Java 流 API - 按多个字段分组

Question

I have java POJO with following fields

class Product{
    private String productName;
    private String productCode;
    private String price;
    private String productId;
    private String country;
    private List<Comments> comments;
}

class Comments {
   private String productCode;
   private String languageCode;
   private String comment;

}

When I retrieve data from the database. I get the data in the following format.

productName, productCode, price, productId, country, languageCode, comment

iPhone, 1XBA22, 1000, 134, USA, EN, comment in English

iPhone, 1XBA22, 1000, 134, USA, CN, comment in Chinese

laptop, 1234, 2000, 145, UK, EN, comment in English

laptop, 1234, 2000, 145, UK, CN, comment in Chinese

laptop, 1234, 2000, 145, UK, FR, comment in French

This result from db is stored in following data structure.

class ProductWithComments{
    private String productName;
    private String productCode;
    private String price;
    private String productId;
    private String country;
    private String comment;
    private String languageCode;
}

now, as you can see, the product with comments has duplicate products. because each product has comments in many languages, using Java Streams API, how can I convert the above list of data into List into List.

meaning, I group by Product and each product has many Comments. So basically grouping by needs to happen using many columns (productName, productCode, price, productId, country) then all the Comments for one Group should be listed in List<Comments> . Thanks in advance for any guide on this.

Answer 1

You need to pull out a class to use as a key:

class ProductKey {
  private String productName;
  private String productCode;
  private String price;
  private String productId;
  private String country;
  private String languageCode;
  // constructor, equals, hashCode, etc.
  // leave out the fields we're not grouping by
}

Then you just need to do:

products.stream().collect(
  Collectors.groupingBy(
    product -> new ProductKey(product.getProductName(), ...),
    Collectors.mapping(Product::getComment, Collectors.toList())));