向使用BoxLayout的JFrame添加JLabel

JFrame frame = new JFrame();
frame.setLayout(new BoxLayout(frame.getContentPane(), BoxLayout.Y_AXIS));

JLabel label = new JLabel("Hello World!");

frame.add(label);

import javax.swing.*;

public class BoxLayoutExample {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setLayout(new BoxLayout(frame.getContentPane(), BoxLayout.Y_AXIS));

        JLabel label = new JLabel("Hello World!");

        frame.add(label);

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }
}