help help, ... :) "game of life"

i had a home work about whats called "Game of Life"

i had to write 4 functions as i did below:

function 1:

Code :

---

	public static boolean isInside(boolean[][] cells, int x, int y) {
 
		if ((cells.length < x) && (cells[0].length < y))
			return false;
 
		return true;
 
	}

---

function 2:

Code :

---

	public static boolean checkCell(boolean[][] cells, int x, int y) {
 
		if (cells[x][y] == true)
			return true;
 
		return false;
 
	}

---

function 3:

Code :

---

	public static int numberOfNeighbors(boolean[][] cells,int x,int y) {
 
		int count=0,j=0,i=1,marker=0;
 
		if (isInside(cells,i,j))
		{
			while (marker < 3)
			{
				if ((checkCell(cells,x+j,y)) && (cells[x+j][y] != cells[x][y]))
					count++;
				if (checkCell(cells,x+j,y+i))
					count++;
				if (checkCell(cells,x+j,y-i))
					count++;
 
				j=1;
 
				if (marker == 2)
					j=-1;
 
				marker++;
			}
		}
 
		return count;
	}

---

and the important function 4:

Code :

---

	public static boolean[][] nextGeneration(boolean[][] cells) {
 
 
		for (int i=0;i<cells.length;i++)
			for (int j=0;j<cells[0].length;j++)
				if (isInside(cells,i,j))
				{
					if (numberOfNeighbors(cells,i,j) == 3)
					{
						cells[i][j] = true;
					}
					if ((checkCell(cells,i,j)) && ((numberOfNeighbors(cells,i,j) == 3) || (numberOfNeighbors(cells,i,j) == 2)))
					{
						cells[i][j] = true;
					}
					else
					{
						cells[i][j] = false;
					}
				}
 
		return cells;
	}

---

,
,

i have to save this file and copy the *.class file to a directory with the prepared files:

GameOfLife.html,CellSpace.class,GameOfLife.class.

the problem is when i copy the file and i run the GameOfLife.html.

which opens this window:

http://www.ariel.ac.il/cs/pf/bmboaz/...ameOfLife.html

and works just above.

but when i put on 3 squares and press "start"

the error i get is:

Code :

---

Exception in thread "Thread-11" java.lang.ArrayIndexOutOfBoundsException: -1
	at NextGeneration.checkCell(NextGeneration.java:19)
	at NextGeneration.numberOfNeighbors(NextGeneration.java:38)
	at NextGeneration.nextGeneration(NextGeneration.java:60)
	at CellSpace.next(CellSpace.java:102)
	at GameOfLife.run(GameOfLife.java:96)
	at java.lang.Thread.run(Unknown Source)

---

line 19 = if (cells[x][y] == true)

line 38 = if (checkCell(cells,x+j,y-i))

line 60 = if (numberOfNeighbors(cells,i,j) == 3)

You should safeguard against the edges of the grid. For example, if you are at the first cell [0][0]. Then for line 38:
line 38 = if (checkCell(cells,x+j,y-i))
you will end up checking cell [0][-1], which is out of bounds.

so you mean i have to check if the x,y already on the board
everytime i call for this function:

before:

Code :

---

	public static boolean checkCell(boolean[][] cells, int x, int y) {
 
			if (cells[x][y] == true)
				return true;
 
 
		return false;
 
	}

---

after:

Code :

---

	public static boolean checkCell(boolean[][] cells, int x, int y) {
 
 
		if (isInside(cells,x,y))
		{
			if (cells[x][y] == true)
				return true;
 
		}
		return false;
 
	}

---

Yes, but something even more fundamental: examine the isInside function, what if x = 0 and y = -1? You need to safeguard against values greater than the size of the grid, but also less than the size (eg < 0 ). Also, you should safeguard against just x > cells.length or y > cells[0].length