Java：对嵌套列表的简单访问 <Object> 在实体上

Question

I have an Entity with multiple nested lists like this :

public class DataFile  {
    @Id
    private int id;
    private List<DataObject> datas = new ArrayList<>();
}

public class DataObject {
    @Id
    private String type;
    private List<DataValue> values = new ArrayList<>();
}


public class DataValue  {
    @Id
    private int id;
    private String dataValue;
    private LocalDateTime dataDate = LocalDateTime.now();
}

If I want a specific dataValue with DataObject.type = "speType" and DataValue.id = 2, I need this:

String value = dataFile.getDatas().forEach(t -> {
                 if(t.getType().equals("speType")){
                    t.getValues().forEach(v -> {
                        if(v.getId(2))
                            return v.getDataValue();
                    });
                  }
               });

Is it possible to create a simple method ?

Thanks

Answer 1

You could try something like that :

Optional<String> optValue =
        dataFile.getDatas()
                .stream()
                .filter(t -> t.getType().equals("specType"))
                .flatMap(t -> t.getValues().stream()
                           .filter(v -> v.getId() == 2)
                           .map(DataValue::getDataValue)
                )
                .findFirst();

Stream.findFirst() allows to exit of the processing early as soon as one element of it matches to the conditions. Similarly to what you did in your actual code.

Note that Stream.findFirst() returns an Optional . Either unwrap the object after the terminal operation such as :

String value = optValue.orElse("default value");

Or do in this stream itself :

String value = 
     ...
     .findFirst();
     .orElse("defaultValue")

Answer 2

For a pure "stream" solution :

dataFile.getDatas()
            .stream()
            .filter(t -> t.getType().equals("speType"))
            .flatMap(t -> t.getValues().stream())
            .filter(v -> v.getId()==2)
            .map(DataValue::getDataValue)
            .findFirst();

I assumed that by "if(v.getId(2))" you meant "if(v.getId()==2)", if not you can easily change the code above.