"Exception in thread "main" java.lang.NullPointerException" help?

[redstripes]

As the title describes, I get an error when I run the "Memory" file. When I compile it nothing's wrong, but when I run the file it starts to complain. To be more specific it says:

Exception in thread "main" java.lang.NullPointerException
at Memory.<init>(Memory.java:49)
at Memory.newGame(Memory.java:353)
at Memory.main(Memory.java:348)

Any ideas on what may be wrong and how I can fix it?

1. Card

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;
 
public class Card extends JButton {
   // instance variable
   private Icon icon;
   private Status status;
 
   // constructors 
   public Card(Icon a) {
      icon = a;
      setStatus(Status.MISSING); // sets the cards background and icon 
   }
 
   public Card(Icon a, Status status) {
      icon = a;
      setStatus(status); 
   }
 
   // enum
   public enum Status  {
      HIDDEN, VISIBLE, MISSING;
   }
 
   // methods
   public void setStatus (Status status) {
      this.status = status;
      if (this.status == Status.HIDDEN) { 
         this.setBackground(Color.blue);
         this.setIcon(null);
      } else { 
         if (this.status == Status.MISSING) { 
            this.setBackground(Color.white);
            this.setIcon(null);
         } else {
            if (this.status == Status.VISIBLE) { 
               this.setBackground(Color.blue);
               this.setIcon(icon);
            }
           }
        } 
   }
 
   public Status getStatus () {
      return this.status; 
   } 
 
   public Card copy () {
      return new Card (icon, this.status);
   }
 
   public boolean equalIcon (Card a) {
      return icon == a.icon;
   }      
 
}

2. Tools

public class Tools {
	// adds object in an array in a random order
	public static void randomOrder(Object[] a) {
		Object[] temp = new Object[a.length];
		for (int i = 0; i < a.length; i=i+1) {
			temp[i] = a[i];
			a[i] = null;
		}
 
 
		int i = 0;
		while (i < a.length) {
 
			int f = (int) (Math.random()*a.length);
			if (a[f] == null) {
				a[f] = temp[i];
				i=i+1;
			}
		}
 
	}
 
}

3. Player

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Font;
 
import javax.swing.BorderFactory;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
 
 
// the class Player used to describe the game's players
public class Player extends JPanel {
 
	private String name;
	private int points;
	private boolean active;
	private JLabel nameLabel;
	private JLabel pointsLabel;
	private Font nameFont, pointsFont;
 
	// constructor for player - recieves names, adds labels and initializes points and activities
	public Player(String name) {
 
		setLayout(new BorderLayout());
 
		nameFont = new Font("Default", Font.BOLD, 10);
		pointsFont = new Font("Default", Font.BOLD, 25);
 
		this.name = name;
		nameLabel = new JLabel(name, JLabel.CENTER);
		nameLabel.setFont(nameFont);
 
		points = 0;
		pointsLabel = new JLabel(((Integer) points).toString(), JLabel.CENTER);
		pointsLabel.setFont(pointsFont);
 
		active = false;
 
		add(nameLabel, BorderLayout.NORTH);
		add(pointsLabel, BorderLayout.CENTER);
 
		setBackground(Color.GRAY);
		setBorder(new EmptyBorder(10,10,10,10));
		setBorder(BorderFactory.createLineBorder(Color.BLACK));		
	}
 
	// adds 1 to points and edits the text on its label
	public void addPoint() {
		points=points+1;
		pointsLabel.setText(((Integer) points).toString());
	}
 
	// returns the player's state
	public boolean getActivity() {
		return active;
	}
 
	// changes a player's state to the parameter and then changes the background
	public void setActivity(boolean t) {
		active = t;
		if (t) {
			setBackground(Color.YELLOW);
		}
 
		else {
			setBackground(Color.GRAY);
		}
	}
 
	// returns the player's name
	public String getName() {
		return name;
	}
 
	// returns the player's points
   	public int getPoints() {
		return points;
	}
 
	// resets the player's points
	public void resetPoints() {
		points = 0;
		pointsLabel.setText("  "+((Integer) points).toString());
	}
}

4. The memory game

import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.Timer;
 
public class Memory extends JFrame implements ActionListener {
	private Timer t;
 
	// game's size is size * size
	private int size;
 
	// all available cards
	private Card[] allCards;
 
	// all active cards
   private Card[] playCards; 
 
