GridBagLayout：如何移动指定组件

上面的代码暗示的误解都weightx/y & gridx/y是如何工作的。此MCVE将数字更正为将按钮与图像居中，而将标签和文本字段留在左上角。

import java.awt.*; 
import java.awt.image.*; 
import javax.swing.*; 
import javax.swing.border.*; 
public class CenteredImageInGridBag { 
    private JComponent ui = null; 
    CenteredImageInGridBag() { 
     initUI(); 
    } 
    public final void initUI() { 
     if (ui != null) { 
      return; 
     } 
     ui = new JPanel(new BorderLayout(4, 4)); 
     ui.setBorder(new EmptyBorder(4, 4, 4, 4)); 
     JPanel center = new JPanel(); 
     ui.add(center); 
     center.setLayout(new GridBagLayout()); 
     GridBagConstraints gbc = new GridBagConstraints(); 
     Border innerC = BorderFactory.createTitledBorder("Travel details"); 
     Border outsideC = BorderFactory.createEmptyBorder(5, 5, 5, 5); 
     center.setBorder(BorderFactory.createCompoundBorder(outsideC, innerC)); 
     JLabel fromLabel = new JLabel("From : "); 
     JTextField fromField = new JTextField("to", 30); 
     gbc.gridx = 0; 
     gbc.gridy = 0; 
     gbc.weightx = 0; 
     gbc.weighty = 0; 
     gbc.insets = new Insets(5, 4, 5, 5); 
     gbc.fill = GridBagConstraints.NONE; 
     gbc.anchor = GridBagConstraints.LINE_END; 
     center.add(fromLabel, gbc); 
     gbc.gridx = 1; 
     gbc.gridy = 0; 
     gbc.weightx = 0; 
     gbc.weighty = 0; 
     gbc.fill = GridBagConstraints.NONE; 
     gbc.anchor = GridBagConstraints.LINE_START; 
     gbc.gridwidth = 1; 
     center.add(fromField, gbc); 
     JButton trainImage = new JButton(); 
     BufferedImage image = 
       new BufferedImage(40,10,BufferedImage.TYPE_INT_RGB); 
     trainImage.setIcon(new ImageIcon(image)); 
     gbc.gridx = 1; 
     gbc.gridy = 1; 
     gbc.weightx = 1; 
     gbc.weighty = 1; 
     gbc.fill = GridBagConstraints.NONE; 
     gbc.anchor = GridBagConstraints.CENTER; 
     gbc.gridwidth = 3; 
     gbc.ipadx = 5; 
     center.add(trainImage, gbc); 
    } 
    public JComponent getUI() { 
     return ui; 
    } 
    public static void main(String[] args) { 
     Runnable r =() -> { 
      try { 
       UIManager.setLookAndFeel(
         UIManager.getSystemLookAndFeelClassName()); 
      } catch (Exception useDefault) { 
      } 
      CenteredImageInGridBag o = new CenteredImageInGridBag(); 
      JFrame f = new JFrame(o.getClass().getSimpleName()); 
      f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE); 
      f.setLocationByPlatform(true); 
      f.setContentPane(o.getUI()); 
      f.pack(); 
      f.setMinimumSize(f.getSize()); 
      f.setVisible(true); 
     }; 
     SwingUtilities.invokeLater(r); 
    } 
}