I'm trying to map a model TradeValidationData to an entity TradeValidatioEntity using map() operation is the stream.

Here are the models:

@Data
@JsonIgnoreProperties(ignoreUnknown = true)
public class TradeValidationData {
    @JsonProperty("orderId")
    public Integer orderId;
    
    @JsonProperty("tradeValidations")
    public List<TradeValidation> tradeValidations;
}

@Data
@JsonIgnoreProperties(ignoreUnknown = true)
public class TradeValidation {
    @JsonProperty("clientReferenceId")
    public String clientReferenceId;
    
    @JsonProperty("tradeValidationMessages")
    public List<TradeValidationMessage> tradeValidationMessages;
}

@Data
@JsonIgnoreProperties(ignoreUnknown = true)
public class TradeValidationMessage {
    @JsonProperty("validationCode")
    public Integer validationCode;
    
    @JsonProperty("validationMessage")
    public String validationMessage;
}

Here is the entity:

public class TradeValidationEntity implements Serializable {
    
    private static final long serialVersionId = 1L;
    
    @EmbeddedId
    private TradeValidationCompositeKey tradeValidationCompositeKey;
    
    @Column(name = "validation_message")
    private String validationMessage;
}

public class TradeValidationCompositeKey implements Serializable {
    private static final long serialVersionId = 1L;
    
    @Column(name = "order_id")
    private Integer orderId;
    
    @Column(name = "client_reference_id")
    private String clientReferenceId;
    
    @Column(name = "function_type_id")
    private Integer functionTypeId;
    
    @Column(name = "validation_id")
    private Integer validationId;
}

I have a model object TradeValidationData which in turn has a List<TradeValidation>. I need to transform each model TradeValidation into an entity TradeValidationEntity.

Each TradeValidation contains a list of messages List<TradeValidationMessage>.

In order to initialize TradeValidationEntity I need to create a composite key TradeValidationCompositeKey, which requires data from an instance of TradeValidation and a message.

Here's my previous code which makes use of the Stream.forEach() which is working fine. I hope it would help to convey my intention.

var tradeValidationEntities = new ArrayList<TradeValidationEntity>();

tradeValidationData.tradeValidations.stream().forEach(tradeValidation -> {
    tradeValidation.tradeValidationMessages.stream().forEach(tradeValidationMessage -> {
        var tradeValidationCompositeKey = new TradeValidationCompositeKey(orderId,
            tradeValidation.clientReferenceId, functionTypeId, tradeValidationMessage.getValidationCode());
            
        TradeValidationEntity tradeValidationEntity = new TradeValidationEntity();
        tradeValidationEntity.setTradeValidationCompositeKey(tradeValidationCompositeKey);
        tradeValidationEntity.setValidationMessage(tradeValidationMessage.validationMessage);
        
                tradeValidationEntities.add(tradeValidationEntity);
    });
});

And here's the current problematic attempt to map TradeValidationData to TradeValidatioEntity:

List<TradeValidationEntity> tradeValidationEntities = tradeValidationData.tradeValidations.stream()
    .map(tradeValidation -> {
        var tradeValidationEntity = new TradeValidationEntity();
        tradeValidation.tradeValidationMessages.stream()
            .map(tradeValidationMessage -> {
                var tradeValidationCompositeKey = new TradeValidationCompositeKey();
                tradeValidationCompositeKey.setOrderId(orderId);
                tradeValidationCompositeKey.setClientReferenceId(tradeValidation.clientReferenceId);
                tradeValidationCompositeKey.setFunctionTypeId(functionTypeId);
                tradeValidationCompositeKey.setValidationId(tradeValidationMessage.validationId);
                tradeValidationEntity.setTradeValidationCompositeKey(tradeValidationCompositeKey);
                tradeValidationEntity.setValidationMessage(tradeValidationMessage.getValidationMessage());
                return tradeValidationEntity;
            });
        return tradeValidationEntity;
    })
    .collect(Collectors.toList());

I think the issue is with the second return statement I have for tradeValidationEntity. But I'm not sure what to return instead.

CodePudding user response：

You need to transform each TradeValidation object in the stream into multiple TradeValidationEntity instances.

map() operation is not the right tool for that purpose, it's meant for one-to-one transformation. Its function take one object and produces only one object.

To perform one-to-many transformation, you can use flatMap() operation. It expects a function, which take an element and produces a stream of elements.

That's how it might be implemented.

List<TradeValidationEntity> tradeValidationEntities = tradeValidationData.tradeValidations.stream()
    .flatMap(tradeValidation -> tradeValidation.tradeValidationMessages.stream()
        .map(tradeValidationMessage -> {
            var tradeValidationCompositeKey = new TradeValidationCompositeKey(orderId,
                tradeValidation.clientReferenceId, functionTypeId, tradeValidationMessage.getValidationCode());
    
            TradeValidationEntity tradeValidationEntity = new TradeValidationEntity();
            tradeValidationEntity.setTradeValidationCompositeKey(tradeValidationCompositeKey);
            tradeValidationEntity.setValidationMessage(tradeValidationMessage.validationMessage);
            
                    return tradeValidationEntity;
        })
    )
    .toList(); // for Java 16  or collect(Collectors.toList())

Note:

• map() - is an intermediate operation, it doesn't produce a result, it spawns another stream.
• A stream which doesn't ends with a terminal operation will not be executed (unless it's consumed by another stream via some operations like concat(), or flatMap()).
• Multiline lambda expressions are difficult to read. Readability and conciseness is a main weapon of streams. I would advise considering extracting heavy logic from a function into a separate method with a self-explanatory name and replacing the multiline lambda with method reference.