This question already has an answer here:
I need to sort a list of Points. First I need to compare the x value, then if the x values are equal, the y value. So I thought I'd use the thenComparing method:
Comparator<Point> cmp = Comparator.comparingInt(p -> p.x).thenComparingInt(p -> p.y);
But I keep getting the message: Incompatible types: Comparator<Object> cannot be converted to Comparator<Point>.
There are other ways I can make this comparison, and it works, but I don't understand what I'm doing wrong here.
This code does work:
Comparator<Point> cmp = Comparator.<Point> comparingInt(p -> p.x)
.thenComparingInt(p -> p.y);
I only added <Point> before comparingInt, which explicitly declares the type of p in the lambda. This is necessary, since Java cannot infer the type, due to the method chain.
See also Generic type inference not working with method chaining?
Here is another alternative:
Comparator<Point> cmp = Comparator.comparingDouble(Point::getX)
.thenComparingDouble(Point::getY);
Here, the type can be inferred without any problems. However, you need to use the double comparison, because getX and getY return double values. I personally prefer this approach.
Try changing:
Comparator<Point> cmp = Comparator.comparingInt(p -> p.x).thenComparingInt(p -> p.y);
to
Comparator<Point> cmp = Comparator.comparingInt((Point p) -> p.x).thenComparingInt((Point p) -> p.y);