Question

修改

我已清理.equals方法以获取字符串相等性，并将ContactsCollection初始化更改为：

public static ArrayList<Contact> contactList = new ArrayList<Contact>();

我还改变了动作执行方法，希望显示联系人＆＃39;会显示多个联系人。

    if (contactInput.equals("Display contacts"))
    {
        ContactsCollection.read();
        for (int i = 0; i < contactList.size(); i++)
        {
            contact = (Contact)contactList.get(i);
            for (int j =0; j < contactList.size(); j++)
            {
                textArea.append(contact.getName() + "," + contact.getNumber() + "\n");
            }
        }
    }

最终写入.dat文件，但不包含通过GUI添加的任何数据。

结束编辑

我正在编写一个模拟手机GUI，充当一个非常基本的联系人管理器。还有其他几个类没有处理正在按预期工作的ArrayList。

尝试向文件添加联系人时，我在ContactsCollection类的第13行收到空指针异常：

for (int i = 0; i < contactList.size(); i++)

联系人（GUI）类的

和第93行：

contactList.add(contact);

我感觉在编写Contacts和ContactsCollection类时我做错了。我希望程序运行如下：用户单击添加联系人并输入成为对象Contact的信息并添加到contactList ArrayList并写入（序列化）到文件＆＃34; contactList.dat＆＃ 34 ;.当用户单击显示联系人时，将读入文件并在GUI中显示每个联系人。

我认为我设置ArrayList的方式有几个问题，但我认为我非常接近让程序按照我的希望运行。非常感谢任何帮助！

联系人课程：

import java.util.*;
import java.io.*;

public class Contact implements Serializable
{
public static final long serialVersionUID = 42L;
public String name, number;

Contact()
{
    name = "No name";
    number = "No number";
}

Contact (String theName, String theNumber)
{
    this.name = theName;
    this.number = theNumber;
}

public void setName(String aName)
{
    this.name = aName;
}

public void setNumber(String aNumber)
{
    this.number =aNumber;
}

public String getName()
{
    return name;
}

public String getNumber()
{
    return number;
}

public String toString()
{
    return name + ": " + number;
}

public boolean equals(Contact other)
{
   if (name.equals(other.getName()) && number.equals(other.getNumber()))
   {
      return(true);
   }
   else
   {
      return(false);
   }
}   
}

联系人集合类

import java.io.*;
import java.util.*;

class ContactsCollection
{
public static ArrayList<Contact> contactList;

public static void write()
{
    try 
    {
        ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("contactList.dat"));
        for (int i = 0; i < contactList.size(); i++)
        {
            out.writeObject(contactList.get(i));
        }
        out.close();
    } 
    catch(IOException e)
    {
        e.printStackTrace();
    }
}

public static void read()
{
contactList = new ArrayList<Contact>();
try
{
    ObjectInputStream in = new ObjectInputStream(new FileInputStream("contactList.dat"));
    Contact temp;
    while (in.available()!=0)
    {
        temp = (Contact)in.readObject();
        contactList.add(temp);
    }
    in.close();
}
catch(FileNotFoundException e)
{
    e.printStackTrace();
}
catch(IOException e)
{
    e.printStackTrace();
}
catch(ClassNotFoundException e)
{
    e.printStackTrace();
}       

}
}

联系人（GUI）类

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

class Contacts extends JFrame implements ActionListener, WindowListener
{
public static final int WIDTH = 400;
    public static final int HEIGHT = 600;
    public static final int SMALL_WIDTH = 200;
    public static final int SMALL_HEIGHT = 100;

private static final Dimension stdBtn = new Dimension(150, 50);    

    JPanel centerPanel, northPanel, southPanel;
ImageIcon icon;
JLabel picture;
JButton addContact, displayContacts;
JScrollPane scroll;
JTextArea textArea;
Clock clock;
Background background;
Contact contact;
ArrayList<Contact> contactList;


public Contacts()
{
    super("Contacts");
    this.setLayout(new BorderLayout());
    this.setSize(WIDTH, HEIGHT);
    this.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
    addWindowListener(this);
    this.setLocationRelativeTo(null);

    centerPanel = new JPanel();
    northPanel = new JPanel();
    southPanel = new JPanel();

    centerPanel.setBackground(Color.BLACK);
    southPanel.setBackground(Color.BLACK);      

    clock = new Clock();
    northPanel.add(clock);

    icon = new ImageIcon("ContactsBackground.jpg");
    picture = new JLabel(icon);
    centerPanel.add(picture);

    textArea = new JTextArea("", 10, 30);
    textArea.setEditable(false);
    JScrollPane scroll = new JScrollPane(textArea, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
            JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);    
    centerPanel.add(scroll);    

    JButton displayContacts = new JButton("Display contacts");
    displayContacts.addActionListener(this);
    southPanel.add(displayContacts);

    JButton addContact = new JButton("Add contact");
    addContact.addActionListener(this);
    southPanel.add(addContact);

    this.add(northPanel, BorderLayout.NORTH);
    this.add(centerPanel, BorderLayout.CENTER);
    this.add(southPanel, BorderLayout.SOUTH);       

    setResizable(false);        
}

public void actionPerformed(ActionEvent e)
{
    contactList = new ArrayList<Contact>();
    JButton source = (JButton)e.getSource();
    String contactInput = source.getText();

    if (contactInput == "Display contacts")
    {
        ContactsCollection.read();
        for (int i = 0; i < contactList.size(); i++)
        {
            contact = (Contact)contactList.get(i);
            textArea.setText(contact.getName() + "," + contact.getNumber() + "\n");
        }
    }
    if (contactInput == "Add contact")
    {
        String name = JOptionPane.showInputDialog(null, "Enter Name");
        String number = JOptionPane.showInputDialog(null, "Enter Number");
        contact = new Contact(name, number);
        contactList.add(contact);
        ContactsCollection.write();
    }
}

public void windowOpened(WindowEvent e)
{}

public void windowClosing(WindowEvent e)
{
    this.setVisible(false);
    background = new Background();
    background.setVisible(true);        
}

public void windowClosed(WindowEvent e)
{}

public void windowIconified(WindowEvent e)
{}

public void windowDeiconified(WindowEvent e)
{}

public void windowActivated(WindowEvent e)
{}

public void windowDeactivated(WindowEvent e)
{}      
}

Answer 1

更改

public static ArrayList<Contact> contactList;

到

public static ArrayList<Contact> contactList = new ArrayList<>();

您的contactList版本中的

为null，因为您从未对其进行初始化，而是在write()方法中尝试在其上调用size()。

此外，您的GUI类中存在一些严重缺陷（在actionPerformed()方法中），请阅读此问题以修复它们：How do I compare strings in Java?

另请注意textArea.setText(...)将为textArea设置完整的文本，因为这是代码中的循环，textArea将包含上一次迭代的输出那个循环只。在您的情况下，它将只是最后一次联系。

Java ArrayList空指针异常

1 个答案: