import java.awt.Component;
import java.awt.Dimension;
import javax.swing.BorderFactory;
import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Main {
public static void main(String[] args) {
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().setLayout(
new BoxLayout(frame.getContentPane(), BoxLayout.Y_AXIS));
JPanel a = new JPanel();
a.setAlignmentX(Component.CENTER_ALIGNMENT);
a.setPreferredSize(new Dimension(100, 100));
a.setMaximumSize(new Dimension(100, 100)); // set max = pref
a.setBorder(BorderFactory.createTitledBorder("aa"));
JPanel b = new JPanel();
b.setAlignmentX(Component.CENTER_ALIGNMENT);
b.setPreferredSize(new Dimension(50, 50));
b.setMaximumSize(new Dimension(50, 50)); // set max = pref
b.setBorder(BorderFactory.createTitledBorder("bb"));
frame.getContentPane().add(a);
frame.getContentPane().add(b);
frame.pack();
frame.setVisible(true);
}
}