在运行时将组件添加到jpanel [英] add component to jpanel on runtime

问题描述

我在JFrame上有一个JPanel和JButton.
在运行时，将JLabel添加到JPanel时，单击JButton.

I have a JPanel and JButton on the JFrame.
on runtime add JLabel to JPanel When click JButton.

我使用以下代码:

I use of the following code:

 panel.setLayout(null);

 jLabel _lbl=new jLabel();
 _lbl.setText("Label");
 panel.add(_lbl);
 panel.validate();

，但在JPanel中不显示任何JLabel.

推荐答案

我看到您创建了一个名为_lbl的JLabel:

I see you create a JLabel called _lbl:

 JLabel _lbl=new JLabel();

，但是您永远不会将其添加到面板中.相反，您将没有文本的新JLabel添加到面板中:

but you never add it to your panel. Instead you add a new JLabel with no text to your panel:

 panel.add(new JLabel());

这当然会构建一个不可见的空标签.

This would ofcourse construct an empty label which wont be visible.

还应尝试在添加JLabel之后在JPanel实例上调用revalidate()和repaint()，如下所示:

Also try calling revalidate() and repaint() on your JPanel instance after adding the JLabel like so:

JLabel _lbl=new JLabel("Label");//make label and assign text in 1 line

panel.add(_lbl);//add label we made

panel.revalidate();
panel.repaint();

为此，您可能还需要在框架实例上调用pack()，以便调整JFrame的大小以适合新组件.

With this you may also need to call pack() on your frames instance so as to resize the JFrame to fit the new components.

也请不要使用null/Absolute布局，这是非常不好的做法(除非制作动画)，并且可能会出现问题并且很难使用.

Also please never use a null/Absolute layout this is very bad practice (unless doing animation) and may prove to be problematic and very hard to use.

宁可使用LayoutManager:

• 布局管理器的可视指南

，或者如果JPanel上只有一个组件，则只需调用add(label);，因为它会扩展到JPanel的大小.

or if you only have a single component on the JPanel simply call add(label); as it will stretch to the JPanel size.

更新:

这是一个小样本.只需在每次按下JButton时将JLabel添加到JPanel:

Here is a small sample. Simply adds JLabels to the JPanel each time JButton is pressed:

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

public class JavaApplication116 {

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

            @Override
            public void run() {
                new JavaApplication116().createAndShowUI();
            }
        });
    }

    private void createAndShowUI() {
        JFrame frame = new JFrame("Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        initComponents(frame);

        frame.setResizable(false);
        frame.pack();
        frame.setVisible(true);
    }

    private void initComponents(final JFrame frame) {
        final JPanel panel = new JPanel(new FlowLayout());
        JButton button = new JButton("Add label");
        button.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                JLabel _lbl = new JLabel("Label");//make label and assign text in 1 line

                panel.add(_lbl);//add label we made

                panel.revalidate();
                panel.repaint();

                frame.pack();//so our frame resizes to compensate for new components
            }
        });
        frame.getContentPane().add(panel, BorderLayout.CENTER);
        frame.getContentPane().add(button, BorderLayout.SOUTH);
    }
}

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