Warning

A super-simple program, that displays a single message and quits.

Important Commands

extends JOptionPane
Allows Dialog commands to be executed.

showMessageDialog(this,"Warning ...");
Pop-up Dialog showing message.

System.exit(0);
Ends the program.

Comments

Real programs are never this short - this does nothing useful.  Extending JOptionPane is not the normal way to start a program - rather extend JFrame would be normal.