Solution 3

But, there is still a dependency on the notion of writing to standardout. What if we wanted to pop up a window?
Specifically, what if we wanted to maintain a console and a GUI versions of this application?
For the GUI, the function needs a reference to the widget. How does it get it?

Define a reporter interface

public interface Reporter {
  void report (String msg);
}

Each method that needs it gets a reference to this reporter, as in:

void download (Reporter r,....){
  r.report ( "Starting downloading" );
  ...;
}

Implement the standardout reporter:

public class StandardOutReporter implements Reporter {
  public void report (String msg){
    System.out.println (msg + " at: " +new Date ());
  }
}

Implement the GUI (Swing) reporter:

public class JTextComponentReporter implements Reporter {
  JTextComponent comp;
  public JTextComponentReporter (JTextComponent c){
    comp =c;
  }
  public void report (String msg){
    comp.setText (msg + " at: " +new Date ());
  }
}

The parts that use the Reporter interface are de-coupled from its implementation. They don't care if its writing to stdout or a GUI.
Notice that the GUI implementation creates its own widget reference and keeps a reference to it.