueFileFilter.TRUE, FalseFileFilter.INSTANCE.not());
+        assertSame(TrueFileFilter.INSTANCE, FalseFileFilter.INSTANCE.not());
     }
 
     @Test