java - 奇怪的 Swing 编译时可访问性错误

Question

这是代码-

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;

public final class SetLabelForDemo {
    public static void main(String[] args){
        SwingUtilities.invokeLater(new Runnable(){
            @Override
            public void run() {
                createAndShowGUI();             
            }
        });
    }

    private static void createAndShowGUI(){
        final JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(new JLabeledButton("foo:")); // new JLabeledButton("foo:") is the problem
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    private final class JLabeledButton extends JButton{
        public JLabeledButton(final String s){
            super();
            JLabel label = new JLabel(s);
            label.setLabelFor(this);
        }
    }
}

这是错误信息-

No enclosing instance of type SetLabelForDemo is accessible. Must qualify the allocation with an enclosing instance of type SetLabelForDemo (e.g. x.new A() where x is an instance of SetLabelForDemo).

我完全不明白这个错误。对我来说，一切似乎都是完全正确的。我错过了什么吗？

Answer 1

您必须将您的类 JLabeledButton 声明为静态的，因为您是在静态上下文中实例化它的:

private static final class JLabeledButton extends JButton {
    ...
}

因为您的方法 createAndShowGUI 是静态的，所以编译器不知道您正在为哪个 SetLabelForDemo 实例创建封闭类。