How to dynamically add JButton to JPanel?

Question

In NetBeans, I used the GUI editor to create a JFrame, and I put the JPanel in the frame. At the moment, I'm trying to create a new button on the panel when creating the class. This is the code that I have, but I cannot get it to work. (The first line makes the button, the other lines show it.)

this.jPanel2.add(new JButton("Test"),BorderLayout.NORTH); this.jPanel2.validate(); this.jPanel2.revalidate(); this.jPanel2.repaint(); this.jPanel2.setVisible(true); this.revalidate(); this.setVisible(true); this.repaint(); 

I walked all night, but I can’t get it to work.

Answer 1

Several times when you do not see a button, this is a layout manager problem (since you are not setting the correct properties for the layout manager). You can verify this by disabling it:

 this.jPanel2.setLayoutManager(null); 

And setting the button borders ( JButton.setBounds() ).

If the above problem fixes your problem, you need to examine the requirements set by your LayoutManager (see also Robin's answer).

All calls to validate() , revalidate() and repaint() are not required for this.

Answer 2

Usually just calling add enough.

Note: a BorderLayout can contain only one component in each location. Therefore, if you add another component to 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 installed BorderLayout on this particular panel? Otherwise, the BorderLayout#NORTH constraint is incorrect.

All calls to validate , revalidate , repaint can be removed

Edit

It seems that some kind of validation is needed in the end. I got the impression that Swing should be smart enough to listen to the event when something is added to the Container and update everything it needs (a bit like updating TableModel updates JTable based on events, without the need to call repaint or JTable on JTable ) .

However, when you try to do this in SSCCE, I came to the following code (different versions, only publish the most complex version)

• without a scroll pane, validate calls seem ineffective. I really need to call pack again to make the new shortcuts visible (not included in SSCCE, but removing scroll from code is trivial)

• using the scroll pane, calling validate 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 ); } } ); } } 

Answer 3

Create dynamic JButton with Image and ActionListener - Java Swing

Dynamically create JButton with Image and ActionListener. You can change the height of the button, the width of the horizontal gap and the vertical gap in one place.

You can find more information here.