	// panels for cards and playerspanel
	private JPanel cardsPanel, playersPanel;
 
	// array holding the players
   private Player[] players;
 
	// menubars
	private JMenuBar menuBar;
	private JMenu menuGame, menuSettings;
	private JMenuItem nytt, sluta, spelareSetting, sizeSetting;
 
	// constructors for memory where the headframe and the menu initialise withe the available cards
	public Memory() {
		t = new Timer(1200, this);
 
		setTitle("Memory");
 
		// adds all the cards to mypictures in allCards
		File folder = new File("mypictures");
		File[] pictures = folder.listFiles();
 
		allCards = new Card[pictures.length];
 
		for(int i = 0; i < pictures.length; i=i+1) {
			ImageIcon icon = new ImageIcon(pictures[i].getPath());
			allCards[i] = new Card(icon, Card.Status.HIDDEN);
		}
 
		// placing all the graphic components
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setLayout(new BorderLayout());
 
		menuBar = new JMenuBar();
		setJMenuBar(menuBar);
 
		menuGame = new JMenu("Game");
		menuSettings = new JMenu("Settings");
 
		menuBar.add(menuGame);
		menuBar.add(menuSettings);
 
		nytt = new JMenuItem("New");
		menuGame.add(nytt);
 
		sluta = new JMenuItem("Stop");
		menuGame.add(sluta);
 
		spelareSetting = new JMenuItem("Players");
		menuSettings.add(spelareSetting);
 
		sizeSetting = new JMenuItem("Size");
		menuSettings.add(sizeSetting);
 
		nytt.addActionListener(this);
		sluta.addActionListener(this);
		sizeSetting.addActionListener(this);
		spelareSetting.addActionListener(this);
 
		pack();
		setSize(600, 600);
		setVisible(true);
 
		// sets players and game size
		setSize();
		setPlayers();
 
	}
 
	// opens a dialogue where you get to decide new game size
	public int newGameSize() {
		boolean exit = false;
		int sizet = 0;
 
		while (!exit) {
			sizet = Integer.parseInt(JOptionPane.showInputDialog(null, 
														"Difficulty? 4 or 6."));
			if (4 == sizet || sizet == 6) {
				exit = true;
			}
			else {
				JOptionPane.showMessageDialog(null, "error");
			}
		}
		return sizet;
 
	}
 
	// opens a dialogue where you get to decide the number of players
	public Player[] nrOfPlayers(){
 
		boolean exit = false;
		int sizet = 0;
 
		while (!exit) {
			sizet = Integer.parseInt(JOptionPane.showInputDialog(null,
												"Number of Players? 1 - 4"));
			if (1 <= sizet && sizet <= 4) {
				exit = true;
			}
			else {
				JOptionPane.showMessageDialog(null, "error");
			}
		}
 
		players = new Player[sizet];
 
		for(int i = 0; i < sizet; i=i+1){
			players[i] = new Player(JOptionPane.showInputDialog(null, "Name?"));
		}
 
		return players;
	}
 
	// sets new game size and places out all the cards in random order
	public void setSize() {
		size = newGameSize();
 
		if (cardsPanel != null) {
			cardsPanel.setVisible(false);
		}
 
		Tools.randomOrder(allCards);
		playCards = new Card[size*size];
 
		for (int i = 0; i < ((size*size)); i+=2) {
			playCards[i] = allCards[i];
			playCards[i+1] = allCards[i].copy();
		}
 
		Tools.randomOrder(playCards);
 
		cardsPanel = new JPanel();
		cardsPanel.setLayout(new GridLayout(size,size));
 
 
		for(int i = 0; i < playCards.length; i=i+1) {
			cardsPanel.add(playCards[i]);
			playCards[i].addActionListener(this);
		}
 
		add(cardsPanel, BorderLayout.CENTER);
		setVisible(true);
	}
 
	// sets number of players and places out labels for each player
   	public void setPlayers() {
		players = nrOfPlayers();
 
		if (playersPanel != null) {
			playersPanel.setVisible(false);
		}
 
		players[0].setActivity(true);
 
		playersPanel = new JPanel();
		playersPanel.setLayout(new BoxLayout(playersPanel, 
												BoxLayout.Y_AXIS));
		playersPanel.setSize(50, 600);
 
		for(int i = 0; i < players.length; i=i+1) {
			playersPanel.add(players[i]);
		}
 
		add(playersPanel, BorderLayout.WEST);
 
		setVisible(true);
	}
 
	// listener, listens for every action performed
	public void actionPerformed(ActionEvent e){
 
