// use setForeground and setBackground for the colors
sclass JColorBar > JComponentWithChangeListeners {
  settableWithVar double value;
  settableWithVar double max = 100;
  
  *() {
    setForeground(Color.red);
  }

  public void paintComponent(Graphics g) {
    super.paintComponent(g); // paint background
    int w = getWidth(), h = getHeight();
    fillRect(g, 0, 0, w*doubleRatio(value, max), h, getForeground());
  }
  
  selfType max aka setMax(double max) {
    this.max = max;
    repaint();
    this;
  }
  
  selfType value aka setValue(double value) {
    this.value = value;
    repaint();
    this;
  }
}