🚀 go-pugleaf

Asger,

unfortunately that is not as easy as you had expected.

1. 
You create a JOptionPane instance and set it's font. 
Then you call the static(!) showMessageDialog method which 
returns an entirely new JOptionPane instance with the default 
font.

2. 
Even if it worked wouldn't you get what you want since setting 
the pane's font does not automatically cause all containing 
components to adopt that font. You want to change the font of 
the message label, right?

3. 
You can't directly control how the pane that showMessageDialog
returns looks like apart from the parameter argument options. It's ready-made, use it as it is or provide your own custom 
MessageDialog.

4. 
You *can* indirectly control how components are set up because 
they get their font, color, border and so on from a list of ui 
defaults. If you (temporarily) change that defaults you might 
get want you want.
The defaults is a hashtable and is available through the 
UIManager class: UIManager.getDefaults().

In your case:
  UIDefaults uiDefaults = UIManager.getDefaults();
  Font originalFont = uiDefaults.getFont("Label.font");
  uiDefaults.put("Label.font", originalFont.deriveFont(30.0f));
  JOptionPane.showMessageDialog(parent, "Test");
  uiDefaults.put("Label.font", originalFont);//restore

5. 
The UIDefaults are not documented and may change in a future 
release. Consider tweaking them as a last ressort ;-)
You might be better off providing your own MessageDialog.

Torsten