如何更改Jframe里Jpanel的大小

先贴代码：

package xiya;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

class Fram extends JFrame{
	public Fram(){
		setTitle("Test");
		setSize(800, 600);
		setResizable(false);
		setVisible(true);
		setBackground(Color.WHITE);
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		
		vgaPanel vgaPanel = new vgaPanel();
		add(vgaPanel, BorderLayout.WEST);
		
		regPanel regPanel = new regPanel();
		add(regPanel, BorderLayout.EAST);
		
		JPanel northJPanel = new JPanel();
		northJPanel.setPreferredSize(new Dimension(800, 0));
		add(northJPanel, BorderLayout.NORTH);
		
		JPanel buttonJPanel = new JPanel();
		buttonJPanel.setPreferredSize(new Dimension(800, 40));
		buttonJPanel.add(new JButton("Run"));
		buttonJPanel.add(new JButton("Step"));
		buttonJPanel.add(new JButton("Restart"));
		add(buttonJPanel, BorderLayout.SOUTH);
	}
}

class vgaPanel extends JPanel{
	public vgaPanel() {
		setLayout(null);
		//setSize(400, 600);
		setPreferredSize(new Dimension(640, 300));
		setBackground(Color.RED);
	}
}

class regPanel extends JPanel{
	public regPanel() {
		setLayout(null);
		//setSize(400, 600);
		setPreferredSize(new Dimension(160, 300));
		setBackground(Color.GREEN);
	}
}
public class Test {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Fram fram = new Fram();
	}

}

Jframe默认布局为：BorderLayout

Jpanel默认布局为：FlowLayout

默认布局会屏蔽掉我们的设置大小(如setSize)

Jpanel中使用setPreferredSize更改大小,如果想使用自己的布局方式,必须setLayout(null);