How to “do something” on Swing component resizing?

Question

I\'ve a class which extends JPanel. I overwrote protected void paintComponent(Graphics g).

There is a variable which has to be recalculate

Answer 1

It resizes automatically if it's

1. inside a BorderLayout panel and
2. put there as BorderLayout.CENTER component.

If it doesn't work, you probably have forgotten one of these two.

Answer 2

If the calculation isn't time consuming, I would just re-calculate the value each time in paintComponent().

Otherwise, you can save a value that is the size of the component and check it against the new size in paintComponent. If the size changed, then recalculate, otherwise don't.

private Dimension size;

protected void paintComponent(Graphics g){
    if (!size.equals(getSize())){
        size = getSize();
        // recalculate value
    }
}

Or, you can do the calculation on a resize event.

private CompoentListener resizeListener = new ComponentAdapter(){
    public void componentResized(ActionEvent e){
        // recalculate value
    }
};

//in the constructor add the line
addComponentListener(resizeListener);