Question

given the following code:

public class MainFrame extends JFrame{

  public MainFrame() throws HeadlessException {
    super();
    this.setSize(500, 400);
    this.setVisible(true);
    this.setDefaultCloseOperation(DISPOSE_ON_CLOSE);

    JButton myButton = new JButton("Test");

    this.add(myButton);

    this.pack();

  }

  public static void main(String[] args) {     
    new MainFrame();
  }

}

Does the code inside the constructor run on the EDT. I think it does because it’s executed “inside” an instance of a JFrame, but I need a second opinion.

Continuing the idea, If I were to create other controls, for example in the main() function, that code wouldn’t be on the EDT?

Thank you!

Answer 1

No. You are calling the constructor from the main method which runs on the main thread.

Add the usual boilerplate:

public static void main(String[] args) {     
    java.awt.EventQueue.invokeLater(new Runnable() { public void run() {
        new MainFrame();
    }});
}

Also it’s generally a bad idea to extend classes that you don’t need to (including JFrame, JPanel and Thread). There is no need to declare HeadlessException as it is unchecked.