I need a "bit" of help with an 8 Puzzle

Question

My current situation is that I need to create a working 8 puzzle with Java using a GUI. What I've currently done is at http://nale.f2g.net/EightPuzzle.java . My main problem is that when running it I get a NullPointerException at the line containing tile[blankPos].setText(tile[ndx].getText()); in the swap method.

Answer 1

I get a NPE when ever I use the scramble() method.
The source is the line move(rand.nextInt(4)); .Your Random rand is not instantiated nowhere. Thus it is naturally null.
Make rand in the constructor.
Here's a simpler one. You could use random swap() calls for moving/shuffling/scrambling.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
import java.util.Random;
public class ObjectOrientedEightPuzzle extends JFrame implements MouseListener{
  private Piece[] tile;
  private Border etched = BorderFactory.createEtchedBorder();
  private Color tileColor = Color.GRAY;
  private Color blankColor = Color.GREEN;
  private String[] label = {"1", "2", "3", "4", "", "5", "6", "7", "8"};
  private JPanel pane;
  public ObjectOrientedEightPuzzle(){
    setTitle("ObjectOrientedEightPuzzle");
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    pane = (JPanel)getContentPane();
    pane.setLayout(new GridLayout(3,3));
    setResizable(false);
    setSize(400, 400);
    tile = new Piece[label.length];
    for(int ndx = 0; ndx < label.length; ndx++){
      tile[ndx] = new Piece(label[ndx], new Position(ndx));
      tile[ndx].addMouseListener(this);
      pane.add(tile[ndx]);
  public void mouseClicked(MouseEvent e){
    Piece p = (Piece)e.getSource();
    Piece blank = blankPos();
    swap(p, blank);
  public void mousePressed(MouseEvent e){
  public void mouseReleased(MouseEvent e){
  public void mouseEntered(MouseEvent e){
  public void mouseExited(MouseEvent e){
  private void swap(Piece clickp, Piece blankp){
    int i = -1;
    int[] pa;
    int b = blankp.getPosition().getPos();
    pa = clickp.getPosition().getNeighbor();
    for (int p = 0; p < pa.length; ++p){
      if (pa[p] == b){
        i = p;
        break;
    if (i != -1){
      blankp.setText(clickp.getText());
      clickp.setText("");
  private Piece blankPos(){
    int ndx;
    for(ndx = 0; ndx < label.length; ndx++){
      if(tile[ndx].getText().equals("")){
        break;
    return tile[ndx];
  public static void main(String[] args){
    ObjectOrientedEightPuzzle puzzle = new ObjectOrientedEightPuzzle();
    puzzle.setVisible(true);
  // represent the position in the array/grid
  class Position{ // 0 ... 8
    int pos;
    int[] neighbor;
    public Position(int p){
      setPos(p);
    public int getPos(){
      return pos;
    public void setPos(int p){
      pos = p;
      setNeighbor(pos);
    public int[] getNeighbor(){
      return neighbor;
    private void setNeighbor(int p){
      switch (p){
        case 0:
          neighbor = new int[2];
          neighbor[0] = 1;
          neighbor[1] = 3;
          break;
        case 1:
          neighbor = new int[3];
          neighbor[0] = 0;
          neighbor[1] = 2;
          neighbor[2] = 4;
          break;
        case 2:
          neighbor = new int[2];
          neighbor[0] = 1;
          neighbor[1] = 5;
          break;
        case 3:
          neighbor = new int[3];
          neighbor[0] = 0;
          neighbor[1] = 4;
          neighbor[2] = 6;
          break;
        case 4:
          neighbor = new int[4];
          neighbor[0] = 1;
          neighbor[1] = 3;
          neighbor[2] = 5;
          neighbor[3] = 7;
          break;
        case 5:
          neighbor = new int[3];
          neighbor[0] = 2;
          neighbor[1] = 4;
          neighbor[2] = 8;
          break;
        case 6:
          neighbor = new int[2];
          neighbor[0] = 3;
          neighbor[1] = 7;
          break;
        case 7:
          neighbor = new int[3];
          neighbor[0] = 4;
          neighbor[1] = 6;
          neighbor[2] = 8;
          break;
        case 8:
          neighbor = new int[2];
          neighbor[0] = 5;
          neighbor[1] = 7;
          break;
        default:
          throw new IllegalArgumentException(String.valueOf(p));
  // represent the piece of the game == a JLabel disguised
  class Piece extends JLabel{
    private int value;
    private Position pos;
    private String ptext;
    public Piece(String s, Position p){
      super(s);
      setBorder(etched);
      setVerticalAlignment(CENTER);
      setHorizontalAlignment(CENTER);
      setFont(new Font("Courier", Font.BOLD, 30));
      setOpaque(true);
      ptext = s;
      pos = p;
      if (s.equals("")){
        setBackground(blankColor);
      else{
        setBackground(tileColor);
      try{
        value = Integer.parseInt(s);
      catch (NumberFormatException e){
        value = -1;
    public int getValue(){
      return value;
    public Position getPosition(){
      return pos;
    public String getText(){
      return ptext;
    public void setText(String t){
      ptext = t;
      if (t.equals("")){
        setBackground(blankColor);
      else{
        setBackground(tileColor);
      super.setText(t);
    public void setValue(int val){
      value = val;
      ptext = String.valueOf(val);
      if (val < 0){
        ptext = "";
      setText(ptext);
      if (value > 0){
        setBackground(tileColor);
      else{
        setBackground(blankColor);
}