使用 lambda 对 Java 中的列表进行排序

Question

I have a list of list elements, now I want to sort that list using lambda expression, but I am getting compilation error:

Here is working code without lambda:

Collections.sort(positions, new Comparator<List<Integer>>() {
    public int compare(List<Integer> ele1, List<Integer> ele2) {
        // some logic here
        return some_number;
    }
});

Now using lamda, I am trying this:

positions.sort((List<Integer> ele1, List<Integer> ele2) -> {

});

But getting this compilation error:

The method sort(Comparator<? super List<Integer>>) in the type List<List<Integer>> is not applicable for the arguments ((List<Integer> ele1, List<Integer> ele2) -> {})

Answer 1

You are missing a return statement in your lambda:

positions.sort((List<Integer> ele1, List<Integer> ele2) -> {
    // logic...
    return some_number;
});

Note, however, that the idiomatic way to sort such a list in Java 8 is with a Comparator#comparing call:

positions.sort(Comparator.comparing(l -> logic_that_produces_some_comparable));

Answer 2

.sort expects a function that takes in two lists and returns an integer.

The lambda you have provided takes in two lists and returns nothing.

You need to return a value in your lambda.