由于在 Java 8 中用 Collectors.groupingBy 对 List 进行分组时每个组里都必须存在元素，也就是
Stream<Person> stream = Stream.of(new Person("Tom", "male"), new Person("Jerry", "male"));
System.out.println(stream.collect(Collectors.groupingBy(person -> person.getGender)));
//输出：{male=[Tom, Jerry]}
Stream<Person> stream = Stream.of(new Person("Tom", "female"), new Person("Jerry", "male"));
System.out.println(stream.collect(Collectors.groupingBy(person -> person.getGender)));
//输出：{female=[tom], male=[Jerry]}而无法表示存在其他 gender 的可能性，并且 female=[] 的情况，即想要结果
{male=[Tom, Jerry], female=[]}
如果想得到以上的结果该当如何呢? stream.collect() 接受一个 Collector, Collectors 中只是定义了许多常用的 Collector 实现，如果不够用的话我们可以实现自己的 Collector. 下面就来定义一个 GroupingWithKeys, 它需要实现 java.util.stream.Collector 接口，有五个接口方法. 事成之后我们写
Stream<Person> stream = Stream.of(new Person("Tom", "male"), new Person("Jerry", "male"));
System.out.println(stream.collect(new GroupingWithKeys<>(person -> person.gender, "male", "female")));能够得到输出:
{male=[Tom, Jerry], female=[]}
下面是 Person 和 GroupingWithKeys 两个类的完整代码
GroupingWithKeys.java
class GroupingWithKeys<T, K> implements Collector<T, Map<K, List<T>>, Map<K, List<T>>> {
private List<K> possibleKeys = Collections.emptyList();
private Function<T, K> keyGenerator;
public GroupingWithKeys(Function<T, K> keyGenerator, K...possibleKeys) { //构造时传入 Key 生成器和可能的 Keys
if(possibleKeys != null) {
this.possibleKeys = Arrays.asList(possibleKeys);
}
this.keyGenerator = keyGenerator;
}
@Override
public Supplier<Map<K, List<T>>> supplier() {
return () -> {
Map<K, List<T>> map = new LinkedHashMap<>();
possibleKeys.forEach(s -> map.put(s, new ArrayList<T>())); //按 possibleKeys 依次用空列表填充 Map
return map;
};
}
@Override
public BiConsumer<Map<K, List<T>>, T> accumulator() {
return (map, t) -> {
List<T> list = map.getOrDefault(keyGenerator.apply(t), new ArrayList<T>());
list.add(t);
map.put(keyGenerator.apply(t), list);
};
}
@Override
public BinaryOperator<Map<K, List<T>>> combiner() {
return (map1, map2) -> {
map1.putAll(map2);
return map1;
};
}
@Override
public Function<Map<K, List<T>>, Map<K, List<T>>> finisher() {
return Function.identity();
}
@Override
public Set<Characteristics> characteristics() {
return Collections.unmodifiableSet(EnumSet.of(IDENTITY_FINISH, CONCURRENT));
}
}在上面的 supplier() 方法中为所有可能的 Keys 准备好一个空的 List, 然后填充好 Map .
Person.java
class Person {
public final String name;
public final String gender;
public Person(String name, String gender) {
this.name = name;
this.gender = gender;
}
@Override
public String toString() {
return name;
}
}Java 既然提供了 java.util.stream.Collector 接口让我们扩展，那么想要什么样的 Collector 就自己创建吧。
