public interface File {    public void list();    public void addFile(File file);    public void removeFile(File file);}

public class Folder implements File {    private String name;    private ArrayList<File> files = new ArrayList<File>();    public Folder(String name) {        this.name = name;    }    public void addFile(File file) {        files.add(file);    }    public void removeFile(File file) {        files.remove(file);    }    public void list() {        System.out.println("Folder: " + name);        for (File file : files) {            file.list();        }    }}

public class Document implements File {    private String name;    public Document(String name) {        this.name = name;    }    public void list() {        System.out.println("Document: " + name);    }    public void addFile(File file) {        // do nothing    }    public void removeFile(File file) {        // do nothing    }}

File root = new Folder("root");root.addFile(new Document("one.doc"));File sub = new Folder("sub");sub.addFile(new Document("two.doc"));sub.addFile(new Document("three.doc"));root.addFile(sub);root.list();