#vulnerable code 
                                @Test
    public void testAppender() throws Exception {
        for (int i = 0; i < 8; ++i) {
            logger.debug("This is test message number " + i);
        }
        Thread.sleep(50);
        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);
        for (final File file : files) {
            assertHeader(file);
            assertFooter(file);
        }
        final File logFile = new File(LOGFILE);
        assertTrue("Expected logfile to exist: " + LOGFILE, logFile.exists());
        assertHeader(logFile);
    } 
                            #location 10 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testAppender() throws Exception {
        for (int i = 0; i < 8; ++i) {
            logger.debug("This is test message number " + i);
        }
        Thread.sleep(50);
        final File dir = new File(DIR);
        assertThat(dir, both(exists()).and(hasFiles()));
        final File[] files = dir.listFiles();
        assertNotNull(files);
        for (final File file : files) {
            assertHeader(file);
            assertFooter(file);
        }
        final File logFile = new File(LOGFILE);
        assertThat(logFile, exists());
        assertHeader(logFile);
    }