source: josm/src/org/openstreetmap/josm/gui/MapScaler.java@ 155

package org.openstreetmap.josm.gui;

import java.awt.Graphics;
import java.awt.geom.Rectangle2D;

import javax.swing.JComponent;

import org.openstreetmap.josm.Main;
import org.openstreetmap.josm.actions.HelpAction.Helpful;
import org.openstreetmap.josm.tools.ColorHelper;

public class MapScaler extends JComponent implements Helpful {

        private final MapView mv;

        public MapScaler(MapView mv) {
                this.mv = mv;
                setSize(100,30);
                setOpaque(false);
    }

        @Override public void paint(Graphics g) {
                double circum = mv.getScale()*100*Main.proj.scaleFactor()*40041455; // circumference of the earth in meter
                String text = circum > 1000 ? (Math.round(circum/100)/10.0)+"km" : Math.round(circum)+"m";
                g.setColor(ColorHelper.html2color(Main.pref.get("color.scale", "#ffffff")));
                g.drawLine(0, 5, 99, 5);
                g.drawLine(0, 0, 0, 10);
                g.drawLine(99, 0, 99, 10);
                Rectangle2D bound = g.getFontMetrics().getStringBounds(text, g);
                g.drawString(text, (int)(50-bound.getWidth()/2), 23);
    }

        public String helpTopic() {
            return "MapView/Scaler";
    }
}