无法将任何组件添加到 JFrame - IllegalArgumentException

Question

When I try to add anything to JFrame in constructor and I have IllegalArgumentException.

Similar code is working in documentation: https://docs.oracle.com/javase/tutorial/uiswing/layout/none.html

Why my very simple code doesn't work? It is created in Netbeans 10.

Edit: I also tried add label with position and sieze (setBounds) it doesn't help. I changed the code with setBounds method.

First, main class in JavaTestApp.java:

package javatestapp;

public class JavaTestApp {
    public static void main(String[] args) {
        TestForm mainFrame = new TestForm();

        mainFrame.setLocation(300, 150);
        mainFrame.setVisible(true);

        mainFrame.toFront();
        mainFrame.repaint();
    }

}

Second file is:

package javatestapp;

import javax.swing.JLabel;

public class TestForm extends javax.swing.JFrame {

    /**
     * Creates new form TestForm
     */
    public TestForm() {
        initComponents();

        JLabel label = new JLabel("Test label");
        label.setBounds(10,10,100,25);        

        getContentPane().add(label);
    }

    // GENERATED CODE BELOW
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        getContentPane().setLayout(new org.netbeans.lib.awtextra.AbsoluteLayout());

        pack();
    }// </editor-fold>                        

    public static void main(String args[]) {
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(TestForm.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(TestForm.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(TestForm.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(TestForm.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new TestForm().setVisible(true);
            }
        });
    }

Stacktrace, regarding to @VGR request:

Exception in thread "main" java.lang.IllegalArgumentException
    at org.netbeans.lib.awtextra.AbsoluteLayout.addLayoutComponent(Unknown Source) 
    at java.awt.Container.addImpl(Container.java:1120)
    at java.awt.Container.add(Container.java:410)
    at javatestapp.TestForm.<init>(TestForm.java:16)
    at javatestapp.JavaTestApp.main(JavaTestApp.java:5) 
BUILD STOPPED (total time: 9 seconds)

Link to project uploaded on weetransfer: Project (scanned via my Bitdefender antivir)

Answer 1

I also chosen absolute layout

And that is your problem. You are not using it correctly.

    getContentPane().add(label);

You can't just add the label to the frame without specifying the proper constraints. I've never used AbsoluteLayout (because I believe in proper layout management) but I would guess you need to specify constraints like x, y, width, height.

The layout manager can't guess where you want to position the component so you need to specify all the information. That is why you should be using a layout manager. Then the layout manager will position the component based on the rules of the layout manager. Much easier, once you practice it a little.