在 Java 中使用泛型和类型推断的问题

Question

在此期间，我正在做一个需要某种抽象的项目，但我不知道如何解决与 java 泛型和类型推断相关的一些问题。

这是源代码的简化架构：

public abstract class BaseDataset
{
    // Some data
    // Some methods
}

public class DerivedDataset1 extends BaseDataset
{
    // Some data
    // Implementations of some BaseDataset methods
    // Some additional methods
}

public class DerivedDataset2 extends BaseDataset
{
    // Some data
    // Implementations of some BaseDataset methods
    // Some additional methods
}

public interface BaseMeasure<T extends BaseDataset>
{
    public float evaluate(T dataset);
}

public class DerivedMeasure1 implements BaseMeasure<DerivedDataset1>
{
    @Override
    public float evaluate(DerivedDataset1 dataset)
    {
        // evaluate some measure using DerivedDataset1 methods
        return the evaluated measure;
    }
}

public class DerivedMeasure2 implements BaseMeasure<DerivedDataset2>
{
    @Override
    public float evaluate(DerivedDataset1 dataset)
    {
        // evaluate some measure using DerivedDataset2 methods
        return the evaluated measure;
    }
}

public class SocialNetworkBuilder
{
    public <T extends BaseDataset> void build(T dataset, BaseMeasure<T> measure)
    {
        float weight = measure.evaluate(dataset);

        // ...
    }
}

我的问题是，如果在我的主课中，我会这样写：

public class Application
{
    public static void main(String [] args)
    {
        BaseDataset dataset = new DerivedDataset1();
        BaseMeasure<? extends BaseDataset> measure = new DerivedMeasure1();
        SocialNetworkBuilder socialNetworkBuilder = new SocialNetworkBuilder();
        socialNetworkBuilder.build(dataset, measure);
    }
}

Eclipse 给我错误：“SocialNetworkBuilder 类型中的方法 build(T, BaseMeasure) 不适用于参数 (BaseDataset, BaseMeasure)”

我认为问题在于“构建”方法必须确保数据集和度量属于同一类型 T。

如果我这样做：

public class Application
{
    public static void main(String [] args)
    {
        BaseDataset dataset = new DerivedDataset1();
        BaseMeasure<? extends BaseDataset> measure = new DerivedMeasure1();
        SocialNetworkBuilder socialNetworkBuilder = new SocialNetworkBuilder();
        socialNetworkBuilder.build((DerivedDataset1) dataset, (DerivedMeasure1) measure);
    }
}

它有效，但我无法以这种方式解决我的问题，因为我在编译时不知道我的“数据集”和“测量”实例的派生类型； “dataset”和“measure”应该根据运行时给出的一些参数进行实例化。

我需要在运行时实例化我的“测量”和“数据集”变量，并且仍然能够在它们上调用“构建”方法。

你有解决这个问题的想法吗？

谢谢。

Answer 1

您的应用程序将在您进行如下更改时编译：

public class Application
{
    public static void main(String [] args)
    {
        DerivedDataset1 dataset = new DerivedDataset1();
        BaseMeasure<DerivedDataset1> measure = new DerivedMeasure1();
        SocialNetworkBuilder socialNetworkBuilder = new SocialNetworkBuilder();
        socialNetworkBuilder.build(dataset, measure);
    }
}

Answer 2

我认为您的代码没有问题。原则上应该是正确的，但是类型推断功能不够强大，无法处理BaseMeasure<DerivedDataset1>和DerivedDataset1 extends BaseDataset之间的传递类型依赖。

例如，在 Scala 中，您的原始代码可以正常工作。

object DataSet  extends App {

    abstract class BaseDataset  

    class DerivedDataset1 extends BaseDataset   

    class DerivedDataset2 extends BaseDataset   

    trait BaseMeasure[T <: BaseDataset] {
        def evaluate(dataset:T):Float
    }

    class DerivedMeasure1 extends BaseMeasure[DerivedDataset1] {
        override def evaluate(dataset: DerivedDataset1) = 0
    }

    class DerivedMeasure2 extends BaseMeasure[DerivedDataset2]  {
        override def evaluate(dataset: DerivedDataset2) = 1
    }

    class SocialNetworkBuilder {
        def  build[T <: BaseDataset](dataset: T, measure:BaseMeasure[T] ) : Unit = {
            val weight = measure.evaluate(dataset);
            println(weight)
        }
    }

    val dataset1 = new DerivedDataset1()
    val measure1 = new DerivedMeasure1()
    val dataset2 = new DerivedDataset2()
    val measure2 = new DerivedMeasure2()
    val socialNetworkBuilder = new SocialNetworkBuilder()
    socialNetworkBuilder.build(dataset1, measure1)
    socialNetworkBuilder.build(dataset2, measure2)
}