Change preferredSize of a JLabel after setVisible the JFrame

I have a problem when I try to resize a JLabel. In my application appears the next strucutre. Understand every list item like something inside the previous list item.

• JFrame (Layout null, fixed size, not resizeable, used by different people).

• JPanel (Layout null, with a size of all the window, the place were I put my work).

• various JPanel with different Layouts (the areas of content inside the main panel, you can think about it like html divs...).

• Inside one of this "divs" with Layout null there are extended Classes of JPanel with Flow Layout.

• Inside every one of this extedend Classes are labels with preferredSizes.

The thing is that when after create all of this i call theFrame.setVisible(true); and all works propertly.

But in a moment of the flow of my application I have to change the size of one of the labels. Then, I simply call label.setPreferredSize(d) and the change doesn't change. The function works propertly if I call it before set visible the frame, but not after.

I have the feeling that the problem is that I don't use nothing like pack(), validate(), repaint(), etc. Because I don't know what it works. I try call repaint and validate to the jlabel, and repaint the main panel, but doesn't works.

I'm relative new with awt and swing, and this is for homework. Sorry for my bad use 开发者_如何学Cof English language, and thank you for your help.

After you change the size of your JLabel, call revalidate(). This will cause the JLabel to be resized without waiting for an event that triggers a re-layout (such as resizing the parent Frame, etc...).

JLabel lbl_test;

lbl_test.setPreferredSize(new Dimension(100, 100) );
lbl_test.revalidate();

One other thing to keep in mind, as I'm not certain which class you're having problems with. Null layouts (absolute positioning) mixed with layout managers are going to cause some strange things to happen.