How to display an alert in java?

Example:

// Using Java's JOptionPane
import javax.swing.JOptionPane;

public class Main {
    public static void main(String[] args) {
        JOptionPane.showMessageDialog(null, "This is an alert!");
    }
}

Solution:

// Use Java's 'JOptionPane' to display GUI alerts.
import javax.swing.JOptionPane;

public class AlertDemo {
    public static void main(String[] args) {
        JOptionPane.showMessageDialog(null, "Alert: Something went wrong!");
    }
}