Exception in thread "main" java.lang.IllegalArgumentException: cannot add to layout: constraints must be a GridBagConstraint

Question

Im righting a simple text based game for my computing coursework but I'm having a few problems grasping how to use the AWT GridBagLayout and GridBagConstraints.

When ever I run the code i get this error message:

Here is a copy of my GUI code:

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;

import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Toolkit;
import java.awt.*;
@SuppressWarnings("unused")

public class GUI extends JFrame
{

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

    public GUI()
    {

        JFrame AG = new JFrame("Adventure Game");
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();

        setBounds(0, 0, screenSize.width, screenSize.height);
        setResizable(true);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        JPanel p=new JPanel();
        p.setLayout (new GridBagLayout());

        AG.add(p);
        GridBagConstraints GBC = new GridBagConstraints();
        AG.getContentPane().add(p, BorderLayout.NORTH);

        JButton saveButton =new JButton("Save");
        JButton loadButton =new JButton("Load");
        JButton optionsButton = new JButton("Options");

        JLabel textBox= new JLabel("Story line will go here.");

        JLabel label11 = new JLabel("Test 1");
        GBC.gridx = 0;
        GBC.gridy = 1;
        p.add(label11,AG);
        JLabel label12 = new JLabel("Test 2");
        GBC.gridx = 0;
        GBC.gridy = 2;
        p.add(label12,AG);
        JLabel label13 = new JLabel("Test 3");
        GBC.gridx = 0;
        GBC.gridy = 3;
        p.add(label13,AG);
        JLabel label14 = new JLabel("Test 4");
        GBC.gridx = 0;
        GBC.gridy = 4;
        p.add(label14,AG);

        add(p);
    }
}

I think the problem is me trying to use GridBagConstraints without using GridBagLayout but I'm quite new to java so don't how to use GridBagLayout to fix the problem, most of this code is from various different tutorials I've watched on youtube, but not copied just taking how various people do it and trying to mix it badly

Thanks for the help - Tom

Answer 1

Main Problem

You are passing AG, which is an instance of JFrame as the constraint when adding your components...

p.add(label11, AG);

Instead, you should be using GBC...

p.add(label11, GBC);

Other problems...

Your GUI is extending from JFrame, but you are creating an instance of another JFrame within in, this is very confusing...

It is normally recommended that you don't extend from top level containers, like JFrame, as you're not really adding an new value to the class and it locks you into a single container, reducing the reusability of your UI's

Instead, stick with the new instance of a JFrame (AG)...

public class GUI {

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

    public GUI() {

        JFrame AG = new JFrame("Adventure Game");
        AG.setResizable(true);
        AG.setVisible(true);
        AG.setDefaultCloseOperation(EXIT_ON_CLOSE);
        //...

---

You should really call setVisible last, after you have build the core interface, this will reduce potential issues with the way that the API updates the components

    public GUI() {

        JFrame AG = new JFrame("Adventure Game");
        AG.setResizable(true);
        AG.setDefaultCloseOperation(EXIT_ON_CLOSE);
        //...
        AG.setVisible(true);
    }

---

This...

    Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
    setBounds(0, 0, screenSize.width, screenSize.height);

is a horrible way to maximise a window, just use JFrame#setExtendedState and pass it JFrame.MAXIMIZED_BOTH...

setExtendedState(JFrame.MAXIMIZED_BOTH);

---

You should learn and use the common naming conventions of the lanuage, have a read through Code Conventions for the Java TM Programming Language, it will make it easier for people to read your code and for you to read others

---

Swing has some important rules, one of which is to ensure that you always create and modify your UI from within the context of the Event Dispatching Thread.

public GUI() {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            try {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                ex.printStackTrace();
            }

            new GUI();
        }
    });
}

See Initial Threads for more details

---

You main method should be declared as static, for example...

public static void main(String[] args) {

otherwise the class will not be runnable as a main entry point for Java