How to get a list of all files relative to a disk in Java?

Question

Java

Hello. I have below code:

private static ArrayList<String> fileList = new ArrayList<String>();
  
    public static void main(String[] args) throws IOException, InterruptedException, ParserConfigurationException, ClassNotFoundException, InstantiationException, IllegalAccessException, UnsupportedLookAndFeelException {
    	displayAll(new File("D://"));
    	PrintWriter out = new PrintWriter(C://test.txt);
    	for (String fileName : fileList) {
    	   out.println(fileName);
    	}
    }

    public static void displayAll(File path){      
        if(path.isFile()){
        	fileList.add(path.getAbsolutePath());
        }else{
        	fileList.add(path.getAbsolutePath());       
            File files[] = path.listFiles();
            for(File dirOrFile: files){
                displayAll(dirOrFile);
            }
        }
    }

When starting the program, an error appears in the console:

Exception in thread "main" java.lang.NullPointerException
  at ru.fixer.Main.displayAll(Main.java:75)
  at ru.fixer.Main.displayAll(Main.java:76)
  at ru.fixer.Main.displayAll(Main.java:76)
  at ru.fixer.Main.main(Main.java:61)

If you change the path from new File("D://") to new File ("D://Desktop"), then a list of all files is quietly displayed.
What needs to be changed so that the path is from the disk, and not from the folder in the disk?