Project Euler "Largest product in a grid" (#11) in Java 8

I have come up with a Java 8 solution for the following problem:


In the 20×20 grid below, four numbers along a diagonal line have been marked in red (bold here).


08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08

49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00

81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65

52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91

22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80

24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50

32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70

67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21

24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72

21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95

78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92

16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57

86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58

19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40

04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66

88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69

04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36

20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16

20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54

01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48


The product of these numbers is 26 × 63 × 78 × 14 = 1788696.


What is the greatest product of four adjacent numbers in the same direction (up, down, left, right, or diagonally) in the 20×20 grid?

I would comments on everything:

```
public class Problem11 extends Problem {
/** The n of the n x n grid. */
private final int n;

/** The grid. */
private final Grid grid;

/**
* Constructs this class.
*
* @param n The n of the n x n grid
* @param gridString The String representation of the grid
*

Your code looks great and I can see that you have put some thought and time into it. Also, you've used pretty fancy concepts that I didn't know (which is not so hard as my Java is pretty rusty). However, it looks slightly over-engineered to me so I'll try to make things more simple.

• Don't mess with anyone's brain

IntBinaryOperator sumOperator = (x, y) -> x * y; : calling sum a product is one of the most confusing thing you could possibly do :-).

• Do not repeat yourself - Keep it simple, stupid

In many places, things could have been done in a more straightforward way.

• First example : addIfNotEmpty

You don't need to have two return doing the same thing in :

private List addIfNotEmpty(final List list, final OptionalInt optionalInt) {
    if (!optionalInt.isPresent()) {
        return list;
    }
    list.add(optionalInt.getAsInt());
    return list;
}

It could easily be written :

private List addIfNotEmpty(final List list, final OptionalInt optionalInt) {
    if (optionalInt.isPresent()) {
        list.add(optionalInt.getAsInt());
    }
    return list;
}

• Second example : n

Your Problem11 class has a private final int n and a Grid.
Your Grid has a private final int n; and an Array.
An Array has a length attribute.

Do you see the pattern here ? Good news is that you are considering squares because if we were to have 2 dimensions here, it would probably be a mess.

This shows some kind of problem with your design. Maybe you should rely on the length attribute whenever you need to, maybe you shouldn't even need to (cf Leaky Abstraction).

• Third example : Cell

Your Cell is a structure containing a value and its coordinates in an array of Cells. This reminds me of one of the examples in the "Stop Writing Classes " presentation and I have the feeling that we don't really need it.

I have no time to go on right now. I'll try to edit my answers and provide a working piece of code. In the meantime, as you have solved the Project Euler already, I suggest you have a look at the solutions posted on the boards. They are a great way to learn about algorithms, math and programming style as well.

StackExchange Code Review Q#42744, answer score: 4