Stream.filter(...).count() didn't work [solved]

Newbie & Debugging Questions
#1

Hey, I was creating a replication of a classic game called Minesweeper.
I’m using Java 8 and Stream class to find the number of cells with mine around a cell.

Here’s a bit of the code :-


		public void reveal() {
			this.revealed = true;
			
			// Reveal current cell
			removeAll();
			this.value = (hasMine) ?  Res.MINE_ICON : Res.EMPTY_LABEL;
			add(this.value);
			setBackground(Color.WHITE);
			repaint();
			revalidate();
			
			if(hasMine(this)) return;  // Don't need to proceed since you clicked on a mine!
			
			// Reveal neighbors
			final List<Object> neighbors = Arrays.asList(grid.getNeighborsAt(row,col));
			neighbors.forEach(obj -> {
				CellPanel cell = (CellPanel) obj;
				if(!hasMine(cell) && !cell.hasRevealed())
					cell.reveal();
			});
			
			Stream<Object> mineCells = neighbors.stream().filter(CellPanel::hasMine);
			int totalMinesNearby = (int) (mineCells.count());
			System.out.println(totalMinesNearby + " mines nearby " + this);
			System.out.println(mineCells + " nearby " + this);
			setValue(totalMinesNearby);
		}

I tried that hasMine with the inverse (Exclamation mark) and it worked out just fine.
May I know what’s the problem? :slight_smile: :slight_smile:

#2

Check if the neighbors list of empty, if it isn’t, it’s that no neighbors satisfy the condition in the filter (hasMine).

#3

Hey, thank you.

I just found out that I just need to tweak this part a little bit :-


		public Object[] getNeighborsAt(final int row,final int col){
			final Stream<CellPanel> neighbors = cells.stream().filter(cell -> 
				(cell.getRow() == row - 1 && (cell.getCol() >= col - 1 && cell.getCol() <= col + 1) ||
				cell.getRow() == row + 1 && (cell.getCol() >= col - 1 && cell.getCol() <= col + 1) ||
				cell.getCol() == col - 1 && (cell.getRow() >= row - 1 && cell.getRow() <= row + 1) ||
				cell.getCol() == col + 1 && (cell.getRow() >= row - 1 && cell.getRow() <= row + 1)) 
				// && !CellPanel.hasMine(cell) // This part is the problem  
			);
			return neighbors.toArray();
		}
#4

Hey

I advise you not to create a list from an array just to use it as a stream. Rather use a stream builder to build a stream or use map() and filter() directly on the stream build from grid.getNeighborsAt(row,col). You lose some performance enhancements of streams when you create lots of temporary structures to build them.

#5

Use [icode]Arrays.stream[/icode] instead. It will become


Arrays.stream(grid.getNeighborsAt(row, col))
      .filter(CellPanel::hasMine)
      .count();

By the way, why convert the list to array in the [icode]getNeighborsAt()[/icode] method in the first place?
Use a list directly there, and use collector to collect the stream contents.


public List<CellPanel> getNeighborsAt(int row, int col) {
    return cells.stream().filter(cell ->
            (cell.getRow() == row - 1 && (cell.getCol() >= col - 1 && cell.getCol() <= col + 1) ||
            cell.getRow() == row + 1 && (cell.getCol() >= col - 1 && cell.getCol() <= col + 1) ||
            cell.getCol() == col - 1 && (cell.getRow() >= row - 1 && cell.getRow() <= row + 1) ||
            cell.getCol() == col + 1 && (cell.getRow() >= row - 1 && cell.getRow() <= row + 1))
    ).collect(Collectors.toList());
}

Should be better.

#6

Er… this is a slightly bonkers implementation isn’t it?

Cas :slight_smile:

#7

Yeah, wouldn’t a 2d array be a better fit? That way you can go straight to the cell you need by using its index coordinates.

#8

[quote]Rather use a stream builder to build a stream or use map() and filter() directly on the stream build from grid.getNeighborsAt(row,col)
[/quote]
Thanks for the suggestion. I’m still new to this

[quote]why convert the list to array in the
getNeighborsAt()
method in the first place?
[/quote]
You got the point. Sorry, I’m dumb ;D

[quote]slightly bonkers implementation isn’t it
[/quote]
Please be nice. I’m still trying to learn about streams

#9

Sorry - was a bit rude there, but the use of streams seemed to imply you are a relatively advanced programmer, which is at odds with the actual problem being discussed, which is about as trivial as can be. I think one lesson to be learned about streams is when not to use them: and this problem is one such case, where streams are like using hammer drills to squash ants. You should have a simple grid array and just look into adjacent coordinates directly; not collect a stream of neighbours. Though if you’re just getting to grips with the whole how streams work thing, it makes sense what you’re doing as an experiment in syntax and technique.

I ramble on.

Cas :slight_smile: