#vulnerable code 
                                @Test
    public void testAppender() throws Exception {
        for (int i=0; i < 100; ++i) {
            logger.debug("This is test message number " + i);
        }
        final File dir = new File(DIR);
        assertTrue("Directory not created", dir.exists() && dir.listFiles().length > 0);
        final File[] files = dir.listFiles();
        assertTrue("No files created", files.length > 0);
        boolean found = false;
        for (final File file : files) {
            final String name = file.getName();
            if (name.startsWith("test1") && name.endsWith(".log")) {
                found = true;
                break;
            }
        }
        assertTrue("No archived files found", found);
    } 
                            #location 9 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testAppender() throws Exception {
        for (int i=0; i < 100; ++i) {
            logger.debug("This is test message number " + i);
        }
        final File dir = new File(DIR);
        assertThat(dir, both(exists()).and(hasFiles()));
        final File[] files = dir.listFiles();
        assertNotNull(files);
        final Matcher<File> withCorrectFileName =
            both(hasName(that(endsWith(".log")))).and(hasName(that(startsWith("test1"))));
        assertThat(files, hasItemInArray(withCorrectFileName));
    }