Java Programming

I've been having some troubles trying to implement a program that generates sudoku problems. It takes a complete puzzle and pulls out the first number and tries to solve it with any of the 8 other numbers and if it can't then that means removing that variable completely will still keep the uniqueness of the solution. Problem is, I'm getting some stack overflow errors while implementing this and I have no idea why. My code is as follows, where the method solve is a program I've already written that can solve any sudoku problem.

public int[][] generate(int [][] board) throws java.io.IOException {
int oldValue;
int[][] tempBoard = new int[9][9];
for (int x = 0; x < 9; x++) {
for (int y = 0; y < 9; y++) {
tempBoard[x][y] = board[x][y];
}
}
for (int x = 0; x < 9; x++) {
for (int y = 0; y < 9; y++) {
oldValue = board[x][y];
for (int z = 1; z <= 9; z++) {
if (z != oldValue) {
board[x][y] = z;
tempBoard = solve(tempBoard);
if (tempBoard != null) {
board[x][y] = oldValue;
break;
}
else {
tempBoard = new int[9][9];
for (int a = 0; a < 9; a++) {
for (int b = 0; b < 9; b++) {
tempBoard[a] = board[a][b];
}
}
}
}
}
board[x][y] = 0;
for (int c = 0; c < 9; c++) {
for (int d = 0; d < 9; d++) {
tempBoard[c][d] = board[c][d];
}
}
}
}
return board;
}
}