泛型用法和stream中collector接口使用

使用場景：在從一個集合對象轉另一個集合對象

使用了jdk1.8後的stream處理

public class PrimaryUtil {
    public static <R extends IPrimaryKey<T>, T> Collection<T> primary(Collection<R> primaryKeyObjs){
        return primaryKeyObjs.stream().collect(new Collector<R, Collection<T>, Collection<T>>() {
            @Override
            public Supplier<Collection<T>> supplier() {
                return ArrayList::new;
            }
            @Override
            public BiConsumer<Collection<T>, R> accumulator() {
                return (c, e) -> {
                    c.add(e.primaryKey());
                };
            }
            @Override
            public BinaryOperator<Collection<T>> combiner() {
                return (a, b) -> {
                    a.addAll(b);
                    return a;
                };
            }
            @Override
            public Function<Collection<T>, Collection<T>> finisher() {
                return (a) -> a;
            }
            @Override
            public Set<Characteristics> characteristics() {
                return Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));
            }
        });
    }


    public static <R extends IPrimaryKey<T>, T> Collection<T> primary1(Collection<R> primaryKeyObjs){
        return primaryKeyObjs.stream().collect(new AbstractCollector<R, Collection<T>>() {
            @Override
            public Supplier<Collection<T>> supplier() {
                return ArrayList::new;
            }
            @Override
            public BiConsumer<Collection<T>, R> accumulator() {
                return (c, e) -> {
                    c.add(e.primaryKey());
                };
            }
        });
    }
}
abstract class AbstractCollector<R , T extends Collection>
        implements Collector<R, T, T> {

    @Override
    public BinaryOperator<T> combiner() {
        return (a, b) -> {
            a.addAll(b);
            return a;
        };
    }
    @Override
    public Function<T, T> finisher() {
        return (a) ->  a;
    }

    @Override
    public Set<Characteristics> characteristics() {
        return Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));
    }
}
           

public interface IPrimaryKey<T>{

    T primaryKey();
}
           

使用demo

propertys.add(new BasicNameValuePair("user_ids", JsonUtil.serialize(PrimaryUtil.primary(getMembers()))));
           

這個裡面是在httpClient中使用，這兒作用是構造post的送出參數。改參數填充的是所有成員的id屬性構成的list序列化得到的字元串。