Question

I am a beginner in Java & I am learning the swing api. I am having trouble with my code, and I can’t figure out how to fix it. Please help.

Below is an example of the code I am using:

import javax.swing.JFrame;
import javax.swing.JLabel;

public class HelloWorldApp extends JFrame {


  public static void main(String args[]) {

    CustomWindow w = new CustomWindow();
    w.setVisible(true);

  }

  public CustomWindow() {

    setSize(500, 500);
    setTitle("Jacob Perkins");
    JLabel lbl = new JLabel("Hello World!");
    add(lbl);

  }

}

My problem is that I get the following error:

invalid method declaration; return type required

Answer 1

public CustomWindow() {
    setSize(500, 500);
    setTitle("Jacob Perkins");
    JLabel lbl = new JLabel("Hello World!");
    add(lbl);
}

should be:

public void CustomWindow() {
    setSize(500, 500);
    setTitle("Jacob Perkins");
    JLabel lbl = new JLabel("Hello World!");
    add(lbl);
}

Or instead of void (for returning nothing) we would put the data type we wanted to return. The method would now have to include a return statement which returns an appropriate data type to match the method signature, or you will get the error you have:

invalid method declaration; return type required

See Returning a Value from a Method.

Unless you are attempting to create a constructor for the HelloWorldApp class than the method must be given the class name (have a read on Providing Constructors for Your Classes):

public HelloWorldApp() {
    setSize(500, 500);
    setTitle("Jacob Perkins");
    JLabel lbl = new JLabel("Hello World!");
    add(lbl);
 }

Other suggestions

• Dont extend JFrame unnecessarily.

• Dont call setSize rather use correct LayoutManager and/or override getPreferredSize and return Dimensions which fit its contents. Than call pack() on JFrame before setting it visible but after adding components.

• Create and manipulate Swing components on Event Dispatch Thread via SwingUtilities.invokeLater(Runnable r) block:

  SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run() {
          //create and show ui here
          CustomWindow w = new CustomWindow();
          w.setVisible(true);
      }
  });