Solving the expression problem in Java using maps.

Command.java

package sandbox;

public interface Command {
}

Formatter.java

package sandbox;

public interface Formatter {
    public String format(Object o);
}

MoveCommand.java

package sandbox;

public class MoveCommand {

    private int x;
    private int y;

    public MoveCommand(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int getX() {
        return x;
    }

    public Object getY() {
        return y;
    }
}

MoveCommandFormatter.java

package sandbox;

public class MoveCommandFormatter implements Formatter {

    @Override
    public String format(Object o) {
        MoveCommand c = (MoveCommand) o;
        return String.format("<MoveCommand %s,%s>", c.getX(), c.getY());
    }

}

Printer.java

package sandbox;

import java.util.HashMap;
import java.util.Map;

public class Printer {

    private static final Map<Class, Formatter> DEFAULT_FORMATTERS = new HashMap();

    static {
        DEFAULT_FORMATTERS.put(MoveCommand.class, new MoveCommandFormatter());
        DEFAULT_FORMATTERS.put(StopCommand.class, new StopCommandFormatter());
    }

    private Map<Class, Formatter> formatters;

    public Printer() {
        this(DEFAULT_FORMATTERS);
    }

    public Printer(Map<Class, Formatter> formatters) {
        this.formatters = formatters;
    }

    public void print(Object o) {
        System.out.println(format(o));
    }

    public String format(Object o) {
        if (o == null) {
            return "<null>";
        } else if (formatters.containsKey(o.getClass())) {
            return formatters.get(o.getClass()).format(o);
        } else {
            return "<unknown object " + o + ">";
        }
    }

    public static void main(String[] args) {
        MoveCommand mc = new MoveCommand(10, 20);
        StopCommand sc = new StopCommand();
        java.util.Date d = new java.util.Date(1348040520000L);
        Printer p = new Printer();
        p.print(mc);
        p.print(sc);
        p.print(d);
    }

}

StopCommand.java

package sandbox;

public class StopCommand implements Command {

}

StopCommandFormatter.java

package sandbox;

public class StopCommandFormatter implements Formatter {
    @Override
    public String format(Object o) {
        StopCommand c = (StopCommand) o;
        return String.format("<StopCommand>");
    }

}