java 比较器比较Int
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Comparator comparingInt
提问by fwend
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:
我需要对点列表进行排序。首先我需要比较 x 值，然后如果 x 值相等，则 y 值。所以我想我会使用 thenComparing 方法：
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>.
但我不断收到消息：不兼容的类型：Comparator<Object> 无法转换为 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.
我还有其他方法可以进行这种比较，并且它有效，但我不明白我在这里做错了什么。
回答by Stefan Dollase
This code does work:
此代码确实有效：
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 pin the lambda. This is necessary, since Java cannot infer the type, due to the method chain.
我只添加了<Point>before comparingInt，它p在 lambda 中明确声明了 的类型。这是必要的，因为由于方法链的原因，Java 无法推断类型。
See also Generic type inference not working with method chaining?
另请参阅通用类型推断不适用于方法链？
Here is another alternative:
这是另一种选择：
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 getXand getYreturn double values. I personally prefer this approach.
在这里，可以毫无问题地推断类型。但是，您需要使用双重比较，因为getX并getY返回双重值。我个人更喜欢这种方法。
回答by Darshan Mehta
Try changing:
尝试改变：
Comparator<Point> cmp = Comparator.comparingInt(p -> p.x).thenComparingInt(p -> p.y);
to
到
Comparator<Point> cmp = Comparator.comparingInt((Point p) -> p.x).thenComparingInt((Point p) -> p.y);
