调用创建JButton的代码时出错(不可见)

Question

我创建了一个刽子手游戏，并想在它周围添加一个JFrame。我开始制作主屏幕，但在启动JButton时遇到了问题。由于某种原因，它根本没有出现，我尝试了多种解决方案。

import java.util.*;
import javax.swing.*;
public class mainScreen extends JFrame {
JPanel mainPanel = new JPanel();
JButton startButton = new JButton("Start");

public mainScreen() {
JFrame mainFrame = new JFrame("Main Screen");
mainFrame.add(new JLabel(new ImageIcon("C:/Users/Owner/Desktop/Java Projects/hangman_image.jpg")));
mainFrame.setSize(1920, 1080);
mainFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE );
mainPanel = new JPanel();
startButton = new JButton("Start");
mainPanel.add(startButton);
mainFrame.setVisible(true);
}
} 

框架出现了，但按钮本身没有出现(我试着去掉图像，看看它是否覆盖了它(。

Answer 1

主要问题是mainPanel没有添加到可见容器中。请参阅代码注释中的进一步提示。

import java.awt.*;
import javax.swing.*;
// no need to extend JFrame ..
//public class mainScreen extends JFrame {
public class mainScreen {
// this is never addded to a visible container so .. (1)
JPanel mainPanel = new JPanel();
JButton startButton = new JButton("Start");
public mainScreen() {
JFrame mainFrame = new JFrame("Main Screen");
mainFrame.add(new JLabel("C:/../hangman_image.jpg"));
// (1) .. add it!
mainFrame.add(mainPanel, BorderLayout.PAGE_START);
// don't guess.. 
//mainFrame.setSize(1920, 1080);
// .. instead
mainFrame.setExtendedState(JFrame.MAXIMIZED_BOTH);
mainFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
// it is already instantiated, no need to do it twice! 
//mainPanel = new JPanel();
startButton = new JButton("Start");
mainPanel.add(startButton);
mainFrame.setVisible(true);
}
public static void main(String[] args) {
Runnable r = () -> {
mainScreen ms = new mainScreen();
};
SwingUtilities.invokeLater(r);
}
}

Answer 2

您已经创建了面板，但即使您的按钮已经添加到面板中，也没有将其添加到大型机中。

import javax.swing.*;
class Test1 extends JFrame {
JPanel mainPanel = new JPanel();
JButton startButton = new JButton("Start");

public Test1() {
JFrame mainFrame = new JFrame("Main Screen");
mainFrame.add(new JLabel(new ImageIcon("/filelocation")));
mainFrame.setSize(1920, 1080);
mainFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE );
mainPanel = new JPanel();
startButton = new JButton("Start");
mainPanel.add(startButton);
//here you add your panel to mainFrame
mainFrame.add(mainPanel);
mainFrame.setVisible(true);
}
}

public class Test{
public static void main(String args[]) {
new Test1();
}
} 

希望这能有所帮助。