不能使用setLocationRelativeTo(Null)使Jframe居中；

import java.awt.Dimension;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;

import javax.swing.JFrame;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("My Frame");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        // 设置窗口大小
        frame.setSize(400, 300);
        
        // 获取屏幕的宽度和高度
        GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
        GraphicsDevice gd = ge.getDefaultScreenDevice();
        int screenWidth = gd.getDisplayMode().getWidth();
        int screenHeight = gd.getDisplayMode().getHeight();
        
        // 计算窗口的位置
        int x = (screenWidth - frame.getWidth()) / 2;
        int y = (screenHeight - frame.getHeight()) / 2;
        
        // 设置窗口位置
        frame.setLocation(x, y);
        
        frame.setVisible(true);
    }
}

import java.awt.Dimension;
import java.awt.Toolkit;

import javax.swing.JFrame;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("My Frame");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        // 设置窗口大小
        frame.setSize(400, 300);
        
        // 获取屏幕的大小
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        int screenWidth = screenSize.width;
        int screenHeight = screenSize.height;
        
        // 计算窗口的位置
        int x = (screenWidth - frame.getWidth()) / 2;
        int y = (screenHeight - frame.getHeight()) / 2;
        
        // 设置窗口位置
        frame.setLocation(x, y);
        
        frame.setVisible(true);
    }
}