Java, grafiskt..

package appet;

public class ConsoleLogger implements Logger
{
    public void log(String message)
    {
        System.out.println(message);
    }
}

package appet;

import java.io.*;

public class FileLogger implements Logger
{
    private PrintWriter logstream;
    
    public FileLogger()
    {
        try
        {
            logstream = new PrintWriter(new FileWriter("logga.txt"), true);
        } catch (IOException ioe)
        {
            System.out.println("Kunde inte logga!");
            ioe.printStackTrace();
        }
    }
    
    public void log(String message)
    {
        logstream.println(message);
    }
}

package appet;

public class LoggHandler
{
    private Logger logger;

    public LoggHandler(Logger logger)
    {
        this.logger = logger;
    }
    
    public void setLogger(Logger logger)
    {
        this.logger = logger;
    }

    public void messageTest(int nummer)
    {
        logger.log("Något nummer: " + nummer);
    }
}

package appet;

public interface Logger
{
    void log(String message);
}

package appet;

public class Main
{
    public static void main(String[] args)
    {
        NewFrame nyRam = new NewFrame ("Testar", new NewPanel());
        LoggHandler client = new LoggHandler(new FileLogger());
        client.messageTest(1);
	client.messageTest(2);
	client.messageTest(3);

	client.setLogger(new ConsoleLogger());
	client.messageTest(4);
	client.messageTest(5);
	client.messageTest(6);

        nyRam.setVisible(true);
    }
}

package appet;

import javax.swing.*;

public class NewFrame extends JFrame
{
    public NewFrame(String title, JPanel panel)
    {
        super(title);
        this.setSize(640, 480);
        this.setLocationRelativeTo(null);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        
        this.add (panel);
    }
}

package appet;

import javax.swing.*;

public class NewPanel extends JPanel implements Logger
{
    private JTextArea jta;
    
    public NewPanel()
    {
        jta = new JTextArea();
        add(jta);
    }

    public void log(String message)
    {
        jta.setText(message);
    }
}