Question

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

public class BMICalc extends JApplet
{
   private final String PROGRAM_NAME = "BMI Calculator";
   JTextField weightText = new JTextField(3);
   JTextField ftHeightText = new JTextField(3);
   JTextField inHeightText = new JTextField(3);

   DecimalFormat bmiFmt = new DecimalFormat("##0.0");

   public void init()
   {
      JLabel label;
      JPanel panel;

      setLayout(new GridLayout(4, 1));

      //Heading cell
      panel = new JPanel(new FlowLayout(FlowLayout.LEFT));
      label = new JLabel(PROGRAM_NAME);
      label.setFont(new Font("Verdana", Font.BOLD, 18));
      panel.add(label);
      add(panel);

      //Weight cell
      panel = new JPanel(new FlowLayout(FlowLayout.LEFT));
      panel.add(new JLabel("Weight: "));
      panel.add(weightText);
      panel.add(new JLabel("pounds"));
      add(panel);

      //Button cell
      panel = new JPanel(new FlowLayout(FlowLayout.CENTER));
      JButton calculateButton = new JButton("Calculate");
      panel.add(calculateButton);
      add(panel);

      calculateButton.addActionListener(new CalcButtonListener());
   }

   private class CalcButtonListener implements ActionListener
   {
      public void actionPerformed(Action e)
      {
         double weight;
         double feet;
         double inches;
         double bmi;
         String result;

         //Weight error message
         try
         {
            weight = Double.parseDouble(weightText.getText());
         }
         catch(NumberFormatException ex)
         {
            JOptionPane.showMessageDialog(BMICalc.this,
               "Weight must be a number in pounds.",
               PROGRAM_NAME,
               JOptionPane.ERROR_MESSAGE);
            return;
         }
         //Height error message
         try
         {
         feet = Double.parseDouble(ftHeightText.getText());
         }
         catch(NumberFormatException ex)
         {
            JOptionPane.showMessageDialog(BMICalc.this,
               "Height must be a number in feet.",
               PROGRAM_NAME,
               JOptionPane.ERROR_MESSAGE);
            return;
         }
         //Height error message
         try
         {
         inches = Double.parseDouble(inHeightText.getText());
         }
         catch(NumberFormatException ex)
         {
            JOptionPane.showMessageDialog(BMICalc.this,
               "Height must be a number in inches.",
               PROGRAM_NAME,
               JOptionPane.ERROR_MESSAGE);
            return;
         }

      }
   }

错误：

BMICalc.java:56: 
error: BMICalc.CalcButtonListener is not abstract and does not override  
       abstract method actionPerformed(ActionEvent) in ActionListener
private class CalcButtonListener implements ActionListener
       ^

Answer 1

您的类中的函数应为public void actionPerformed(ActionEvent e)，以便它定义ActionListener中概述的抽象方法。

为了更直接地回答你的问题，编译器说类CalcButtonListener也应该是抽象的，因为它仍然有一个抽象方法来实现ActionListener。

Answer 2

抽象方法actionPerformed（ActionEvent）存在于ActionListener接口中。此方法的输入是ActionEvent而不是Action。

您实际上是使用Action而不是ActionEvent e覆盖该方法，因此错误。

为什么我会收到此错误？ BMICalc.CalcButtonListener不是抽象的

2 个答案: