Only single item is visible in the JFrame Window

Question

I wrote this after seeing videos of thenewboston and then, when I ran, only the last item added was visible and all other textFields [textField1, textField2 and textField3] are not visible. It worked perfectly in his videos but when I tried, only the passwordField was visible. I'm a beginner and I was unable to find what's the problem. Please help me out what is the problem and what should I do to mitigate this error. One small request, please explain a bit detail because I a newbie to Java and GUI.

package learningPackage;

import java.awt.FlowLayout;
//gives the layout
import java.awt.event.ActionListener;
//this makes the field wait for an event.
import java.awt.event.ActionEvent;
import javax.swing.JOptionPane;
import javax.swing.JFrame;
import javax.swing.JTextField;
//creates a field where we can type text.
import javax.swing.JPasswordField;
//creates a text field where the text typed is hidden with asterics.

class tuna extends JFrame
{
    private JTextField textField1;
    private JTextField textField2;
    private JTextField textField3;
    
    private JPasswordField passwordField;
    
    public tuna()
    {
        super("Title Text");
        
        textField1 = new JTextField(10);
        //sets a the default value of 10. 
        add(textField1);
        
        textField2 = new JTextField("Enter a Text");
        //sets default text of "Enter a Text" without the quotes
        add(textField2);
        
        textField3 = new JTextField("Uneditable", 20);
        //Displays Uneditable with a default value of 20.
        //To make this Uneditable, you must do this...
        textField3.setEditable(false);
        //this makes the textField uneditable.
        add(textField3);
        
        passwordField = new JPasswordField("myPassword");
        add(passwordField);
        
        thehandler Handler = new thehandler();      
        textField1.addActionListener(Handler);
        textField2.addActionListener(Handler);
        textField3.addActionListener(Handler);
        passwordField.addActionListener(Handler);
        /*
         * The addActionListener method takes an object of Event Handler class. 
         * 
         * Therefore, we must create an object of Event Handler Class.
         * 
         * 
         * The addActionListener method takes an object because, sometimes, we might
         * have different classes with different code to execute. So, we pass the object to 
         * identify which class code is to be executed. 
         */
    }
    
    class thehandler implements ActionListener
    {
        /*
         * In order to handle events in Java, you need Event handler Class.
         * and, that Event Handler Class must implement ActionListener.
         * 
         * What the ActionListener does is that 
         * 
         * It will wait for some Event to happen and after that particular event happens, 
         * it will implement some piece of code.
         */
        public void actionPerformed(ActionEvent event)
        {
            String string = "";
            
            if(event.getSource()==textField1)
                string = String.format("Text Field 1 : %s", event.getActionCommand());
            else if(event.getSource()==textField2)
                string = String.format("Text Field 2 : %s", event.getActionCommand());
            else if(event.getSource()==textField3)
                string = String.format("Text Field 3 : %s", event.getActionCommand());
            else if(event.getSource()==passwordField)
                string = String.format("Password Field : %s", event.getActionCommand());
            
            //when the user presses enter key after entering text, this actually stores the 
            //thing entered into the String. 
            
            JOptionPane.showConfirmDialog(null, string);
        }
    }
    
}

public class Apples {

    public static void main(String[] args) 
    {
        
        tuna Srivathsan = new tuna();
        Srivathsan.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //this would close the window when X button is pressed.
        
        Srivathsan.setSize(500, 500);
        Srivathsan.setVisible(true);
        Srivathsan.setLocationRelativeTo(null);
        
    }

}

Here is the image of the window :

Answer 1

import java.awt.FlowLayout;
//gives the layout

That statement imports the layout & makes it available to the code, but does not set the layout on any container.

While a JPanel has a default layout of FlowLayout, the default layout of the (content pane of the) JFrame is BorderLayout. A BorderLayout accepts up to 5 components or containers (like JPanel) in each of 5 constraints. If no constraint is given, it defaults to the CENTER. If more than one component is added to the CENTER (or any other area of a BorderLayout), all but one of those components will fail to appear. I cannot recall if it is the first or last that appears, but it's a moot point, because the code should not do that.

My advice would be as follows:

1. Don't extend JFrame (or JPanel).
2. Add one JPanel to the JFrame.
3. Add the components (and/or containers) to that JPanel.

Here is an example implementing points 2 & 3. Point 1 is not implemented (batteries not included).

import javax.swing.*;

public class SingleComponentLayoutProblem extends JFrame {

    private final JTextField textField1;
    private final JTextField textField2;
    private final JTextField textField3;

    private final JPasswordField passwordField;

    public SingleComponentLayoutProblem() {
        super("Title Text");
        
        JPanel panel = new JPanel(); // uses FlowLayout be DEFAULT
        add(panel);

        textField1 = new JTextField(10);
        //sets a the default value of 10. 
        panel.add(textField1);

        textField2 = new JTextField("Enter a Text");
        //sets default text of "Enter a Text" without the quotes
        panel.add(textField2);

        textField3 = new JTextField("Uneditable", 20);
        //Displays Uneditable with a default value of 20.
        //To make this Uneditable, you must do this...
        textField3.setEditable(false);
        //this makes the textField uneditable.
        panel.add(textField3);

        passwordField = new JPasswordField("myPassword");
        panel.add(passwordField);
    }

    public static void main(String[] args) {
        Runnable r = () -> {
            SingleComponentLayoutProblem sclp = 
                    new SingleComponentLayoutProblem();
            sclp.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            //this would close the window when X button is pressed.
            
            // don't guess sizes! ..
            //sclp.setSize(500, 500); 
            // .. instead ..
            sclp.pack();
            sclp.setVisible(true);
            sclp.setLocationRelativeTo(null);
        };
        SwingUtilities.invokeLater(r);
    }
}