java小程序，编译通过了，运行时提示nullPointerException。请大神解答

Question

public class StringToDouble {
public static void main(String []args) {
String s = "1,2;3,4,5;6,7,8";
double[][] array =StringToDouble.f(s);

  for (int i =0;i<array.length;i++) {
     for (int j = 0;j<array[i].length;j++) {
        System.out.println(array[i][j]);
     }
  }

}

public static double[][] f(String s) {
String [] s1 = s.split(";");

double[][] d = new double [s1.length][];

  for (int i = 0;i<s1.length;i++) {
     String [] s2 = s1[i].split(",");

     for (int j = 0;j<s2.length;j++) {
        d[i][j] = Double.parseDouble(s2[j]);

     }
  }

  return d;

}
}

将一个字符串转换为double型二维数组，编译通过了，运行的时候提示nullPointerException。java提示错误发生在第4行和第21行。

Answer 1

 String [] s2 = s1[i].split(",");
后面加上
d[i] = new double[s2.length];