Java 8 Stream：使用HashMap中的值填充实例化的对象列表

Question

So I have a HashMap of key-value pairs and would like to create a list of new objects instantiated using each key-value pair. For example:

//HashMap of coordinates with the key being x and value being y
Map<Integer, Integer> coordinates = new HashMap<Integer, Integer>();
coordinates.put(1,2);
coordinates.put(3,4);

List<Point> points = new ArrayList<Point>();

//Add points to the list of points instantiated using key-value pairs in HashMap
for(Integer i : coordinates.keySet()){
     points.add(new Point(i , coordinates.get(i)));
}

How could I go about doing this same thing using Java 8 streams.

Answer 1

    List<Point> points = coordinates.entrySet().stream()
            .map(e -> new Point(e.getKey(), e.getValue()))
            .collect(Collectors.toList());

Note: I have not used forEach(points::add) , because it could result in concurrency issues. In general you should be wary of streams with side-effects.

Answer 2

List<Point> points = new ArrayList<Point>();
coordinates.forEach((i, j) -> points.add(new Point(i, j)));

Answer 3

Here is the possible solution:

Map<Integer, Integer> coordinates = new HashMap<Integer, Integer>();
coordinates.put(1,2);
coordinates.put(3,4);

List<Integer> list = coordinates.entrySet().stream()
        .map(entry -> entry.getValue())
        .collect(Collectors.toList());