Java ArrayList空指针异常

Question

EDIT

I've cleaned up the .equals method for string equality and changed the ContactsCollection initialization to:

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

I've also changed the action performed method in hopes that 'Display contacts' would show more than one contact.

    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");
            }
        }
    }

Ultimately the .dat file is written but does not contain any data that is added through the GUI.

END EDIT

I am writing a mock cellphone GUI that acts as a very basic contacts manager. There are several other classes that do not deal with the ArrayList that are working as intended.

When attempting to add a contact to the file I receive a null pointer exception at line 13 of the ContactsCollection class:

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

and line 93 of the Contacts (GUI) class:

contactList.add(contact);

I have a feeling that I did something wrong when coding the Contacts and ContactsCollection classes. I'm hoping the program to run as follows: The user clicks add contact and enters the information which becomes an object Contact and is added to the contactList ArrayList and written (serialized) to a file "contactList.dat". When the user clicks display contacts the file is read in and each contact is displayed in GUI.

I think that there are several issues with the way I set up the ArrayList, but I think I'm very close to having the program run as I had hoped. Any help is greatly appreciated!

Contacts class:

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);
   }
}   
}

Contacts Collection class

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();
}       

}
}

Contacts (GUI) class

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

Change

public static ArrayList<Contact> contactList;

to

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

in your version contactList is null because you never initialize it and in the write() method you are trying to call size() on it.

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Also, there are some serious flaws in your GUI class (in the actionPerformed() method), read this question to fix them: How do I compare strings in Java?

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Also remember that textArea.setText(...) will set the complete text for the textArea , so because this is in a loop in your code, the textArea will contain the output of the last iteration of that loop only. In your case it will be only the last contact.