如何从Jbutton actionlistener创建JLabel实例？

Question

I'm trying to create a JLabel WITHIN the actionlistener event so please don't post any new jlabel(); instances outside of the actionlistener event. Main is the JFrame and Credits is the Jbutton and Oracle is the instance Jlabel i want to create.

Here is the code:

 Credits.addActionListener(new ActionListener() {
    public  void actionPerformed (ActionEvent e)
    { 
      JLabel Oracle = new JLabel();
      Oracle.setBounds(100,300,300,300);
      Oracle.setText("HI");
     Main.add(Oracle);
     }
 });

Answer 1

Suggestions:

• First and foremost, don't set the bounds of anything. Using setBounds(...) suggests that you're using null layouts, often a sign of a newbie Swing program, and those programs are very rigid, hard to update, enhance and debug. Use the layout managers instead.
• Next the layout manager of the container (here "Main") that is accepting the JLabel is key. Some don't take too well to added components, for example GroupLayout, while others do better, such as BoxLayout.
• Call revalidate() and repaint() on your container after adding or removing components.
• As a side recommendation (one not directly affecting your problem), to help us now and to help yourself in the future, please edit your code and change your variable names to conform with Java naming conventions : class names all start with an upper-case letter and method/variable names with a lower-case letter.
• If you still need more help, consider creating and posting a minimal example program .

---

For example, code with explanatory comments:

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import javax.swing.*;

@SuppressWarnings("serial")
public class AddALabel extends JPanel {
   private static final int PREF_W = 300;
   private static final int PREF_H = 300;

   // The JPanel that gets the JLabels. GridLayout(0, 1) will stack them
   // in a single column
   private JPanel receivingPanel = new JPanel(new GridLayout(0, 1));

   // wrap the above JPanel in the below one, and add it 
   // BorderLayout.PAGE_START so that the newly added JLabels 
   // bunch to the top
   private JPanel wrapPanel = new JPanel(new BorderLayout());

   // put the outer JPanel, the wrapPanel into a JScrollPane
   private JScrollPane scrollpane = new JScrollPane(wrapPanel);

   // Holds text that goes into the newly created JLabel
   private JTextField textField = new JTextField("Foo", 10);

   public AddALabel() {
      // wrap the receiving panel.
      wrapPanel.add(receivingPanel, BorderLayout.PAGE_START);
      scrollpane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

      JPanel northPanel = new JPanel();
      northPanel.add(textField);
      // add a JButton that does our work. Give it an Action/ActionListener
      northPanel.add(new JButton(new AddLabelAction("Add a Label")));

      setLayout(new BorderLayout());
      add(northPanel, BorderLayout.PAGE_START);
      add(scrollpane, BorderLayout.CENTER);
   }

   @Override
   public Dimension getPreferredSize() {
      if (isPreferredSizeSet()) {
         return super.getPreferredSize();
      }
      return new Dimension(PREF_W, PREF_H);
   }

   // the code for our Action/ActionListener
   private class AddLabelAction extends AbstractAction {
      public AddLabelAction(String name) {
         super(name);
      }

      @Override
      public void actionPerformed(ActionEvent e) {
         // get text from JTextField
         String text = textField.getText();

         // create JLabel and insert above text
         JLabel label = new JLabel(text);

         // add JLabel to receiving JPanel
         receivingPanel.add(label);

         // revalidate and repaint receiving JPanel
         receivingPanel.revalidate();
         receivingPanel.repaint();
      }
   }

   // create our GUI in a thread-safe way
   private static void createAndShowGui() {
      AddALabel mainPanel = new AddALabel();

      JFrame frame = new JFrame("AddALabel");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

Answer 2

I think all you were missing was a call to the pack method, here how I was able to get your wanted result :

public class Main extends JFrame {

    final JButton credits;

    public Main()
    {
        super("JLabel from JButton actionlistener");
        setSize(100, 100);        
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationByPlatform(true);

        credits = new JButton("Credits");

        credits.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae)
            {
                JLabel oracle = new JLabel();
                oracle.setText("Oracle licenses are out of price");
                Main.this.add(oracle);
                Main.this.pack();
            }
        });

        getContentPane().setLayout(new FlowLayout());
        add(credits);
        setVisible(true);
    }

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

}