在 Java 中以递归方式复制文件时的 NullPointerException [duplicate]

问：

我们有以下练习：给定一个目录和文件列表，然后是目标，复制所有这些目录（递归），然后 文件添加到目标中。

这是我写的代码：

public static void copyFile(File source, File dest) throws IOException {

    //use DataOutputStream for its method size() to measure amount of bytes copied
    try(FileInputStream is = new FileInputStream(source);
        FileOutputStream os = new FileOutputStream(dest);
        DataOutputStream dos = new DataOutputStream(os)) {
            byte[] buffer = new byte[1000];
            int res;
            while ((res = is.read(buffer)) !=-1) {
                dos.write(buffer, 0, res);
            }
            int size1 = dos.size();
            if(size1<1024){
                System.out.println("Total of " + size1 + "B" + " were copied");
            }
            if(size1>1024){
                System.out.println("Total of " + size1/1024 + "KB" + " were copied");
            }
            if(size1>1e-6){
                System.out.println("Total of " + size1/1024/1024 + "MB" + " were copied");
            }
        }
    }

    static void recursiveCopy(File file, File Destination){
        if(file.isFile()){
            try {
                System.out.println(">Started: " + file.getAbsolutePath());
                copyFile(file, Destination);
                System.out.println(">Finished FILE : " + file.getAbsolutePath());
                return;
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        for(File sub:file.listFiles()){
            String indent2 = "";
            System.out.println( indent2+ ">Started: " + sub.getAbsolutePath());
            recursiveCopy(sub, Destination);
            indent2+= "  ";
        }
    }

    static void copyAll(File[] listOfFiles, File dest) throws IOException{
        for(File f:listOfFiles){
            recursiveCopy(f, dest);
            System.out.println("        >Finished FOLDER: " + f.getAbsolutePath());
        }
    }

    public static void main(String[] args) throws IOException {
        File[] listOfFiles = new File[] {new File("resources/docs/books"),
                                         new File("resources/docs/books/some citations.txt")};
        copyAll(listOfFiles, new File("dest"));
    }

它抛出以下消息：NullPointerException

线程“main”java.lang.NullPointerException中的异常：无法读取数组长度，因为“<local2>”为空

问题从行开始for(File sub:file.listFiles())

有谁知道我该如何解决这个问题？所有文件和目录都存在，所以我不知道数组的哪个元素可能是空的。也许还有别的什么？