在BorderLayout.CENTER中设置JPanel的最大大小

import javax.swing.*;
import java.awt.*;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("BorderLayout.CENTER Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);

        JPanel centerPanel = new JPanel();
        centerPanel.setPreferredSize(new Dimension(300, 200));
        centerPanel.setBackground(Color.CYAN);

        frame.getContentPane().setLayout(new BorderLayout());
        frame.getContentPane().add(centerPanel, BorderLayout.CENTER);

        frame.setVisible(true);
    }
}