JLabel wont change color twice

Question

Hi,

I have the following code:

   public class Test extends JFrame implements ActionListener{
 private static final Color TRANSP_WHITE = new Color(new Float(1), new Float(1), new Float(1), new Float(0.5)); 
 private static final Color TRANSP_RED = new Color(new Float(1), new Float(0), new Float(0), new Float(0.1));
 private static final Color[] COLORS = new Color[]{ TRANSP_RED, TRANSP_WHITE};
 private int index = 0;

 private JLabel label;
 private JButton button; 
 public Test(){
  super();

  setLayout(new BoxLayout(getContentPane(), BoxLayout.Y_AXIS));
  label = new JLabel("hello world");
  label.setOpaque(true);
  label.setBackground(TRANSP_WHITE);

  getContentPane().add(label);

  button = new JButton("Click Me");
  button.addActionListener(this);

  getContentPane().add(button);

  pack();
  setVisible(true);
 }

 @Override
 public void actionPerformed(ActionEvent e) {
  if(e.getSource().equals(button)){
   label.setBackground(COLORS[index % (COLORS.length - 1)]);
index++;
      }
     }

 public static void main(String[] args) {
  new Test();
 }
    }

When I run it I get the label with the TRANSP_WHITE background and then when I click the button this color changes to TRANSP_RED but when I click it again I see no change in color. Does anyone know why?

Thanks

Answer 1

Well, what were you expecting to happen?

label.setBackground(COLORS[index % (COLORS.length - 1)]);

The index variable is hard coded to 0. and COLORS.length -1 is essentially a constant. So every time you click your setting the background to COLORS[0];

If you change your action method to the following you'll get the results you are looking for:

 @Override
 public void actionPerformed(ActionEvent e) {
  if(e.getSource().equals(button)){
   label.setBackground(COLORS[index % COLORS.length]);
   index++;
  }
 }

First: The modulo operator will always return a value between 0 and one less than the value passed to it. So

index % COLORS.length

Will always return a value between 0 and COLORS.length -1.

Second: You were forgetting to increment index after every call.

Answer 2

Hey! You forgot to increment index. In this expression:

label.setBackground(COLORS[index % (COLORS.length - 1)]);

index % (COLORS.length - 1) is always 0.

BTW. you don't have to use new Float(1) when creating Color. 1F should work too.

Answer 3

Here is the code you have to use

label.setBackground(COLORS[index % (COLORS.length)]);
index++;

Answer 4

You are doing it wrong. It should be done like that

label = new JLabel("hello world"){
     public void paintComponent(Graphics g)
     {
         //draw background
         Color old=g.getColor();
         g.setColor(getBackground());
         g.fillRect(0,0,getWidth(),getHeight());
         g.setColor(old);
         super.paintComponent(g);
     }
};
label.setOpaque(false); // your component is not opaque!