叉/连接示例

Question

我正在尝试使用 Fork / Join机制（List＆gt;＆gt; forks ）机制来搜索磁盘“D：/”上的所有文件。

这是代码。我从JLS实现了这个机制。

public class FileFound_02 {
            public static void main(String[] args) {
                String searchword = null;
                System.out.println("Andrey3k");
                // Get Search word from arguments
                if( args.length > 0 ) {
                    searchword = args[0];
                    System.out.println("____ args.length " + searchword);
                }
                // Get Disk Drives
                File[] rootDrive = File.listRoots();
                for(File sysDrive : rootDrive){
                    System.out.println("Drive : " + sysDrive);
                    System.out.println("Drive getAbsolutePath: " + sysDrive.getAbsolutePath());

                    // Scanning D Disk
                    if( sysDrive.getAbsolutePath().startsWith("D:") ) {
                        System.out.println("sysDrive.getAbsolutePath(): " + sysDrive.getAbsolutePath());

                        if( searchword != null ) {
                            ForkJoinPool forkJoinPool = new ForkJoinPool() ;

                            List<File> directories = forkJoinPool.invoke(new ScanningDirectory( new File( sysDrive.getAbsolutePath() ), searchword) );

                            for (int i = 0; i < directories.size(); i++) {
                                File file = (File) directories.get(i);
                                System.out.println(file.getAbsolutePath());
                            }
                        }
                    }
                }
            }
        }


        public class ScanningDirectory extends RecursiveTask<List<File>> {
            File file;
            String searchword = "";
            public ScanningDirectory(File file, String searchword) {
                this.file = file;
                this.searchword = searchword;
            }
            @Override
            protected List<File> compute() {
                //
                List<RecursiveTask<List<File>>> forks = new LinkedList<>(); 
                List files = new ArrayList();

                 if (file.isDirectory()) {
                     for (File childFile : file.listFiles()) {
                         ScanningDirectory task = new ScanningDirectory(childFile, searchword);
                         forks.add(task);
                         task.fork();// запусткаем
                     }
                     for (RecursiveTask<List<File>> task : forks) {
                         files.addAll(task.join()); // ждем выполнения задач и результат
                     }
                 }
                 return files ;
            }
        }

控制台说：

 *Exception in thread "main" java.lang.NullPointerException
    at andrey3k.FileFound_02.main(FileFound_02.java:40)
Caused by: java.lang.NullPointerException
    at andrey3k.ScanningDirectory.compute(ScanningDirectory.java:34)
    at andrey3k.ScanningDirectory.compute(ScanningDirectory.java:1)
    ... 7 more**

有什么问题？我该如何解决？