Exception in thread “main” java.lang.NullPointerException but int is parsed in main method

Question

I get the following when I run the code, however I am calling solve(4) in the main method. So I am not sure why the null is there? Originally I had NQueensProblem q = new NQueensProblem(4); but that threw an error on compile. Any Guidance would be greatly appreciated.

Exception in thread "main" java.lang.NullPointerException at NQueensProblem.canPlace(NQueensProblem.java:60) at NQueensProblem.placeQueens(NQueensProblem.java:32) at NQueensProblem.solve(NQueensProblem.java:at at NQueensProblem.main(NQueensProblem.java:76)

public class NQueensProblem {
public int[][] solution;
public NQueensBT(int N) {
    solution = new int[N][N];
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            solution[i][j] = 0;
        }
    }
}
public void solve(int N) {
    if (placeQueens(0, N)) {
        //print the result
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < N; j++) {
                System.out.print(" " + solution[i][j]);
            }
            System.out.println();
        }
    } else {
        System.out.println("NO SOLUTION EXISTS");
    }
}
public boolean placeQueens(int queen, int N) {
    // will place the Queens one at a time, for column wise
    if (queen == N) {
        //if we are here that means we have solved the problem
        return true;
    }
    for (int row = 0; row < N; row++) {
        // check if queen can be placed row,col
        if (canPlace(solution, row, queen)) {
            // place the queen
            solution[row][queen] = 1;
            // solve for next queen
            if (placeQueens(queen + 1, N)) {
                return true;
            }
            //if we are here that means above placement didn't work
            //BACKTRACK
            solution[row][queen] = 0;
        }
    }
    //if we are here that means we haven't found solution
    return false;
}
// check if queen can be placed at matrix[row][column]
public boolean canPlace(int[][] matrix, int row, int column) {
    // since we are filling one column at a time,
    // we will check if no queen is placed in that particular row
    for (int i = 0; i < column; i++) {
        if (matrix[row][i] == 1) {
            return false;
        }
    }
    // we are filling one column at a time,so we need to check the upper and
    // diagonal as well
    // check upper diagonal
    for (int i = row, j = column; i >= 0 && j >= 0; i--, j--) {
        if (matrix[i][j] == 1) {
            return false;
        }
    }
    // check lower diagonal
    for (int i = row, j = column; i < matrix.length && j >= 0; i++, j--) {
        if (matrix[i][j] == 1) {
            return false;
        }
    }
    // if we are here that means we are safe to place Queen at row,column
    return true;
}
public static void main(String[] args) {
    int N = 4;
    NQueensProblem q = new NQueensProblem(N);
    q.solve(N);
}

}

Answer 1

If I understand the code and what's the issue here, try the following.

public class NQueensProblem {
public int[][] solution;
public NQueensProblem(int N) {
    solution = new int[N][N];
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            solution[i][j] = 0;
        }
    }
}
public void solve(int N) {
    if (placeQueens(0, N)) {
        //print the result
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < N; j++) {
                System.out.print(" " + solution[i][j]);
            }
            System.out.println();
        }
    } else {
        System.out.println("NO SOLUTION EXISTS");
    }
}
public boolean placeQueens(int queen, int N) {
    // will place the Queens one at a time, for column wise
    if (queen == N) {
        //if we are here that means we have solved the problem
        return true;
    }
    for (int row = 0; row < N; row++) {
        // check if queen can be placed row,col
        if (canPlace(solution, row, queen)) {
            // place the queen
            solution[row][queen] = 1;
            // solve for next queen
            if (placeQueens(queen + 1, N)) {
                return true;
            }
            //if we are here that means above placement didn't work
            //BACKTRACK
            solution[row][queen] = 0;
        }
    }
    //if we are here that means we haven't found solution
    return false;
}
// check if queen can be placed at matrix[row][column]
public boolean canPlace(int[][] matrix, int row, int column) {
    // since we are filling one column at a time,
    // we will check if no queen is placed in that particular row
    for (int i = 0; i < column; i++) {
        if (matrix[row][i] == 1) {
            return false;
        }
    }
    // we are filling one column at a time,so we need to check the upper and
    // diagonal as well
    // check upper diagonal
    for (int i = row, j = column; i >= 0 && j >= 0; i--, j--) {
        if (matrix[i][j] == 1) {
            return false;
        }
    }
    // check lower diagonal
    for (int i = row, j = column; i < matrix.length && j >= 0; i++, j--) {
        if (matrix[i][j] == 1) {
            return false;
        }
    }
    // if we are here that means we are safe to place Queen at row,column
    return true;
}
public static void main(String[] args) {
    int N = 4;
    NQueensProblem q = new NQueensProblem(N);
    q.solve(N);
}
}