Looping and Streams - how to do it in this case?

Question

Given:

@Entity
public class Pixel {
    @Id
    Point position;
    String color;

I store pixels like this:

for (int row = 0; row < 20; row++) {
    for (int col = 0; col < 20; col++) {
        repository.save(new Pixel(new Point(row, col), "w3-flat-turquoise"));
    }
}

This represents a square of 20x20

Now I need to display that list using layouts.

I can display easily a square like this (working):

Now I would like to retrieve the pixels from the database and display them... but I can't find a proper way.

So far, I have this (not working because final 'i' can't be change):

VerticalLayout lines = new VerticalLayout();
HorizontalLayout line;
int width = 20;
List<Pixel> pixels = pixelRepository.findAll();

for(final int i=0;i<width;++i) {
    line = new HorizontalLayout();
    Stream<Pixel> pixelLine = pixels.stream().filter(pixel -> pixel.getPosition().x == i);
    for(int j=0;j<width;++j) {
        addNewColorButton(line, pixelLine.filter(pixel -> pixel.getPosition().y == j).findFirst().get().color);
    }
    lines.add(line);
}
add(lines);

What would be a proper algorithm to do it ?

Working solution with final variables:

VerticalLayout lines = new VerticalLayout();
HorizontalLayout line;
int width = 20;
List<Pixel> pixels = pixelRepository.findAll();

for(int i=0;i<width;++i) {
    int finalI = i;
    line = new HorizontalLayout();
    Supplier<Stream<Pixel>> pixelLine = () -> pixels.stream().filter(pixel -> pixel.getPosition().x == finalI);
    for(int j=0;j<width;++j) {
        int finalJ = j;
        addNewColorButton(line, pixelLine.get().filter(pixel -> pixel.getPosition().y == finalJ).findFirst().get().color);
    }
    lines.add(line);
}
add(lines);

Warning: `findFirst().get()` might result in an exception if `findFirst` returns `Optional.empty` — Thiyagu, Dec 12 '18 at 04:42

score 1 · Accepted Answer · answered Dec 12 '18 at 04:27

1

You can just create a final object from the i that changes i's value. finalI value is final and never changes.

int finalI = i;
int finalJ = j;

and here is your code after adding the final objects

VerticalLayout lines = new VerticalLayout();
HorizontalLayout line;
int width = 20;
List<Pixel> pixels = pixelRepository.findAll();

for (int i = 0; i < width; ++i) {
    int finalI = i;
    line = new HorizontalLayout();
    Stream<Pixel> pixelLine = pixels.stream().filter(pixel -> pixel.getPosition().x == finalI);
    for (int j = 0; j < width; ++j) {
        int finalJ = j;
        addNewColorButton(line, pixelLine.filter(pixel -> pixel.getPosition().y == finalJ).findFirst().get().color);
    }
    lines.add(line);
}
add(lines);

answered Dec 12 '18 at 04:27

SamHoque

2,978
2
13
43

It gives the error "stream has already been operated upon or closed" on the 'addNewColorButton' line. that odd, because filter is not supposed to consume the stream... – Tyvain Dec 12 '18 at 04:51
@Tyvain Did you close the Stream? – SamHoque Dec 12 '18 at 04:52
https://stackoverflow.com/questions/23860533/copy-a-stream-to-avoid-stream-has-already-been-operated-upon-or-closed – SamHoque Dec 12 '18 at 04:53

score 0 · Answer 2 · answered Dec 12 '18 at 04:26

0

for(final int i=0;i<width;++i) is pretty bogus for obvious reasons.

If you need to get your i into the lambda you can transform it to an array, which will be treated as effectively final by the compiler like this:

int[] wrapped_i = new int[]{i}

and access it from within the lambda with wrapped_i[0]. This approach is somewhat error-prone and no cure-all, especially when you're using parallel streams (which sorta-kinda is the whole selling point).

answered Dec 12 '18 at 04:26

nitowa

1,079
2
9
21

1

Array's look ugly if used in this way IMO – SamHoque Dec 12 '18 at 04:28
That's why I said `IMO` – SamHoque Dec 12 '18 at 04:31

Looping and Streams - how to do it in this case?

2 Answers2