Set Matrix Zeroes (Constant Space)

public void setZeroes(int[][] matrix) {
    int rownum = matrix.length;
    if (rownum == 0)  return;
    int colnum = matrix[0].length;
    if (colnum == 0)  return;

    boolean hasZeroFirstRow = false, hasZeroFirstColumn = false;

    // Does first row have zero?
    for (int j = 0; j < colnum; ++j) {
        if (matrix[0][j] == 0) {
            hasZeroFirstRow = true;
            break;
        }
    }

    // Does first column have zero?
    for (int i = 0; i < rownum; ++i) {
        if (matrix[i][0] == 0) {
            hasZeroFirstColumn = true;
            break;
        }
    }

    // find zeroes and store the info in first row and column
    for (int i = 1; i < matrix.length; ++i) {
        for (int j = 1; j < matrix[0].length; ++j) {
            if (matrix[i][j] == 0) {
                matrix[i][0] = 0;
                matrix[0][j] = 0;
            }
        }
    }

    // set zeroes except the first row and column
    for (int i = 1; i < matrix.length; ++i) {
        for (int j = 1; j < matrix[0].length; ++j) {
            if (matrix[i][0] == 0 || matrix[0][j] == 0)  matrix[i][j] = 0;
        }
    }

    // set zeroes for first row and column if needed
    if (hasZeroFirstRow) {
        for (int j = 0; j < colnum; ++j) {
            matrix[0][j] = 0;
        }
    }
    if (hasZeroFirstColumn) {
        for (int i = 0; i < rownum; ++i) {
            matrix[i][0] = 0;
        }
    }
}