		// when the timer isn't running
		if (!t.isRunning()) {		
			// pressing the button "new"
			if ("New".equals(e.getActionCommand())) {
				dispose();
				newGame();
			}
 
			// pressing the button "stop"
         else if ("Stop".equals(e.getActionCommand())) {
				System.exit(0);
			}
 
			// pressing the button "size"
			else if ("Size".equals(e.getActionCommand())) {
 
				for(int i = 0; i < players.length; i=i+1) {
					players[i].resetPoints();
					players[i].setActivity(false);
				}
 
				for(int i = 0; i < playCards.length; i=i+1) {
					playCards[i].setStatus(Card.Status.HIDDEN);
				}
 
				players[0].setActivity(true);
 
				setSize();
 
				setVisible(false);
				setVisible(true);
			}
 
			// pressing the button "players"
         	else if ("Players".equals(e.getActionCommand())) {
 
				for(int i = 0; i < playCards.length; i=i+1) {
					playCards[i].setStatus(Card.Status.HIDDEN);
				}
 
				setPlayers();
				setVisible(false);
				setVisible(true);
			}
 
			// if you're done initializing players and cards
			else if (!(((Card) e.getSource()).getStatus() 
					                      == Card.Status.MISSING)) {
 
				int temp = 0;
 
				((Card) e.getSource()).setStatus(Card.Status.VISIBLE);
 
				for(int i = 0; i < size*size; i=i+1) {
					if (playCards[i].getStatus() == Card.Status.VISIBLE) {
						temp++;
					}	
				}
 
				// if there's more than 1 visible card the timer will set for an intervall of 1.5 seconds
				if (temp > 1){			
					t.start();								
				}					
			}
		}
 
		// if actionevent is timer t
		else if (e.getSource() == t) {
 
			t.stop();
 
			Card[] tempCard = new Card[2];
			int temp = 0;
			int f = 0;
 
			for(int i = 0; i < size*size; i=i+1){
 
				// copies visible cards
				if (playCards[i].getStatus() == Card.Status.VISIBLE) {
					tempCard[temp] = playCards[i];
					temp=temp+1;
				}
				// the variable f is added to decide when the game is over
				if (playCards[i].getStatus() == Card.Status.MISSING) {
					f=f+1;
				}	
			}
 
			// if the two visible cards are equal
			if (tempCard[0].equalIcon(tempCard[1])) {
 
				tempCard[0].setStatus(Card.Status.MISSING);
				tempCard[1].setStatus(Card.Status.MISSING);
 
				// adds 1 point to the player that is active
				for(int i = 0; i < players.length; i=i+1) {
 
					if (players[i].getActivity()) {
 
						players[i].addPoint();
 
					}
				}
			}
 
			// decides who's turn it is
			else {
 
				tempCard[0].setStatus(Card.Status.HIDDEN);
				tempCard[1].setStatus(Card.Status.HIDDEN);
 
				for(int i = 0; i < players.length; i=i+1) {
 
					if (players[i].getActivity()) {
 
						players[i].setActivity(false);
 
						if (i == players.length-1) {
							players[0].setActivity(true);
							break;
						}
						else {
							players[i+1].setActivity(true);
							break;
						}
					}
				}
			}
 
			// decides who the winner is
			if(f >= size*size-2) {
				Player winner = players[0];
 
				for(int i = 1; i < players.length; i=i+1) {
					if (winner.getPoints() < players[i].getPoints()) {
						winner = players[i];
					}
				}
 
				JOptionPane.showMessageDialog(null, winner.getName() + " won");
 
				dispose();
				newGame();
			}	
		}
	}
 
 
	public static void main(String[] args) {
		newGame();
	}
 
	// Creates a new Memory
	public static void newGame() {
		new Memory();
	}
}

[PhHein]

Something in line 49 of your Memory.java file is null.

[andbin]

Any ideas on what may be wrong and how I can fix it?

Line 49 is this:

allCards = new Card[pictures.length];

pictures may be null, it happens on I/O error or more typically if the File on which you invoke listFiles() is not a directory or doesn't exist.

[redstripes]

I still don't get how I can fix the problem?

[Norm]

First you need to find the variable with the null value. Add a println() statement that prints out the value of all the variables on line 49.
When you know what variable, then work of finding why it does not have a valid value.