Adding a JPanel created from class to JtabbedPane

Question

I'm trying to add a JPanel to a tab. The panel is created with the LanguageTab class but I can't figure out why it doesn't work. It maybe stupid but self taught is not enough here. Hope someone can give me a little hand. thanks!
I have my code down here for you to understand what I'm going through!

public class MainTab extends JPanel {
JTabbedPane mainTab;
JPanel languageTab;
JFrame mainFrame;
JPanel mainPanel;

public MainTab(){
    mainFrame = new JFrame();
    mainTab = new JTabbedPane();

    mainPanel = new JPanel();
    //mainPanel.add(new JTextField("ciao"));
    mainPanel.add(new JLabel(new ImageIcon("C:\\Users\\angelica\\Desktop\\developed.jpg")));

    //languageTab = new LanguageTab();


    mainTab.add("main",mainPanel);
    mainTab.add("Language Tab",languageTab);
    add(mainTab);

    mainFrame.add(mainTab);
    mainFrame.setVisible(true);
    mainFrame.setDefaultLookAndFeelDecorated(true);
}

public static void main(String args[]){
    MainTab mt = new MainTab();
}

}

and this is my LanguageTab

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JPanel;

public class LanguageTab extends JPanel implements ActionListener{

public LanguageTab(){

    ImageIcon icona = new ImageIcon("C:\\Users\\angelica\\workspace\\whatever\\src\\whatever\\ita.jpg");
    ImageIcon icona_DSA = new ImageIcon("C:\\Users\\angelica\\workspace\\whatever\\src\\whatever\\ita2.jpg");
    ImageIcon iconb = new ImageIcon("C:\\Users\\angelica\\workspace\\whatever\\src\\whatever\\brit.jpg");
    ImageIcon iconb_DSA =new ImageIcon("C:\\Users\\angelica\\workspace\\whatever\\src\\whatever\\brit2.jpg");

    JPanel langTab= new JPanel();
    langTab.setLayout(new GridLayout(2,2));

    JButton ADHDbutton = new JButton("ADHD ENGLISH");
    ADHDbutton.setIcon(iconb);
    ADHDbutton.setActionCommand("adhd_english");
    ADHDbutton.addActionListener(this);

    JButton ADHDbutton1 = new JButton("ADHD \n ITALIANO");
    ADHDbutton1.setIcon(icona);
    ADHDbutton1.setActionCommand("adhd_italiano");
    ADHDbutton1.addActionListener(this);

    JButton DSAbutton = new JButton("DSA ENGLISH");
    DSAbutton.setIcon(iconb_DSA);
    DSAbutton.setActionCommand("dsa_english");
    DSAbutton.addActionListener(this);

    JButton DSAbutton1 = new JButton("DSA ITALIANO");
    DSAbutton1.setIcon(icona_DSA);
    DSAbutton1.setActionCommand("dsa_italiano");
    DSAbutton1.addActionListener(this);

    langTab.add(ADHDbutton);
    langTab.add(ADHDbutton1);
    langTab.add(DSAbutton);
    langTab.add(DSAbutton1);

    //return toModify;
}

@Override
public void actionPerformed(ActionEvent arg0) {
    // TODO Auto-generated method stub

}

}

Answer 1

You need to set the size of your JFrame so that it's large enough to see its contents.

Add the following as the last line of your MainTab constructor:

mainFrame.setSize(new Dimension(300, 300));

Additionally, inside your LanguageTab constructor, you have a variable called langTab , when the class itself ( this ) is supposed to represent the language tab. You then add all your buttons to langTab and never do anything with it.

Delete the variable langTab replace every call to it with this in the LanguageTab Constructor. Your constructor should then look something like this:

public LanguageTab() {
    // button initialization goes here...

    this.add(ADHDbutton);
    this.add(ADHDbutton1);
    this.add(DSAbutton);
    this.add(DSAbutton1);
}

---

Now that your question has been answered, if you don't mind I would like to suggest a few things that might improve your future code quality.

1. Only use global variables when it's necessary to.

   • You declare four global variables for different parts of your UI, all of which can be declared instead as local variables. Working with Swing, it is often necessary to declare components as global so that they can be accessed in various places within the class, and that may eventually be the case for you. But you should always prefer local scoped variables when it's possible.

2. Give your variables the smallest possible scope

   • You don't have any access level (scope) modifiers on your global variables, which gives them a package scope by default. As I mentioned before it is often necessary to make swing components global for various reasons, but it is almost never necessary for them to have a scope larger than private (so they can only be accessed from within the class in which they are declared.

3. Access static methods in a "static way".

   • You call JFrame 's setDefaultLookAndFeelDecorated() method using your variable mainFrame . mainFrame is an instance of the JFrame class (you declared it by saying mainFrame = new JFrame() ), so you only use it to call instance methods. setDefaultLookAndFeelDecorated() is a static method (declared with the keyword static ) and should therefore be accessed with the class name: JFrame.setDefaultLookAndFeelDecorated(true)

4. Be careful with your reference types.

   • You have declared your languageTab variable (in the MainTab class) as a JPanel . JPanel does not implement the ActionListener interface like your LanguageTab does, so by declaring your variable as a JPanel , you lose the ability to call actionPerformed() on it (or more likely set it as an action listener of some swing component).

5. Variables in Java, by convention, should begin with a lower case letter.

   • You gave your four buttons in the LanguageTab class names that start with capital letters. The Java convention is for variable names to begin with a lower case letter, in the same way that classes should begin with an upper case letter.