GridBagLayout：仅当我在宽度方向展开JFrame时才会出现JPanels

Question

我发布了一个简短的示例代码和2个屏幕截图。第一个没有在panels内显示mainPanel（frame内部），我认为它们只是没有出现。

当我稍微扩展框架宽度时，面板显示出来，尽管面板的尺寸没有明确设置。 什么是背后的原因以及如何在我认真的申请中避免这种情况？

public class test extends JFrame {
    public test() {
    }
    public static void main(String [] args){
        JFrame frame= new JFrame();
        Dimension size= frame.getPreferredSize();
        size.width=200;
        size.height=300;
        frame.setPreferredSize(size);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true); 

        frame.getContentPane().setLayout(new BorderLayout()); 

        JPanel mainPanel= new JPanel();
        mainPanel.setBorder(BorderFactory.createTitledBorder(
                new EtchedBorder(), "Details", TitledBorder.CENTER,
                TitledBorder.DEFAULT_POSITION, new Font("calibri", Font.PLAIN,
                        10)));
        frame.getContentPane().add(mainPanel, BorderLayout.CENTER);

        mainPanel.setLayout(new GridBagLayout()); 

        JPanel panel1= new JPanel();
        panel1.setBorder(BorderFactory.createTitledBorder(
                new EtchedBorder(), "Panel1", TitledBorder.CENTER,
                TitledBorder.DEFAULT_POSITION, new Font("calibri", Font.PLAIN,
                        10)));
        //check...panel1.setBackground(Color.black);
        GridBagConstraints c= new GridBagConstraints();
        c.gridx= 0;
        c.gridy=0;
        c.weightx=1;
        c.weighty=1;
        c.fill= c.BOTH;
        mainPanel.add(panel1, c); 

        JPanel panel2= new JPanel();
        panel2.setBorder(BorderFactory.createTitledBorder(
                new EtchedBorder(), "Panel2", TitledBorder.CENTER,
                TitledBorder.DEFAULT_POSITION, new Font("calibri", Font.PLAIN,
                        10)));
        //check..panel2.setBackground(Color.red); 
        c= new GridBagConstraints();
        c.gridx=0;
        c.gridy=1;
        c.weightx=1;
        c.weighty=2;
        c.fill= c.BOTH;
        mainPanel.add(panel2, c);

        JPanel panel3= new JPanel();
        panel3.setBorder(BorderFactory.createTitledBorder(
                new EtchedBorder(), "Panel3", TitledBorder.CENTER,
                TitledBorder.DEFAULT_POSITION, new Font("calibri", Font.PLAIN,
                        10)));
        //check...panel3.setBackground(Color.blue);
        c= new GridBagConstraints();
        c.gridx=0;
        c.gridy=2;
        c.weightx=1;
        c.weighty=6;
        c.fill=c.BOTH;
        mainPanel.add(panel3, c);

    }
}

Answer 1

frame.setVisible(true); 

必须在将所有组件添加到框架及其子面板后执行上述声明。

  

当我稍微扩展框架宽度时，面板出现了，

调整框架大小时，会调用布局管理器，以便可以正确地设置和绘制组件。