来自init（）的方法调用-Java

Question

I have been teaching myself Java for the past week or so. I'm brand new to programming. My code compiles just fine, but the applet does not display any text fields and the genRandomNumbers() method is not being invoked. So the question prompting the user to multiply two numbers is not displayed on the applet.

In my code, genRandomNumber() is being invoked from init() . Could that cause an issue? I tried to do this from paint() instead. That caused bigger issues - none of the text fields were displaying on the applet. So I moved the call to genRandomNumber() back to init() .

The applet window shows "Start: applet window not initialized" in the Status area.

Could you point me in the right direction? Any help is greatly appreciated!

Here is my code:

 //Generate 2 random numbers
 //Post a question to multiply the two numbers
 //Verify the answer entered
 //Post a new question if the solution is correct

import java.awt.event.*;
import java.awt.*;
import javax.swing.*;

public class LearnMultiplication extends JApplet implements ActionListener
{

   JLabel answerLabel;
   JTextField answerTextField, commentTextField, questionTextField;
   int random1, random2;

   public void init()
   {
   Container c = getContentPane();
   c.setLayout(new FlowLayout() );

   JTextField questionTextField = new JTextField(30);
   c.add(questionTextField);

   JLabel answerLabel = new JLabel("Enter you answer here");
   c.add(answerLabel);

   JTextField answerTextField = new JTextField(5);
   answerTextField.addActionListener(this);
   c.add(answerTextField);   

   JTextField commentTextField = new JTextField(30);
   c.add(commentTextField);

   genRandomNumbers(); // invoke method to generate 2 random numbers
   }



   public void actionPerformed (ActionEvent e)
   {
      int a = Integer.parseInt(e.getActionCommand() ); 
      verifyAnswer(a); // invoke method to verify the product
   }

   //method to generate 2 random numbers
   public void genRandomNumbers()
   {
        random1= 1 + (int)(Math.random() * 9    );    
        random2 = 1 + (int)(Math.random() * 9   );
        questionTextField.setText("Multiply " + Integer.toString(random1) + "and " + Integer.toString(random2) + ".");

  }  

   // method to verify the product of the 2 random numbers
   public void verifyAnswer(int answer)
   {
      int correctAnswer = random1 * random2;

      if ( correctAnswer == answer)
      {
         commentTextField.setText("Very Good!");
         genRandomNumbers(); //call the method again to generate 2 new random numbers
      }  
      else
      {
         commentTextField.setText("No, try again!!");
      }  

    }
}   

Answer 1

Basically, you have a series of NullPointerException s. This is one of the reasons why applets aren't a great place to get started with programming, because unless you have the Java console enabled, you won't get any output from your program which might be useful...

Any way...

You declare the following instance variables...

JTextField answerTextField, commentTextField, questionTextField;

Then in you init method you do this...

public void init() {
    Container c = getContentPane();
    c.setLayout(new FlowLayout());

    // Re-decleared/shadowed variable...
    JTextField questionTextField = new JTextField(30);
    c.add(questionTextField);

    JLabel answerLabel = new JLabel("Enter you answer here");
    c.add(answerLabel);

    // Re-decleared/shadowed variable...
    JTextField answerTextField = new JTextField(5);
    answerTextField.addActionListener(this);
    c.add(answerTextField);

    // Re-decleared/shadowed variable...
    JTextField commentTextField = new JTextField(30);
    c.add(commentTextField);

    genRandomNumbers(); // invoke method to generate 2 random numbers
}

You declare the questionTextField , answerTextField and commentTextField as local variables. This is commonly known as shadowing. Basically, when you think the instance variables have being initialised, they haven't, because you've used local variables instead...

If, instead, you do something more like...

public void init() {
    Container c = getContentPane();
    c.setLayout(new FlowLayout());

    questionTextField = new JTextField(30);
    c.add(questionTextField);

    JLabel answerLabel = new JLabel("Enter you answer here");
    c.add(answerLabel);

    answerTextField = new JTextField(5);
    answerTextField.addActionListener(this);
    c.add(answerTextField);

    commentTextField = new JTextField(30);
    c.add(commentTextField);

    genRandomNumbers(); // invoke method to generate 2 random numbers
}

You should now find your instance variables have being intialised and are now accessiable as expected from the other parts of your class...