在子 jpanel 上重新绘制

Question

好吧，假设我在另一个 jpanel 中有一个 JPanel“控件”和 jpanel“graphPanel”，

public class outer extends JPanel implements ActionListener{
   private JPanel controls,graphPanel;
   private JButton doAction

   public outer(){
      JPanel controls = new JPanel();
      JButton doAction = new JButton("Do stuff");
      doAction.addActionListener(this);
      controls.add(doAction);

      JPanel graphPanel = new JPanel();
      this.add(controls);
      this.add(graphPanel);
   }



public void actionPerformed(ActionEvent e) {
    if(e.getSource()==doAction){
        //How do I fire paintComponent of controls JPanel on this click
}

单击按钮后如何重新绘制“graphPanel”

Answer 1

doAction 和 graphPanel 声明了两次 - 一次在类级别，然后在方法中再次声明：

   private JPanel controls,graphPanel; // declared here first
   private JButton doAction; // declared here first

   public outer(){
      JPanel controls = new JPanel(); // Whoops, declared here again
      JButton doAction = new JButton("Do stuff"); // Whoops, declared here again
      doAction.addActionListener(this);
      controls.add(doAction);

      JPanel graphPanel = new JPanel(); // Whoops, declared here again
      ...

在方法中删除声明，并使它们成为简单的赋值，如下所示：

controls = new JPanel(); // no leading 'JPanel'

这样做，附加的重绘代码将不会抛出 NPE