You have forgotten a ";" after the lastName variable. Also make sure at the top of the program to include "import javax.swing.JOptionPane;"

String firstName;

String lastName;

firstName = JOptionPane.showInputDialog("Enter your first name.");

lastName = JOptionPane.showInputDialog("Enter your last name.")

JOptionPane.showMessageDialog(null, "Hello " + firstName + " " + lastName );

System.exit(0);

The first prompt appear, but after that nothing happens.