java - 我可以按属性分组并映射到 Java 8 中的新对象吗

Question

您好，我有一个对象数组，其中类别和子类别详细信息位于同一个对象中。所以它就像，

public class MyObject {
    String categoryCode;
    String categeoryCodeDescription;
    String subCategoryCode;
    String subCategoryCodeDescription;

    public MyObject(String categoryCode, String categeoryCodeDescription, String subCategoryCode, String subCategoryCodeDescription) {
        this.categoryCode = categoryCode;
        this.categeoryCodeDescription = categeoryCodeDescription;
        this.subCategoryCode = subCategoryCode;
        this.subCategoryCodeDescription = subCategoryCodeDescription;
    }
}

List<MyObject> collection = new ArrayList<MyObject>;
collection.add(new My Object("A1", "descA1", "A1A", "descA1A"));
collection.add(new My Object("A1", "descA1", "A1B", "descA1B"));
collection.add(new My Object("A1", "descA1", "A1C", "descA1C"));
collection.add(new My Object("A2", "descA1", "A2A", "descA2A"));
collection.add(new My Object("A2", "descA1", "A2B", "descA2B"));

您能否按类别代码分组，但同时映射到包含描述的对象。因此，如果我有两个类，例如..

public class Category {
    String categoryCode;
    String categoryDesc;
    public Category (String categoryCode, String categoryDesc) {
        this.categoryCode = categoryCode;
        this.categoryDesc = categoryDesc;
    }
}

public class SubCategory {
    String subCategoryCode;
    String subCategoryDesc;
    public SubCategory (String subCategoryCode, String subCategoryDesc) {
        this.subCategoryCode = subCategoryCode;
        this.subCategoryDesc = subCategoryDesc;
    }
}

..我想将集合列表分组到 Map<Category,List<SubCategory>> 中.我可以对类别代码进行分组，但看不到如何创建新的类别实例作为映射键。这在一个类轮中可能是不可能的。

Map<String, List<MyObject>> map = collection.stream().collect(Collectors.groupingBy(MyObject::getCategoryCode));

Answer 1

如果将 mapping 收集器链接到 groupingBy 就可以做到这一点。

您使用 mapping() 将 MyObject 实例转换为 SubCategory 实例。

Map<Category,List<SubCategory>> map =
    collection.stream().collect(Collectors.groupingBy(mo -> new Category(mo.getCategoryCode(),mo.getCategoryDesc()),
                                                      Collectors.mapping(mo->new SubCategory(mo.getSubCategoryCode(),mo.getSubCategoryDesc()),
                                                                         Collectors.toList())));

请注意，Category 必须覆盖 equals 和 hashCode 才能使该分组起作用。