#vulnerable code 
                                public void solve(Path path) throws IOException {
        File file = path.toFile();
        if (file.isDirectory()) {
            for (File f : file.listFiles()) {
                solve(f.toPath());
            }
        } else {
            if (file.getName().endsWith(".java")) {
                if (printFileName) {
                    out.println("- parsing " + file.getAbsolutePath());
                }
                CompilationUnit cu = JavaParser.parse(file);
                List<Node> nodes = collectAllNodes(cu);
                nodes.forEach(n -> solve(n));
            }
        }
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public void solve(Path path) throws IOException {
        Files.walkFileTree(path, new SimpleFileVisitor<Path>() {
            @Override
            public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
                if (file.toString().endsWith(".java")) {
                    if (printFileName) {
                        out.println("- parsing " + file.toAbsolutePath());
                    }
                    CompilationUnit cu = JavaParser.parse(file);
                    List<Node> nodes = collectAllNodes(cu);
                    nodes.forEach(n -> solve(n));
                }
                return FileVisitResult.CONTINUE;
            }
        });
    }