在流操作期间转换列表

Question

Here's my scenario:

private List<Entity> getPlanets() {
   return entities.values()
                  .stream()
                  .filter(x -> x instanceof Planet)
                  .collect(Collectors.toList());
}

• Entity is the super class of Planet
• entities is a HashMap<Entity>
• As the method is called "getPlanets" I would like it to return a List<Planet> but it appears to me that the stream expression is going to return a List<Entity>
• I tried some casting expressions but none seem to work out.

I am brand new with Java 8 streams so maybe someone can point out what I am missing?

Answer 1

return entities.values()
               .stream()
               .filter(x -> x instanceof Planet)
               .map(x -> (Planet) x)
               .collect(Collectors.toList());

Answer 2

A variation with method references:

return entities.values().stream()
    .filter(Planet.class::isInstance)
    .map(Planet.class::cast)
    .collect(Collectors.toList());