Folder1
    Folder1.1
        File1.1.1
    Folder1.2
Folder2
    File2.1
    File2.2
...

public static void filetree(File mainfolder, int indent){

    Stack<String> filesanddirectories = new Stack<>();

        for (File file : mainfolder.listFiles()){
            if (file.isDirectory()){
                filesanddirectories.push(file.getName());
                filetree(file, 0);
            }
            else if (file.isFile()){
                filesanddirectories.push(file.getName());
            }
        }

    for (int i = 0; i < filesanddirectories.size(); i++){
        System.out.println(filesanddirectories.pop());
    }
}

public static void filetree(File mainfolder, int indent) {

        Stack<String> filesanddirectories = new Stack<>();
        int temp;

        for (File file : mainfolder.listFiles()) {
            for(int i = 0; i<indent; i++) {
                System.out.print("  ");
            }
            temp = indent;
            if (file.isDirectory()) {
                filesanddirectories.push(file.getName());
                indent++;
                System.out.println(file.getName());

                filetree(file, indent);
                indent--;

            } else if (file.isFile()) {

                filesanddirectories.push(file.getName());

                System.out.println(file.getName());
                indent = temp;
            }
        }
    }