我先用 groupingBy 接近 mapping 值的收集器，然后 map 结果Map到所需对象。

import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.toList;

import java.util.List;
import java.util.stream.Stream;

class x
{
    public static void main(String[] args) {
        Stream<ProductTitle> sampleData= Stream.of(
            new ProductTitle(1, "product1", "seller1", 2, 25, 1, "title1"),
            new ProductTitle(1, "product1", "seller1", 2, 23, 1, "title2"),
            new ProductTitle(1, "product1", "seller1", 2, 45, 1, "title3"),
            new ProductTitle(2, "product2", "seller2", 5, 67, 0, "title4"),
            new ProductTitle(2, "product2", "seller2", 5, 11, 1, "title5"),
            new ProductTitle(3, "product3", "seller3", 9, 10, 1, "title6"),
            new ProductTitle(6, "product4", "seller4", 7, 36, 1, "title7"));

        List<Aggregate>
                aggregateKeyListMap =
                sampleData.collect(collectingAndThen(groupingBy(ProductTitle::toKey, mapping(ProductTitle::toTitleInfo, toList())),
                        groupedByKey -> groupedByKey.entrySet()
                                .stream()
                                .map(entry -> new Aggregate(entry.getKey(), entry.getValue()))
                                .collect(toList())));
        aggregateKeyListMap.forEach(System.out::println);
    }

    static class ProductTitle
    {
        private Long   productId;
        private String productName;
        private String sellerName;
        private Long   categoryId;
        private Long   titleId;
        private int    status;
        private String title;
        private Long   titlePrice;

        public ProductTitle(long productId, String productName, String sellerName, long categoryId, long titleId, int status, String title) {
            this.productId = productId;
            this.productName = productName;
            this.sellerName = sellerName;
            this.categoryId = categoryId;
            this.titleId = titleId;
            this.status = status;
            this.title = title;
        }

        public AggregateKey toKey() {
            return new AggregateKey(productId, productName, sellerName, categoryId);
        }

        public TitleInfo toTitleInfo() {
            return new TitleInfo(titleId, status, title);
        }
    }

    @lombok.ToString(includeFieldNames = false)
    @lombok.EqualsAndHashCode
    @lombok.AllArgsConstructor
    static class AggregateKey
    {
        Long   productId;
        String productName;
        String sellerName;
        Long   categoryId;
    }

    @lombok.ToString(includeFieldNames = false)
    @lombok.AllArgsConstructor
    static class Aggregate
    {
        AggregateKey        key;
        List<TitleInfo> titles;
    }

    @lombok.ToString(includeFieldNames = false)
    @lombok.AllArgsConstructor
    static class TitleInfo
    {
        Long   titleId;
        int    status;
        String title;
    }

}

对于样本数据，它产生：

x.Aggregate(x.AggregateKey(6, product4, seller4, 7), [x.TitleInfo(36, 1, title7)])
x.Aggregate(x.AggregateKey(1, product1, seller1, 2), [x.TitleInfo(25, 1, title1), x.TitleInfo(23, 1, title2), x.TitleInfo(45, 1, title3)])
x.Aggregate(x.AggregateKey(3, product3, seller3, 9), [x.TitleInfo(10, 1, title6)])
x.Aggregate(x.AggregateKey(2, product2, seller2, 5), [x.TitleInfo(67, 0, title4), x.TitleInfo(11, 1, title5)])