Puzzle Game coding

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

import java.util.Random;
public class GameOfFifteen extends JPanel
{
private int size;
private int nbTiles;
private int dimension;
private static final Color FOREGROUND_COLOR=new Color(239,83,80);
private static final Random RANDOM=new Random();
private int[] tiles;
private int tileSize;
private int blankPos;
private int margin;
private int gridSize;
private boolean gameOver;
public GameOfFifteen(int size, int dimension, int margin)
{
super();
this.size = size;
this.dimension = dimension;
this.margin = margin;
nbTiles=size*size-1;
tiles=new int[size*size];
gridSize=(dimension-2*margin);
tileSize=gridSize/size;
setPreferredSize(new Dimension(dimension,dimension+margin));
setBackground(Color.WHITE);
setForeground(FOREGROUND_COLOR);
setFont(new Font("SansSerif",Font.BOLD,60));
gameOver=true;
addMouseListener(new MouseAdapter(){

public void mousePressed(MouseEvent e)
{

if(gameOver)
{
newGame();
}
else
{
int dir=0;
int ex=e.getX()-margin;
int ey=e.getY()-margin;
if(ex<0||ex>gridSize||ey<0||ey>gridSize)
return;
int c1=ex/tileSize;
int r1=ey/tileSize;
int c2=blankPos%size;
int r2=blankPos/size;
int clickPos=r1*size+c1;
if(c1==c2 && Math.abs(r1-r2)>0)

dir=(r1-r2)>0 ? size:-size;

else if(r1==r2 && Math.abs(c1-c2)>0)
dir=(c1-c2)>0? 1:-1;

if(dir!=0)
{
do {
int newBlankPos=blankPos+dir;
tiles[blankPos]=tiles[newBlankPos];
blankPos=newBlankPos;
}while(blankPos!=clickPos);
tiles[blankPos]=0;
}
gameOver=isSolved();
}
repaint();
}
});
newGame();

}
private void newGame() {
do {
reset();
shuffle();
}while(!isSolvable());
gameOver=false;
}
private void reset() {
for(int i=0;i<tiles.length;i++) {
tiles[i]=(i+1)%tiles.length;
}
blankPos=tiles.length-1;
}
private void shuffle()
{
int n=nbTiles;
while(n>1)
{
int r=RANDOM.nextInt(n--);
int tmp=tiles[r];
tiles[r]=tiles[n];
tiles[n]=tmp;
}
}
private boolean isSolvable()
{
int countInversions=0;
for(int i=0;i<nbTiles;i++)
{
for(int j=0;j<i;j++)
{
if(tiles[j]>tiles[i])
countInversions++;
}
}
return countInversions%2==0;
}
private boolean isSolved()
{
if(tiles[tiles.length-1]!=0)
return false;
for(int i=nbTiles-1;i>=0;i--)
{
if(tiles[i]!=i+1)
return false;
}
return true;
}
private void drawGrid(Graphics2D g)
{
for(int i=0;i<tiles.length;i++)
{
int r=i/size;
int c=i%size;
int x=margin+c*tileSize;
int y=margin+r*tileSize;
if(tiles[i]==0) {
if(gameOver) {
g.setColor(FOREGROUND_COLOR);
drawCenteredString(g,"|///",x,y);
}
continue;
}
g.setColor(getForeground());
g.fillRoundRect(x, y, tileSize, tileSize, 25, 25);
g.setColor(Color.BLACK);
g.drawRoundRect(x,y,tileSize,tileSize,25,25);
g.setColor(Color.WHITE);
drawCenteredString(g,String.valueOf(tiles[i]),x,y);
}
}
private void drawStartMessage(Graphics2D g) {
if(gameOver) {
g.setFont(getFont().deriveFont(Font.BOLD,18));
g.setColor(FOREGROUND_COLOR);
String s="Click to start new game";
g.drawString(s, (getWidth()-g.getFontMetrics().stringWidth(s))/2, getHeight()-margin);
}
}
private void drawCenteredString(Graphics2D g,String s,int x,int y)
{
FontMetrics fm=g.getFontMetrics();
int asc=fm.getAscent();
int desc=fm.getDescent();
g.drawString(s,x+(tileSize-fm.stringWidth(s))/2,y+(asc+(tileSize-(asc+desc))/2));
}
protected void paintComponent(Graphics g)
{
super.paintComponent(g);
Graphics2D g2D=(Graphics2D)g;
g2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
drawGrid(g2D);
drawStartMessage(g2D);
}
public static void main(String[] args)
{
SwingUtilities.invokeLater(() ->
{
JFrame frame=new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setTitle("Game of Fifteen");
frame.setResizable(false);
frame.add(new GameOfFifteen(4,550,30),BorderLayout.CENTER);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
});
}
}