在Java中使用泛型和類型推斷的問題

Question

在此期間，我正在研究一個需要某種抽象的項目，但我不知道如何解決與Java泛型和類型推理相關的一些問題。

這是源代碼的簡化結構：

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); 

     // ... 
    } 
} 

我的問題是，如果在我的主類，我寫的是這樣的：

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中給我的錯誤：「SocialNetworkBuilder類型中的方法構建（T，BaseMeasure）不適用於參數（BaseDataset，BaseMeasure）」

我認爲問題在於「構建」方法h爲確保數據集和措施是同一類型的T.

的。如果我做這樣的事情：

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); 
    } 
} 

它的工作原理，但我不能這樣解決我的問題，因爲我不在編譯時不知道我的「數據集」和「度量」實例的派生類型; 「數據集」和「度量」應根據運行時給出的一些參數進行實例化。

我需要在運行時實例化我的「測量」和「數據集」變量，並仍然可以調用它們的「構建」方法。

你有解決這個問題的一些想法嗎？

謝謝。

Answer 1

您的應用程序將編譯當你改變它，如下所示：

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

我認爲這沒有什麼不對您的代碼。原則上應該是正確的，但類型推斷的功能不足以處理BaseMeasure<DerivedDataset1>和DerivedDataset1 extends BaseDataset之間的傳遞類型依賴關係。

作爲一個例子，在Scala中，您的原始代碼正常工作。

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) 
}