java-8 - JAVA 8 - 从 3 个 POJO 类检索数据时出现错误的流功能

Question

我有 3 个 POJO 类 products , attribute & value .这三个 POJO 都与其他 POJO 类有关系。现在，我想为 每一个产品。

它与旧方法完美配合，但现在我想用新的 JAVA 8 功能修改我的代码。

请在下面找到我的代码。

listProducts.stream().forEachOrdered(listproducts -> 
listproducts.getProductAttr()
                .stream().forEachOrdered(attr -> 
(((Collection<ProductAttrValue>) attr.getProductAttrValue()
                .stream().filter(attrlabel -> 
attrlabel.getLabel().equalsIgnoreCase("source"))))
                .stream().forEachOrdered(attrval ->
                                {
                                    System.out.println(attrval.getValue());
                                }
                            )       
                        )

            );

请找到我收到的错误消息。

Exception in thread "main" java.lang.ClassCastException: 
java.util.stream.ReferencePipeline$2 cannot be cast to java.util.Collection
at Hibernate.Testing.App.lambda$1(App.java:81)
at java.util.Iterator.forEachRemaining(Unknown Source)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Unknown 
Source)
at java.util.stream.ReferencePipeline$Head.forEachOrdered(Unknown Source)
at Hibernate.Testing.App.lambda$0(App.java:81)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(Unknown Source)
at java.util.stream.ReferencePipeline$Head.forEachOrdered(Unknown Source)
at Hibernate.Testing.App.main(App.java:80)

如果我不进行类型转换 .stream().forEachOrdered(attr -> ((Collection<ProductAttrValue>) attr.getProductAttrValue()我收到以下错误。

请找到我的产品 POJO 类

public class Product {

    private String name;
    private long std_delivery_Time;
    private int min_order_Quantity;

    private String status;
    private long catalog_Id;
    private Date expiration_Date;
    private int rank;
    private String product_Type;
    private int is_Visible;
    private String product_Group;
       private String code;


    @Id
    @Column(name = "ID")
    @GeneratedValue(strategy=GenerationType.AUTO)
    private long id;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


    public void setId(long id) {
        this.id = id;
    }

    public long getId() {
        return id;
    }

    public void setId(int id) {
          this.id = id;
         }


    public String getCode() {
      return code;
     }


    public void setCode(String code) {
      this.code = code;
     }

    @OneToMany( mappedBy = "product", cascade = CascadeType.ALL)
    private List<ProductAttr> productAttr;
    public List<ProductAttr> getProductAttr() {
        return productAttr;
    }

    public void setProductAttr(List<ProductAttr> productAttr) {
        this.productAttr = productAttr;
    }
 }

ProductAttr 类

public class ProductAttr {


    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID", updatable=false, nullable=false)
    private long id;

    @Column(name = "NAME")
    private String name;

    @Column(name = "PRODUCT_ID")
    private long product_id;


    public ProductAttr() {
        super();
        // TODO Auto-generated constructor stub
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public long getProduct_id() {
        return product_id;
    }

    public void setProduct_id(long product_id) {
        this.product_id = product_id;
    }

    public long getId() {
        return id;
    }

    @ManyToOne
    @JoinColumn(name = "PRODUCT_ID",  referencedColumnName = "ID", 
insertable = false, updatable = false)
    private Product product;

    public Product getProduct() {
        return product;
    }

    public void setProduct(Product product) {
        this.product = product;
    }

    @OneToMany( mappedBy = "productAttr", cascade = CascadeType.ALL)
    private Set<ProductAttrValue> productAttrValue;

        public Set<ProductAttrValue> getProductAttrValue() {
            return productAttrValue;
        }

        public void setProductAttrValue(Set<ProductAttrValue> 
productAttrValue) {
            this.productAttrValue = productAttrValue;
        }

}

ProductValue 类

public class ProductAttrValue {

    @Id
    @Column(name = "ATTR_ID", updatable=false, nullable=false)
    private long attr_id;

    @Column(name = "LABEL")
    private String label;

    @Column(name = "VALUE")
    private String value; 

        public long getAttr_id() {
            return attr_id;
        }

        public void setAttr_id(long attr_id) {
            this.attr_id = attr_id;
        }

        public String getLabel() {
            return label;
        }

        public void setLabel(String label) {
            this.label = label;
        }

        public void setValue(String value) {
            this.value = value;
        }

        public String getValue() {
            return value;
        }


      @ManyToOne
      @JoinColumn(name = "ATTR_ID",  referencedColumnName = "ID", insertable 
      = false, updatable = false)

      private ProductAttr productAttr;

      public ProductAttr getProductAttr() {
            return productAttr;
      }

      public void setProductAttr(ProductAttr productAttr) {
            this.productAttr = productAttr;
        }

      public ProductAttrValue() {
        super();
        // TODO Auto-generated constructor stub
    }


}

Answer 1

您的缩进表明错误的心态。这些stream()操作不在一个层次上。你有一个嵌套操作:

listProducts.stream().forEachOrdered(listproducts -> 
    listproducts.getProductAttr().stream().forEachOrdered(attr -> 
        attr.getProductAttrValue().stream()
                                  .filter(av -> av.getLabel().equalsIgnoreCase("source"))
                     /*.stream()*/.forEachOrdered(av -> System.out.println(av.getValue()))
    )
);

(我将误导性名称 attrlabel 和 attrval 更改为 av，因为它们不是标签或值；您正在调用 .getLabel() 和 .getValue() 随后 在这些变量上，所以我使用 av 作为“ ProductAttrValue")

filter 操作是唯一不嵌套的操作，因为它是链接到流的中间操作，而终端操作只需链接到结果流。那个地方不需要 stream() 调用(用 /*.stream()*/ 标记的地方)。

也就是说，你应该避免这种嵌套操作。

你可以使用

listProducts.stream()
            .flatMap(listproducts -> listproducts.getProductAttr().stream())
            .flatMap(attr -> attr.getProductAttrValue().stream())
            .filter(av -> av.getLabel().equalsIgnoreCase("source"))
            .forEachOrdered(av -> System.out.println(av.getValue()));

相反。