How to Dynamically Add JButton to JPanel?

Question

In NetBeans, I have used the GUI editor to make a JFrame and I\'ve put a JPanel in the frame. At the moment, I\'m trying to make a new button in the panel when the class con

Answer 1

Normally the add call is sufficient.

Note: a BorderLayout can only contain one component in each location. So if you add another component in the NORTH location, your button will not be visible.

Second note: by default a JPanel does not have a BorderLayout but a FlowLayout. Have you set a BorderLayout on that specific panel ? Otherwise the BorderLayout#NORTH constraint is incorrect

All the validate,revalidate,repaint calls can be removed

Edit

It seems some sort of validation is needed after all. I was under the impression that Swing should be smart enough to listen for the event when something is added to a Container, and update whatever is necessary (a bit similar to updating a TableModel updates the JTable based on events, without the need to call repaint or the likes on the JTable).

However, when trying this in an SSCCE, I came to the following code (different versions, only post the most elaborate version)

• without the scroll-pane, the validate calls seem to have no effect. I actually need to call pack again to make the new labels visible (not included in the SSCCE, but removing the scrollpane from the code is trivial)

• with the scroll-pane, the validate call has an effect

  import javax.swing.BoxLayout;
  import javax.swing.JButton;
  import javax.swing.JFrame;
  import javax.swing.JLabel;
  import javax.swing.JPanel;
  import javax.swing.JScrollPane;
  import java.awt.BorderLayout;
  import java.awt.Container;
  import java.awt.Dimension;
  import java.awt.EventQueue;
  import java.awt.event.ActionEvent;
  import java.awt.event.ActionListener;
  
  public class AddLabelsAtRuntime {
  
    private int fLabelCounter = 0;
    private JPanel fLabelPanel;
    private final JFrame fTestFrame;
  
    public AddLabelsAtRuntime() {
      fLabelPanel = new JPanel(  );
      BoxLayout boxLayout = new BoxLayout( fLabelPanel, BoxLayout.PAGE_AXIS );
      fLabelPanel.setLayout( boxLayout );
      fTestFrame = new JFrame( "Dynamically add labels" );
    }
  
    private JFrame createUI(){
  
      Container contentPane = fTestFrame.getContentPane();
      contentPane.setLayout( new BorderLayout() );
  
      JScrollPane scrollPane = new JScrollPane( fLabelPanel );
      scrollPane.setPreferredSize( new Dimension( 200, 200 ) );
      contentPane.add( scrollPane, BorderLayout.CENTER );
  
      contentPane.add( createButtonPanel(), BorderLayout.SOUTH );
  
      fTestFrame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      fTestFrame.pack();
      return fTestFrame;
    }
  
    private void addLabel(){
      fLabelPanel.add( new JLabel( "Label " + ++fLabelCounter ) );
    }
  
    private JPanel createButtonPanel(){
      JPanel buttonPanel = new JPanel(  );
      BoxLayout boxLayout = new BoxLayout( buttonPanel, BoxLayout.LINE_AXIS );
      buttonPanel.setLayout( boxLayout );
  
      JButton validateButton = new JButton( "Add + validate" );
      validateButton.addActionListener( new ActionListener() {
        @Override
        public void actionPerformed( ActionEvent e ) {
          addLabel();
          fLabelPanel.validate();
          fTestFrame.validate();
        }
      } );
      buttonPanel.add( validateButton );
  
      JButton noValidateButton = new JButton( "Add" );
      noValidateButton.addActionListener( new ActionListener() {
        @Override
        public void actionPerformed( ActionEvent e ) {
          addLabel();
        }
      } );
      buttonPanel.add( noValidateButton );
  
      JButton packButton = new JButton( "Add + pack" );
      packButton.addActionListener( new ActionListener() {
        @Override
        public void actionPerformed( ActionEvent e ) {
          addLabel();
          fTestFrame.pack();
        }
      } );
      buttonPanel.add( packButton );
  
      return buttonPanel;
    }
  
    public static void main( String[] args ) {
      EventQueue.invokeLater( new Runnable() {
        @Override
        public void run() {
          AddLabelsAtRuntime addLabelsAtRuntime = new AddLabelsAtRuntime();
          addLabelsAtRuntime.createUI().setVisible( true );
        }
      } );
  
    }
  
  }