二话不说 直接上代码
public class Test {
@org.junit.Test
public void test1 () {
Stream<String> names = Stream.of("John", "Paul", "George", "John",
"Paul", "John");
Map<String, List<String>> m = names.collect(new GroupingBy<String,String>(x->x));
System.out.print(m.toString());
// 输出是： {George=[George], John=[John, John, John], Paul=[Paul, Paul]}
}
}
// 模拟 Collectors.groupingBy()
class GroupingBy<T, K> implements Collector<T, Map<K, List<T>>, Map<K,
List<T>>> {
private Function<T, K> tFunction;
GroupingBy(Function<T, K> tFunction){
this.tFunction = tFunction;
}
@Override
public Supplier<Map<K, List<T>>> supplier() {
// 初始化容器
return HashMap::new;
}
@Override
public BiConsumer<Map<K, List<T>>, T> accumulator() {
// 业务操作
return (p,t)-> {
K k = this.tFunction.apply(t);
if (p.containsKey(k)) {
p.get(k).add(t);
} else {
p.put(k, new ArrayList<T>(){{add(t);}});
}
};
}
@Override
public BinaryOperator<Map<K, List<T>>> combiner() {
// 并行操作时 合并子项
return (l, r) -> {
l.putAll(r);
return l;
};
}
@Override
public Function<Map<K, List<T>>, Map<K, List<T>>> finisher() {
// 这里是最终的返回结果
return (p)->p;
}
@Override
public Set<Characteristics> characteristics() {
return Collections.emptySet();
}
}
