Java - 两个代码与 GridBagLayout 的比较 - IllegalArgumentException

Question

我刚刚为 GridBagLayout 测试了这段代码，我想知道为什么这段代码有错误，但它没有另一个代码。此代码出现异常IllegalArgumentException: cannot add to layout: constraint must be a string (or null)：

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Layout extends JFrame
{
    gridbag g=new gridbag();
    public Layout()
    {
        add(g, BorderLayout.CENTER);
    }
    public static void main(String[]args)
    {
        Layout lay=new Layout();
        lay.setSize(500, 500);
        lay.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        lay.setLocationRelativeTo(null);
        lay.setVisible(true);
    }
    class gridbag extends JPanel
    {
        private JLabel label=new JLabel("Test");
        public gridbag()
        {
            setLayout(new GridBagLayout());
            GridBagConstraints gbc=new GridBagConstraints();            
            Container container=getContentPane();           
            addC(label, container, gbc, 0, 0, 1, 4, 0, 0);          
        }
        private void addC(Component c, Container container, GridBagConstraints gbc, int C, int R, int nC, int nR, double wX, double wY )
        {
            gbc.gridx=C;
            gbc.gridy=R;
            gbc.gridwidth=nC;
            gbc.gridheight=nR;
            gbc.weightx=wX;
            gbc.weighty=wY;
            container.add(c, gbc);
        }
    }
}

但是这段代码有效：

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class gridbag extends JPanel
{
    private JLabel label=new JLabel("Test");
    public gridbag()
    {
        setLayout(new GridBagLayout());
        GridBagConstraints gbc=new GridBagConstraints();            
        Container container=this;
        addC(label, container, gbc, 0, 0, 1, 4, 0, 0);          
    }
    private void addC(Component c, Container container, GridBagConstraints gbc, int C, int R, int nC, int nR, double wX, double wY )
    {
        gbc.gridx=C;
        gbc.gridy=R;
        gbc.gridwidth=nC;
        gbc.gridheight=nR;
        gbc.weightx=wX;
        gbc.weighty=wY;
        container.add(c, gbc);
    }
    public static void main(String[]args)
    {
        gridbag g=new gridbag();
        JFrame frm=new JFrame();
        frm.setSize(500, 500);
        frm.add(g, BorderLayout.CENTER);
        frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frm.setLocationRelativeTo(null);
        frm.setVisible(true);
    }
}

区别之一是有一个新的Layout类用于JFrame，第二个代码直接使用JFrame。

Answer 1

问题就在这里，这是两个版本之间的另一个区别。

Container container=getContentPane();

尝试

Container container=this;