Java's setPreferredSize won't resize JPanel

Question

Hi all,

I am struggling ( ! ) with java GUI development. All these small things that make obvious sense just don't work. I'll explain what I am trying to do and where I am failing (this is quite frustrating):

I've got this graphics2d object which resides inside a panel. Now this graphics draws an image, it can go on and draw forever. this drawing expands horrioncally and since I do not know what size to expect I put it inside a scrollpane. Now I want to resize the JPanel in each pain iteration and set the setPreferredSize to a larger value, thus expanding the scrollpane's knob.

The problem: From some reason when I call setPreferredSize from paint it doesn't do anything. I call it on the panel. When I try calling setPreferredSize from the function that calls paint, it performs the resizing operation only once!

What could be the reason for that?

Apart from that I've got another small question, I'd love to solve: Since the graphics draw goes on and increases in size in time, how can I advance the position of scrollpanes? Gradually increasing it?

People, thank you very much. I sure need help here

Answer 1

Sounds to me like you got a couple of things wrong.

• Setting any size attribute (actuallly any attribute of a component at all) inside the paint method will result in possibly strange behavior, because you don't really know when and how often paint gets called

• You normally do not call paint your self. It will get called by Swing itself, once the component becomes visible

• If I got it right you want to have a Component, that grows with time. IMHO the correct approach would be to create a Timer. Using the Timer change the size as required. In the paint method just read this information and use it to decide what to paint.

Answer 2

preffered size is, by its name, not mandatory... maybe pack() or revalidate() would help...

Answer 3

setPreferredSize is one of many hints you can give to the LayoutManager which is managing the layout of your component's container if indeed a LayoutManager has been set. It is up to the given LayoutManager what it does with those hints.

I suggest you check out the book Filthy Rich Clients for an in-depth explanation of how the Swing component rendering works.

Answer 4

You should note that set(Preferred/Minimal/Maximum)Size() are nothing more than hints to the parent component and its layoutmanager how much space the component should get. Further note that theses sizes are only interpreted when the components are layouted (upon first render, invalidate() and so on).