从java中的地图中检索有序列表

Question

let's assume I have a HasMap<String, Student> , where Student has a method double getAverage() , is there a clever way to retrieve a List<String> of keys from that Map ordered by decreasing average?

I was thinking about using streams for compactness, however this does not work:

List<String> ordered  = studentMap.entrySet().stream()
                .sorted((e1, e2) -> e2.getValue().getAverage().compareTo(e1.getValue().getAverage()))
                .map(Map.Entry::getKey)
                .collect(Collectors.toList());

Answer 1

You could use Comparator.comparingDouble and Comparator.reversed() for this purpose:

List<String> ordered  = studentMap.entrySet().stream()
                .sorted(Comparator.<Map.Entry<String, Student>>comparingDouble(e -> e.getValue().getAverage()).reversed())
                .map(Map.Entry::getKey)
                .collect(Collectors.toList());

Answer 2

If your .getAverage() method indeed returns a double , there is no .compareTo() method.

Use this instead:

.sorted((e1, e2) -> Double.compare(e2.getValue().getAverage(), e1.getValue().getAverage()))

Answer 3

If getAverage() returns a primitive (double), you can't use compareTo . Use Double.compare :

List<String> ordered  = studentMap.entrySet().stream()
                .sorted((e1, e2) -> Double.compare(e2.getValue().getAverage(),e1.getValue().getAverage()))
                .map(Map.Entry::getKey)
                .collect(Collectors.toList());