output: #fixed code
                                  @Test
      public void testAppender() throws Exception {
          final Logger logger = ctx.getLogger();
          // Trigger the rollover
          for (int i = 0; i < 10; ++i) {
              // 30 chars per message: each message triggers a rollover
              logger.debug("This is a test message number " + i); // 30 chars:
          }
          Thread.sleep(100); // Allow time for rollover to complete
  
          final File dir = new File(DIR);
          assertTrue("Dir " + DIR + " should exist", dir.exists());
          assertTrue("Dir " + DIR + " should contain files", dir.listFiles().length > 0);
  
          final int MAX_TRIES = 20;
          for (int i = 0; i < MAX_TRIES; i++) {
              final File[] files = dir.listFiles();
              for (File file : files) {
                  System.out.println(file);
              }
              if (files.length == 3) {
                  for (File file : files) {
                      assertTrue("test-4.log should have been deleted",
                              Arrays.asList("test-1.log", "test-2.log", "test-3.log").contains(file.getName()));
                  }
                  return; // test succeeded
              }
              logger.debug("Adding additional event " + i);
              Thread.sleep(100); // Allow time for rollover to complete
          }
          fail("No rollover files found");
      }
input: #vulnerable code 
                                  @Test
      public void testAppender() throws Exception {
          final Logger logger = ctx.getLogger();
          // Trigger the rollover
          for (int i = 0; i < 10; ++i) {
              // 30 chars per message: each message triggers a rollover
              logger.debug("This is a test message number " + i); // 30 chars:
          }
          Thread.sleep(100); // Allow time for rollover to complete
  
          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();
              for (File file : files) {
                  System.out.println(file);
              }
              if (files.length == 3) {
                  for (File file : files) {
                      assertTrue("test-4.log.gz should have been deleted",
                              Arrays.asList("test-1.log.gz", "test-2.log.gz", "test-3.log.gz").contains(file.getName()));
                  }
                  return; // test succeeded
              }
              logger.debug("Adding additional event " + i);
              Thread.sleep(100); // Allow time for rollover to complete
          }
          fail("No rollover files found");
      } 
                              #location 17 
                              #vulnerability type NULL_DEREFERENCE