Set the default size of Java Application

JLabel label = new JLabel("MESSAGE");
label.setFont(new Font("Arial", Font.BOLD, 18));
JOptionPane.showMessageDialog(null,label,"ERROR",JOptionPane.WARNING_MESSAGE);

hope this helps

Seem like it doesn't make any difference... Any idea ?

Are you trying to change the frame title font size?

Message inside JOptionPane...

Try this and let me know

———
import java.awt.BorderLayout;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JDialog;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class DialogueDemo
{
public static void main(String... args)
{
SwingUtilities.invokeLater(() -> {new DialogueDemo().go(args);});
}
private DialogueDemo(){}
private void go(String[] args)
{
JDialog d = new JDialog((java.awt.Frame)null, args[0], true);
d.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
d.setBounds(50, 50, 575, 150);
d.setLayout(new BorderLayout());
// label with original font
JLabel label = new JLabel(args[1]);
label.setFont(new Font("Serif", Font.ITALIC, 32));
d.add(label, BorderLayout.CENTER);
JPanel panel = new JPanel(new BorderLayout());
panel.setBackground(java.awt.Color.RED);
// first button
JButton larger = new JButton("Larger");
larger.addActionListener((ActionEvent evt) -> {label.setFont(new Font(
label.getFont().getName(),
label.getFont().getStyle(),
label.getFont().getSize() + 1
));});
panel.add(larger, BorderLayout.LINE_START);
// second button
JButton smaller = new JButton("smaller");
smaller.addActionListener((ActionEvent evt) -> {label.setFont(new Font(
label.getFont().getName(),
label.getFont().getStyle(),
label.getFont().getSize() - 1
));});
panel.add(smaller, BorderLayout.LINE_END);
d.add(panel, BorderLayout.PAGE_START);
d.setVisible(true);
}
}

———-

java DialogueDemo "Alfred is Brilliant" "This is how you show a Dialogue"

If this does not work I will chek it tomorrow

Compiler successfully but it returns error when running the program...


C:\Temp>javac DialogueDemo.java

C:\Temp>java DialogueDemo
Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 0
        at DialogueDemo.go(DialogueDemo.java:19)
        at DialogueDemo.lambda$main$0(DialogueDemo.java:14)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
        at java.awt.EventQueue.access$500(EventQueue.java:97)
        at java.awt.EventQueue$3.run(EventQueue.java:709)
        at java.awt.EventQueue$3.run(EventQueue.java:703)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Sorry, the above code works ! However, how to  make the default button and the text inside JOptionPane larger and match with the label ?

Thx again.