Question

What happens if the first file this code encounters is a dir. Why does it not ignore all other files in that dir (therefore obscuring the results)?

This is taken from How do I iterate through the files in a directory in Java?

I’m not trying to dispute that this code works but how does it account for above scenario ?

 public static void main(String... args) {
        File[] files = new File("C:/").listFiles();
        showFiles(files);
    }

    public static void showFiles(File[] files) {
        for (File file : files) {
            if (file.isDirectory()) {
                System.out.println("Directory: " + file.getName());
                showFiles(file.listFiles()); // Calls same method again.
            } else {
                System.out.println("File: " + file.getName());
            }
        }
    }

Answer 1

It doesn’t ignore it because it’s just making another recursive call – the original call (with the collection of files from the top level) is still on the stack. It makes a new call with a new list of files, referenced by a new parameter called files in a new stack frame.

So if there’s a directory structure of c:/a/b/c you’ll end up with a stack of:

showFiles([c:/a/b/c])
showFiles([c:/a/b])
showFiles([c:/a])
main(...)

When the “deepest” call (the top of the stack) returns, the next stack frame will still know about any other files at the same level, and may well recurse again.