java - 当我尝试在 net-beans 中运行它时，我收到 "java.lang.reflect.InvocationTargetException"。为什么会发生这种情况？

Question

package airlinep;

/**
 *
 * 
 */
import javax.swing.*; 
import java.awt.*; 
import java.awt.event.*; 
public class NewClass1 extends JApplet{
JButton b1; 
  JButton b2; 

  JLabel lbl; 
    @Override
    public void init()
    {
    try {
        SwingUtilities.invokeAndWait(new Runnable() {
            @Override
            public void run(){
                guiInit();//throw new UnsupportedOperationException("Not supported yet."); 
            }
        }); 
    } catch (Exception ex) {
        System.out.println("could not generate due to "+ex.toString());
    } 
    }

public void start()
{}
public void stop()
{}
public void destroy()
{}
private void guiInit()
{
//setLayout(new FlowLayout());
b1.setText("b1");
b2.setText("b2");
b1.addActionListener(new ActionListener() {

    @Override
    public void actionPerformed(ActionEvent ae) {
         lbl.setText(b1.getText()+" was pressed at ");//To change body of generated methods, choose Tools | Templates.
    }
});
b2.addActionListener(new ActionListener() {

    @Override
    public void actionPerformed(ActionEvent ae) {
         lbl.setText(b2.getText()+" was pressed at ");//To change body of generated methods, choose Tools | Templates.
    }
});
getContentPane().add(b1);
getContentPane().add(b2);
getContentPane().add(lbl);
}
}

当程序在netbeans中运行时，小程序窗口正在显示，但没有像按钮一样可见的gui元素。在控制台中，我可以看到异常已被try catch block 捕获。为什么我会得到这个异常以及我该怎么做才能避免这种情况发生。

Answer 1

您没有看到 NullPointerException，因为您没有正确捕获异常。在尝试使用它们之前，您必须将对象分配给引用变量，包括 JButton 和 JLabel 变量。

换句话说，改变

JButton b1; 
JButton b2; 

JLabel lbl; 

到

JButton b1 = new JButton("something"); 
JButton b2 = new JButton("something else"); 

JLabel lbl = new JLabel("something else entirely"); 

<小时/>

还要捕获错过的目标异常，请考虑执行以下操作:

  try {
     SwingUtilities.invokeAndWait(new Runnable() {
        @Override
        public void run() {
           guiInit();
        }
     });
  } catch (InvocationTargetException e) {
     e.getTargetException().printStackTrace(); // get the target exception
  } catch (InterruptedException e) {
     e.printStackTrace();
  }