output: #fixed code
                                public void download_multiple_files() throws IOException, NoSuchAlgorithmException, InterruptedException, URISyntaxException {
      int numFiles = 50;
      this.tracker.setAcceptForeignTorrents(true);
  
      final File srcDir = tempFiles.createTempDir();
      final File downloadDir = tempFiles.createTempDir();
  
      Client seeder = createClient("seeder");
      seeder.start(InetAddress.getLocalHost());
      Client leech = null;
  
  
      try {
        URL announce = new URL("http://127.0.0.1:6969/announce");
        URI announceURI = announce.toURI();
        final Set<String> names = new HashSet<String>();
        List<File> filesToShare = new ArrayList<File>();
        for (int i = 0; i < numFiles; i++) {
          File tempFile = tempFiles.createTempFile(513 * 1024);
          File srcFile = new File(srcDir, tempFile.getName());
          assertTrue(tempFile.renameTo(srcFile));
  
          Torrent torrent = TorrentCreator.create(srcFile, announceURI, "Test");
          File torrentFile = new File(srcFile.getParentFile(), srcFile.getName() + ".torrent");
          saveTorrent(torrent, torrentFile);
          filesToShare.add(srcFile);
          names.add(srcFile.getName());
        }
  
        for (File f : filesToShare) {
          File torrentFile = new File(f.getParentFile(), f.getName() + ".torrent");
          seeder.addTorrent(torrentFile.getAbsolutePath(), f.getParent());
        }
        leech = createClient("leecher");
        leech.start(new InetAddress[]{InetAddress.getLocalHost()}, 5, null, new SelectorFactoryImpl());
        for (File f : filesToShare) {
          File torrentFile = new File(f.getParentFile(), f.getName() + ".torrent");
          leech.addTorrent(torrentFile.getAbsolutePath(), downloadDir.getAbsolutePath());
        }
  
        new WaitFor(60 * 1000) {
          @Override
          protected boolean condition() {
  
            final Set<String> strings = listFileNames(downloadDir);
            int count = 0;
            final List<String> partItems = new ArrayList<String>();
            for (String s : strings) {
              if (s.endsWith(".part")) {
                count++;
                partItems.add(s);
              }
            }
            if (count < 5) {
  
              System.err.printf("Count: %d. Items: %s%n", count, Arrays.toString(partItems.toArray()));
            }
            return strings.containsAll(names);
          }
        };
  
        assertEquals(listFileNames(downloadDir), names);
      } finally {
        leech.stop();
        seeder.stop();
      }
    }
input: #vulnerable code 
                                public void download_multiple_files() throws IOException, NoSuchAlgorithmException, InterruptedException, URISyntaxException {
      int numFiles = 50;
      this.tracker.setAcceptForeignTorrents(true);
  
      final File srcDir = tempFiles.createTempDir();
      final File downloadDir = tempFiles.createTempDir();
  
      Client seeder = createClient("seeder");
      seeder.start(InetAddress.getLocalHost());
      Client leech = null;
  
  
      try {
        URL announce = new URL("http://127.0.0.1:6969/announce");
        URI announceURI = announce.toURI();
        final Set<String> names = new HashSet<String>();
        List<File> filesToShare = new ArrayList<File>();
        for (int i = 0; i < numFiles; i++) {
          File tempFile = tempFiles.createTempFile(513 * 1024);
          File srcFile = new File(srcDir, tempFile.getName());
          assertTrue(tempFile.renameTo(srcFile));
  
          Torrent torrent = TorrentCreator.create(srcFile, announceURI, "Test");
          File torrentFile = new File(srcFile.getParentFile(), srcFile.getName() + ".torrent");
          saveTorrent(torrent, torrentFile);
          filesToShare.add(srcFile);
          names.add(srcFile.getName());
        }
  
        for (File f : filesToShare) {
          File torrentFile = new File(f.getParentFile(), f.getName() + ".torrent");
          seeder.addTorrent(torrentFile.getAbsolutePath(), f.getParent());
        }
        leech = createClient("leecher");
        leech.start(new InetAddress[]{InetAddress.getLocalHost()}, 5, null);
        for (File f : filesToShare) {
          File torrentFile = new File(f.getParentFile(), f.getName() + ".torrent");
          leech.addTorrent(torrentFile.getAbsolutePath(), downloadDir.getAbsolutePath());
        }
  
        new WaitFor(60 * 1000) {
          @Override
          protected boolean condition() {
  
            final Set<String> strings = listFileNames(downloadDir);
            int count = 0;
            final List<String> partItems = new ArrayList<String>();
            for (String s : strings) {
              if (s.endsWith(".part")) {
                count++;
                partItems.add(s);
              }
            }
            if (count < 5) {
  
              System.err.printf("Count: %d. Items: %s%n", count, Arrays.toString(partItems.toArray()));
            }
            return strings.containsAll(names);
          }
        };
  
        assertEquals(listFileNames(downloadDir), names);
      } finally {
        leech.stop();
        seeder.stop();
      }
    } 
                              #location 35 
                              #vulnerability type NULL_DEREFERENCE