Confused about the lambda expression

Question

I am reading an example code which is to add a 10 pixel wide gray frame replacing the pixels on the border of an image by calling it with the lambda expression:

public static Image transform(Image in, ColorTransformer t) {
    int width = (int) in.getWidth();
    int height = (int) in.getHeight();
    WritableImage out = new WritableImage(width, height);
    for (int x = 0; x < width; x++) {
        for (int y = 0; y < height; y++) {
            out.getPixelWriter().setColor(x, y,
                    t.apply(x, y, in.getPixelReader().getColor(x, y)));
        }
    }
    return out;
}

@Override
public void start(Stage stage) throws Exception {
    Image image = new Image("test_a.png");
    Image newImage = transform(
                       image,
                       (x, y, c) -> (x <= 10 || x >= image.getWidth() - 10 || 
                                     y <= 10 || y >= image.getHeight() - 10) 
                                     ? Color.WHITE : c
                     );
        stage.setScene(new Scene(new VBox(
     new ImageView(image),
     new ImageView(newImage))));
  stage.show();
    stage.show();
 }
}

@FunctionalInterface
interface ColorTransformer {
    Color apply(int x, int y, Color colorAtXY);
}

I am confused about the lambda expression:

Image newImage = transform(
                   image,
                   (x, y, c) -> (x <= 10 || x >= image.getWidth() - 10 || 
                                 y <= 10 || y >= image.getHeight() - 10) 
                                 ? Color.WHITE : c
                 );

To add a frame to a figure, I think "&&" should be more reasonable than "||". But, "&&" does not work here! Could anyone please explain this?
I do not really understand "? Color.WHITE : c". Firstly, why are they outside the previous bracket? Secondly, what does the question mark(?) mean?

Thank you for your help in advance.

Your question/problem is not with lambda but with logic of [conditional operator](http://stackoverflow.com/a/17470669/1393766). — Pshemo, May 24 '15 at 16:24

score 1 · Accepted Answer · answered May 24 '15 at 16:28

1

First off, your question has nothing to do with lambda, but expressions in general.

Starting with the second part: A ? B : C means "if A is considered true, my value is B otherwise it is C. A kind of if statement inside of an expression.

As to the first part: if any of the individual tests is true (each of which is of the form the point is too far left/right/up/down), then the point is (presumably) outside, and is colored white; otherwise, use the color c. You can't have them all be true at once: a point can't both be too far left and too far right, for example.

answered May 24 '15 at 16:28

Scott Hunter

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Thank you so much for your comment. I am clear about this. Just one question: the code does not define "c" at all, therefore, if the condition is FALSE, how will the system get the value of "c"? – SuperDelta May 24 '15 at 16:40
That *is* a question about `lambda`; `(x, y, c)` is the parameter list to your `lambda`, so `c`, `x` and `y` are getting their values from the call to this anonymous function. – Scott Hunter May 24 '15 at 17:08

score 0 · Answer 2 · answered May 24 '15 at 16:46

Your program is equivalent to this: (if you don't use lambda)

public static Image transform(Image in) {
    int width = (int) in.getWidth();
    int height = (int) in.getHeight();
    WritableImage out = new WritableImage(width, height);
    for (int x = 0; x < width; x++) {
        for (int y = 0; y < height; y++) {
            Color c = in.getPixelReader().getColor(x, y);
            if (x <= 10 || x >= image.getWidth() - 10 || 
                y <= 10 || y >= image.getHeight() - 10) 
                c = Color.WHITE;
          /* else
                c = c; */
             out.getPixelWriter().setColor(x, y, c);
         }
    }
    return out;
}

Confused about the lambda expression

2 Answers2