java - 实现 JFrame 时线程 "main"java.lang.NullPointerException 中出现异常

Question

我的任务是创建一个java程序，实现关于BMI转换器的GUI，该转换器可以获取高度和体重。目前尚未完成，但我收到以下错误:

Exception in thread "main" java.lang.NullPointerException
   at BMIGUI.<init>(BMIGUI.java:60)
   at BMITest.main(BMITest.java:7)

在和我的一个 friend 一起调试它时，他建议我的错误是我没有在 BMIGUI 代码中初始化我的变量“frame”。但再看一遍，我觉得这似乎是正确的..

请帮忙。谢谢

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.text.DecimalFormat;
public class BMIGUI extends JFrame
{
   private final int WIDTH = 275;
   private final int HEIGHT = 100;

   DecimalFormat fmt = new DecimalFormat ("0.00");

   private JFrame frame;
   private JPanel panel;
   private JLabel heightLabel, weightLabel, BMILabel, resultLabel;
   private JTextField height, weight;
   private JButton calculate;


   public BMIGUI()
   {
      super("BMI Converter");

      heightLabel = new JLabel ("Your height in meters:");
      weightLabel = new JLabel ("Your weight in kilograms: ");

      //create a "this is your BMI" label
      BMILabel = new JLabel ("Your BMI is ") ;
      //create a result label to hold the BMI value
      resultLabel = new JLabel ("") ;

      //create a JTextField to hold the person's height in inches
      height = new JTextField (5);
      //create a JTextField to hold the person's weight in pounds
      weight = new JTextField (5) ;

      //create a button to press to calculate BMI
      calculate = new JButton ("calculate BMI");
      //create a BMIListener and make it listen for the button to be pressed
      calculate.addActionListener ( new BMIListener () ) ;

      //set up the JPanel to go on the JFrame 
      panel = new JPanel();
      panel.setPreferredSize (new Dimension(WIDTH, HEIGHT));
      panel.setBackground (Color.yellow);

      //add the height label and height textfield to the panel
      panel.add (heightLabel) ;
      panel.add (height) ;
      //add the weight label and weight textfield to the panel
      panel.add (weightLabel) ;
      panel.add (weight) ;
      //add the button to the panel
      panel.add (calculate) ;
      //add the BMI label to the panel
      panel.add (BMILabel) ;
      //add the label that holds the result to the panel
      panel.add (resultLabel) ;

      //add the panel to the frame 
     frame.getContentPane().add (panel);
   }


   public void display()
   {
      frame.pack();
      frame.setVisible(true);
   }


   private class BMIListener implements ActionListener
   {

      public void actionPerformed (ActionEvent event)
      {
         String heightText, weightText;
         double heightVal, weightVal, bmi;

     //get the text from the height and weight textfields
     heightText = height.getText();
     weightText = weight.getText();

     //Use Integer.parseInt to convert the text to integer values
     heightVal = Double.parseDouble (heightText) ;
     weightVal = Double.parseDouble (weightText) ;

     //Calculate the bmi = 703 * weight in pounds / (height in inches)^2
     bmi = weightVal / (heightVal*heightVal) ;

     //Put result in result label.  Use Double.toString to convert double to string.
     resultLabel.setText( fmt.format(bmi) ) ;

      }
   }
}

主类

 import javax.swing.JFrame;

public class BMITest extends JFrame
{
    public static void main(String args[])
    {
        BMIGUI frame = new BMIGUI();

        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE);
        frame.setSize(350,100);
        frame.setVisible(true);
    }
}

Answer 1

尽管您正在引用 BMIGUI 类上下文中声明的 frame，但您尚未初始化它。如:

frame.getContentPane().add (panel);

还有其他地方。由于 BMIGUI 本身就是您的框架窗口，实际上您根本不需要在 BIMGUI 上下文中声明另一个框架实例，只需将 frame 引用替换为 this 即可引用当前引用实例。

这始终被认为是良好的做法:

1. 始终覆盖 getPreferredSize(Dimension) 而不是 setPreferredSize()
2. 调用 pack() 而不是使用 setSize(Dimension) :pack() 方法调整框架大小，以便其所有内容都等于或高于其首选大小。

请，follow java's usual coding convention 。类的名字后面要大写，但不能全部字母都大写，例如BIMGUI应该是BimGui。我们使用全部大写字母来表示常量字段:public static final int DATAMARKER