编辑
我已经清理了字符串相等的 .equals 方法,并将 ContactsCollection 初始化更改为:
public static ArrayList<Contact> contactList = new ArrayList<Contact>();
我还更改了执行操作的方法,希望“显示联系人”能够显示多个联系人。
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,它充当非常基本的联系人管理器。还有其他几个不处理 ArrayList 的类,它们按预期工作。
当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++)
以及 Contacts (GUI) 类的第 93 行:
contactList.add(contact);
我有一种感觉,在编写 Contacts 和 ContactsCollection 类时我做错了什么。我希望程序按如下方式运行:用户单击添加联系人并输入信息,该信息成为对象联系人,并添加到 contactList ArrayList 并写入(序列化)到文件“contactList.dat”。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 的方式存在几个问题,但我认为我已经非常接近让程序按照我的希望运行了。非常感谢任何帮助!
联系人类别:
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);
}
}
}
通讯录集合类
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)
{}
}
最佳答案
改变
public static ArrayList<Contact> contactList;
至
public static ArrayList<Contact> contactList = new ArrayList<>();
在您的版本中 contactList
为 null
因为您从未初始化它并且在您尝试调用的 write()
方法中 size()
就可以了。
此外,您的 GUI 类中存在一些严重缺陷(在 actionPerformed()
方法中),请阅读此问题来修复它们:How do I compare strings in Java?
还要记住 textArea.setText(...)
将为 textArea
设置完整的文本,因此因为这是在代码中的循环中,所以 textArea
将仅包含该循环最后一次迭代的输出。就您而言,这只是最后一次联系。
关于Java ArrayList 空指针异常,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/16557292/