// imports
import greenfoot.*;
import java.awt.Color;

​

// fields
private Actor valueBar;
private int value = 3600;
private int maxValue = value;
   
// in constructor
valueBar = new SimpleActor();
updateValueDisplay();
addObject(valueBar, 80, 20);

​

// changing value
public void adjustValue(int amount)
{
    value += amount;
    if (value < 0) value = 0;
    if (value > maxValue) value = maxValue;
    updateValueDisplay();
}

​

// updating value display image
private void updateValueDisplay()
{
    // fields for dimensions of color bar
    int wide = 100; // width
    int high = 12; // height
    // create full color bar (at maximum value)
    GreenfootImage fullImg = new GreenfootImage(wide, high);
    fullImg.setColor(Color.GREEN);
    fullImg.fill();
    // create color bar (at value)
    GreenfootImage colorBar = new GreenfootImage(wide, high);
    int percentage = wide*value/maxValue;
    colorBar.drawImage(fullImg, percentage-wide, 0);
    // create actor image
    GreenfootImage image = new GreenfootImage(wide+4, high+4);
    image.setColor(Color.WHITE);
    image.fill(); // the background
    image.setColor(Color.BLACK);
    image.drawRect(0, 0, wide+3, high+3); // the frame
    image.drawImage(colorBar, 2, 2); // the color bar
    valueBar.setImage(image);
}