Why this function for JFrame is executed twice?

Question

Introduction

I'm learning how to present GUIs for user 'friendly' input. I will reference below some web pages for those who are interested in the matter.

Code

public class TestGUI{

    private JFrame mainFrame;
    private JLabel headerLabel;
    private JLabel statusLabel;
    private JPanel controlPanel;

    public TestGUI()
    {
        prepareGUI();
    }

    private void prepareGUI()
    {
        mainFrame = new JFrame("TestGUI"); //Header name
        mainFrame.setSize(420, 320); //Size of the frame
        mainFrame.setLayout(new GridLayout(3, 1)); //??

       mainFrame.addWindowListener(new WindowAdapter() //Waits for an user event
       {
         //When the frame is closes, the program does too.
         @Override
         public void windowClosing(WindowEvent windowEvent)
         {
            System.exit(0); //Exit program
         }        
      });

       mainFrame.setVisible(true);//GUI is visible
    }

    public static void main(String[] args) {

        TestGUI test = new TestGUI(); //constructor
        test.prepareGUI(); //Call the method
    }

}

Problem

While running the code I saw that 2 identical frames pop up. I went to debug it and saw that it is executed twice when I call the method!

Why is that?

I only called it once with testGUI.prepareGUI(); in the main function.

Webpages for learning basic GUI in Java

JavaFX

GUI Programming with AWT

Answer 1

You call prepareGUI() in the constructor as well.

public TestGUI()
{
    prepareGUI();
}

When you call new TestGUI() , this constructor gets called and so does the function.

Answer 2

You are calling prepareGui twice

Once here

  public TestGUI()
    {
        prepareGUI();
    }

and once here

    TestGUI test = new TestGUI(); //constructor
    test.prepareGUI(); //Call the method

so first block is executed on new TestGUI() call

Answer 3

You are calling the prepareGui() method twice. One in your constructor and once on your created object (in the main method)

Answer 4

The issue is that constructor (that is TestGUI()) you are already calling prepareGUI(). so just omit the other call to prepareGUI(), which is test.prepareGUI().