Mengapa tipe pengembalian lambda tidak dicentang pada waktu kompilasi?

Referensi metode yang digunakan memiliki tipe kembali Integer. Tapi tidak cocokString diizinkan dalam contoh berikut.

Bagaimana cara memperbaiki withdeklarasi metode untuk mendapatkan tipe referensi metode yang aman tanpa casting secara manual?

import java.util.function.Function;

public class MinimalExample {
  static public class Builder<T> {
    final Class<T> clazz;

    Builder(Class<T> clazz) {
      this.clazz = clazz;
    }

    static <T> Builder<T> of(Class<T> clazz) {
      return new Builder<T>(clazz);
    }

    <R> Builder<T> with(Function<T, R> getter, R returnValue) {
      return null; //TODO
    }

  }

  static public interface MyInterface {
    Integer getLength();
  }

  public static void main(String[] args) {
// missing compiletimecheck is inaceptable:
    Builder.of(MyInterface.class).with(MyInterface::getLength, "I am NOT an Integer");

// compile time error OK: 
    Builder.of(MyInterface.class).with((Function<MyInterface, Integer> )MyInterface::getLength, "I am NOT an Integer");
// The method with(Function<MinimalExample.MyInterface,R>, R) in the type MinimalExample.Builder<MinimalExample.MyInterface> is not applicable for the arguments (Function<MinimalExample.MyInterface,Integer>, String)
  }

}

KASUS PENGGUNAAN: jenis Builder yang aman tetapi generik.

Saya mencoba menerapkan pembangun generik tanpa pemrosesan anotasi (nilai otomatis) atau plugin kompiler (lombok)

import java.lang.reflect.Array;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.HashMap;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Function;

public class BuilderExample {
  static public class Builder<T> implements InvocationHandler {
    final Class<T> clazz;
    HashMap<Method, Object> methodReturnValues = new HashMap<>();

    Builder(Class<T> clazz) {
      this.clazz = clazz;
    }

    static <T> Builder<T> of(Class<T> clazz) {
      return new Builder<T>(clazz);
    }

    Builder<T> withMethod(Method method, Object returnValue) {
      Class<?> returnType = method.getReturnType();
      if (returnType.isPrimitive()) {
        if (returnValue == null) {
          throw new IllegalArgumentException("Primitive value cannot be null:" + method);
        } else {
          try {
            boolean isConvertable = getDefaultValue(returnType).getClass().isAssignableFrom(returnValue.getClass());
            if (!isConvertable) {
              throw new ClassCastException(returnValue.getClass() + " cannot be cast to " + returnType + " for " + method);
            }
          } catch (IllegalArgumentException | SecurityException e) {
            throw new RuntimeException(e);
          }
        }
      } else if (returnValue != null && !returnType.isAssignableFrom(returnValue.getClass())) {
        throw new ClassCastException(returnValue.getClass() + " cannot be cast to " + returnType + " for " + method);
      }
      Object previuos = methodReturnValues.put(method, returnValue);
      if (previuos != null) {
        throw new IllegalArgumentException("Value alread set for " + method);
      }
      return this;
    }

    static HashMap<Class, Object> defaultValues = new HashMap<>();

    private static <T> T getDefaultValue(Class<T> clazz) {
      if (clazz == null || !clazz.isPrimitive()) {
        return null;
      }
      @SuppressWarnings("unchecked")
      T cachedDefaultValue = (T) defaultValues.get(clazz);
      if (cachedDefaultValue != null) {
        return cachedDefaultValue;
      }
      @SuppressWarnings("unchecked")
      T defaultValue = (T) Array.get(Array.newInstance(clazz, 1), 0);
      defaultValues.put(clazz, defaultValue);
      return defaultValue;
    }

    public synchronized static <T> Method getMethod(Class<T> clazz, java.util.function.Function<T, ?> resolve) {
      AtomicReference<Method> methodReference = new AtomicReference<>();
      @SuppressWarnings("unchecked")
      T proxy = (T) Proxy.newProxyInstance(clazz.getClassLoader(), new Class[] { clazz }, new InvocationHandler() {

        @Override
        public Object invoke(Object p, Method method, Object[] args) {

          Method oldMethod = methodReference.getAndSet(method);
          if (oldMethod != null) {
            throw new IllegalArgumentException("Method was already called " + oldMethod);
          }
          Class<?> returnType = method.getReturnType();
          return getDefaultValue(returnType);
        }
      });

      resolve.apply(proxy);
      Method method = methodReference.get();
      if (method == null) {
        throw new RuntimeException(new NoSuchMethodException());
      }
      return method;
    }

