#vulnerable code 
                                @Test
    public void testAppender() throws Exception {
        final Logger logger = ctx.getLogger();
        logger.debug("This is test message number 1");
        Thread.sleep(1500);
        // Trigger the rollover
        for (int i = 0; i < 16; ++i) {
            logger.debug("This is test message number " + i + 1);
        }
        final File dir = new File(DIR);
        assertTrue("Directory not created", dir.exists() && dir.listFiles().length > 0);

        final int MAX_TRIES = 20;
        for (int i = 0; i < MAX_TRIES; i++) {
            final File[] files = dir.listFiles();
            assertTrue("No files created", files.length > 0);
            for (final File file : files) {
                if (file.getName().endsWith(".gz")) {
                    return; // test succeeded
                }
            }
            logger.debug("Adding additional event " + i);
            Thread.sleep(100); // Allow time for rollover to complete
        }
        fail("No compressed files found");
    } 
                            #location 16 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testAppender() throws Exception {
        final Logger logger = ctx.getLogger();
        logger.debug("This is test message number 1");
        Thread.sleep(1500);
        // Trigger the rollover
        for (int i = 0; i < 16; ++i) {
            logger.debug("This is test message number " + i + 1);
        }
        final File dir = new File(DIR);
        assertThat(dir, both(exists()).and(hasFiles()));

        final int MAX_TRIES = 20;
        final Matcher<File[]> hasGzippedFile = hasItemInArray(that(hasName(that(endsWith(".gz")))));
        for (int i = 0; i < MAX_TRIES; i++) {
            final File[] files = dir.listFiles();
            if (hasGzippedFile.matches(files)) {
                return; // test succeeded
            }
            logger.debug("Adding additional event " + i);
            Thread.sleep(100); // Allow time for rollover to complete
        }
        fail("No compressed files found");
    }