Data.java

public interface Data {
    public int getsize();
    public String toString();
}

DataRunner.java

public class DataRunner {
    public static void main(String[] args) {

        Directory directory = new Directory();
        directory.add(new File(10));
        Directory directory1 = new Directory();
        directory.add(directory1);
        directory1.add(new File(20));
        directory1.add(new File(40));
        Directory directory2 = new Directory();
        directory1.add(directory2);
        directory1.add(new File(20));

        System.out.println(directory.toString());

        System.out.println("Size of directory "+directory.getsize());
    }

Directory.java

public class Directory implements Data {
    private ArrayList<Data> files = new ArrayList<Data>();

    public Directory(){

    }

    public void add(Data data){
        files.add(data);
    }
    public int getsize(){
        int size = 0;
        for(Data file: files){
            size += file.getsize();
        }
        return size;
    }
    public String toString(){
        String name="d[";
        for(Data data:files){
            name +=data.toString();
        }
        return name+"] ";
    }
}

File.java

public class File implements Data {
    private int size;

    public File(int size) {
        this.size = size;
    }
    public int getsize(){
        return size;
    }

    public String toString(){
        return "f(" + size + ") ";
    }
}