Java 项目可能因循环错误而崩溃

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

这是我去年用 BlueJ 开始的项目之一，但遇到了问题，所以没有完成。现在我回过头来，确实需要帮助。

这是我的 GUI 类:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
/**
 * 
 * @author TANVEER AHMED
 */
public class HouseAdmin
{
    private JFrame frame = new JFrame();
    private JMenuBar menuBar  = new JMenuBar();
    private JMenu fileMenu = new JMenu("Menu");
    private JMenuItem listAllAppliances;
    private JMenuItem turnOnSingleAppliance;
    private JMenuItem switchOnAllLights;
    private BorderEventListener listener;//this class the inner class to handle the events
    private JTextArea textArea;
    private String printHouse;
    private House myHome;


    /**
     * Constructor for objects of class HouseAdmin
     */
    public HouseAdmin()
    {

        menuBar = new JMenuBar();

       listAllAppliances = new JMenuItem("List All appliances...");
       turnOnSingleAppliance = new JMenuItem("Turn on a single appliance...");
       switchOnAllLights = new JMenuItem("Switch On All Lights...");

       fileMenu.add(listAllAppliances);
       fileMenu.add(turnOnSingleAppliance);
       fileMenu.add(switchOnAllLights);

       menuBar.add(fileMenu);


       textArea = new JTextArea(30,70);
       frame.add(textArea, BorderLayout.CENTER);

       frame.setJMenuBar(menuBar);
       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       frame.setVisible (true);
       frame.pack();

       listener = new BorderEventListener();
       listAllAppliances.addActionListener(listener);
       turnOnSingleAppliance.addActionListener(listener);
       switchOnAllLights.addActionListener(listener);
      setupHouse();



    }



    private void setupHouse(){
        //Sets up a house
        House myHome = new House("Hatfield");

        Light bulb1 = new Light(1,"Bed Light",50,"H");
        Light bulb2 = new Light(2,"Kitchen Light",5,"L");
        Light bulb3 = new Light(3,"Garden Light",30,"E");
        Light bulb4 = new Light(4,"Bathroom Light",50,"H");
        Light bulb5 = new Light(5,"Shower Light",5,"L");
        Light bulb6 = new Light(6,"Solar Light",30,"E");
        Light bulb7 = new Light(7,"Passage Light",5,"L");
        Light bulb8 = new Light(8,"Storage Light",50,"H");
        Light bulb9 = new Light(9,"Garage Light",30,"H");
        Light bulb10 = new Light(10,"Living room Light",30,"E");

        myHome.addAppliance(bulb1);
        myHome.addAppliance(bulb2);
        myHome.addAppliance(bulb3);
        myHome.addAppliance(bulb4);
        myHome.addAppliance(bulb5);
        myHome.addAppliance(bulb6);
        myHome.addAppliance(bulb7);
        myHome.addAppliance(bulb8);
        myHome.addAppliance(bulb9);
        myHome.addAppliance(bulb10);


        printHouse = myHome.toString();

        //textArea.setText(myHome.toString());


    }


    class BorderEventListener implements ActionListener{

        public void actionPerformed (ActionEvent e){

             if(e.getSource() == listAllAppliances){
                textArea.setText(printHouse);

           }

            if (e.getSource () == turnOnSingleAppliance){
                String code  = JOptionPane.showInputDialog("Please Enter Application Code");
                int code2 = Integer.parseInt(code);

                myHome.turnOnASingleAppliance(code2);
                textArea.append("Appliance " + code2 + " Has been turned on!");
                textArea.setText(printHouse);

            }

            if (e.getSource () == switchOnAllLights){
                myHome.turnAllLightsOn();

                textArea.setText(printHouse);
                textArea.append("All lights have been turned on!");

            }

        }
    }

}

http://pastebin.com/dgLwaHSF

您可以看到我在第 105 行调用了一个名为“turnOnSingleAppliance”的方法。现在崩溃并向我抛出终端窗口。

这是该方法的代码:

import java.util.*;
/**
 *
 * @author TANVEER AHMED
 */
public class House {
 private String address;
 private int powerConsumption;
 private ArrayList <ElectricalAppliance> HouseAppliances;
 public House(String address) {
  // initialise instance variables
  this.address = address;
  HouseAppliances = new ArrayList <ElectricalAppliance>();

 }


public ElectricalAppliance turnOnASingleAppliance(int code){
        for (int i = 0; i < HouseAppliances.size(); i++){
            ElectricalAppliance app = HouseAppliances.get(i);
            if (app.getCode() == code ){
                app.lightSwitchOn();
            }
        }
        return null;
    }
}

http://pastebin.com/8krvvVHW

也许你们需要查看更多来源？如果是的话我可以上传整个bluej项目。

我的直觉告诉我循环是错误的。大家觉得怎么样？

Answer 1

更改此行

House myHome = new House("Hatfield");

至此

myHome = new House("Hatfield");

您没有在第一个实例中定义该字段，因此您的错误可能出现在事件监听器内的这一行。

myHome.turnOnASingleAppliance(code2);

您所做的是一个简单的错误，称为“变量遮蔽”，这意味着您正在定义与字段同名的局部变量。