int[][] mat;
// for each row, allocate a slot for a pointer to an array
mat = new int [100][];
for ( int i=0; i<100; i++ )
   {
   // allocate an array for each row
   mat[i] = new int [i+1];
   for ( int j=0; j<=i; j++ )
      {
      mat[i][j] = i * j + 100;
      }
   }