    // R will accep common type Object :-( // see /programming/58337639
    <R, V extends R> Builder<T> with(Function<T, R> getter, V returnValue) {
      Method method = getMethod(clazz, getter);
      return withMethod(method, returnValue);
    }

    //typesafe :-) but i dont want to avoid implementing all types
    Builder<T> withValue(Function<T, Long> getter, long returnValue) {
      return with(getter, returnValue);
    }

    Builder<T> withValue(Function<T, String> getter, String returnValue) {
      return with(getter, returnValue);
    }

    T build() {
      @SuppressWarnings("unchecked")
      T proxy = (T) Proxy.newProxyInstance(clazz.getClassLoader(), new Class[] { clazz }, this);
      return proxy;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) {
      Object returnValue = methodReturnValues.get(method);
      if (returnValue == null) {
        Class<?> returnType = method.getReturnType();
        return getDefaultValue(returnType);
      }
      return returnValue;
    }
  }

  static public interface MyInterface {
    String getName();

    long getLength();

    Long getNullLength();

    Long getFullLength();

    Number getNumber();
  }

  public static void main(String[] args) {
    MyInterface x = Builder.of(MyInterface.class).with(MyInterface::getName, "1").with(MyInterface::getLength, 1L).with(MyInterface::getNullLength, null).with(MyInterface::getFullLength, new Long(2)).with(MyInterface::getNumber, 3L).build();
    System.out.println("name:" + x.getName());
    System.out.println("length:" + x.getLength());
    System.out.println("nullLength:" + x.getNullLength());
    System.out.println("fullLength:" + x.getFullLength());
    System.out.println("number:" + x.getNumber());

    // java.lang.ClassCastException: class java.lang.String cannot be cast to long:
    // RuntimeException only :-(
    MyInterface y = Builder.of(MyInterface.class).with(MyInterface::getLength, "NOT A NUMBER").build();

    // java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    // RuntimeException only :-(
    System.out.println("length:" + y.getLength());
  }

}

Dalam contoh pertama, MyInterface::getLengthdan "I am NOT an Integer"membantu menyelesaikan parameter generik Tdan Rke masing MyInterface- Serializable & Comparable<? extends Serializable & Comparable<?>>masing.

// it compiles since String is a Serializable
Function<MyInterface, Serializable> function = MyInterface::getLength;
Builder.of(MyInterface.class).with(function, "I am NOT an Integer");

MyInterface::getLengthtidak selalu a Function<MyInterface, Integer>kecuali jika Anda secara eksplisit mengatakannya, yang akan menyebabkan kesalahan waktu kompilasi seperti yang ditunjukkan contoh kedua.

// it doesn't compile since String isn't an Integer
Function<MyInterface, Integer> function = MyInterface::getLength;
Builder.of(MyInterface.class).with(function, "I am NOT an Integer");

Ini jenis inferensi yang memainkan perannya di sini. Pertimbangkan generik Rdalam tanda tangan metode:

<R> Builder<T> with(Function<T, R> getter, R returnValue)

Dalam hal ini seperti yang tercantum:

Builder.of(MyInterface.class).with(MyInterface::getLength, "I am NOT an Integer");

jenis Rberhasil disimpulkan sebagai

Serializable, Comparable<? extends Serializable & Comparable<?>>

dan Stringtidak menyiratkan dengan tipe ini, maka kompilasi berhasil.

Untuk secara eksplisit menentukan jenis Rdan menemukan ketidakcocokan, seseorang dapat dengan mudah mengubah baris kode sebagai:

Builder.of(MyInterface.class).<Integer>with(MyInterface::getLength, "not valid");

Itu karena parameter tipe generik Anda Rdapat disimpulkan sebagai Objek, yaitu kompilasi berikut:

Builder.of(MyInterface.class).with((Function<MyInterface, Object>) MyInterface::getLength, "I am NOT an Integer");

Jawaban ini didasarkan pada jawaban lain yang menjelaskan mengapa itu tidak berfungsi seperti yang diharapkan.

LARUTAN

Kode berikut memecahkan masalah dengan memisahkan bifungsi "dengan" menjadi dua fungsi yang lancar "dengan" dan "mengembalikan":

class Builder<T> {
...
class BuilderMethod<R> {
  final Function<T, R> getter;

  BuilderMethod(Function<T, R> getter) {
    this.getter = getter;
  }

  Builder<T> returning(R returnValue) {
    return Builder.this.with(getter, returnValue);
  }
}

<R> BuilderMethod<R> with(Function<T, R> getter) {
  return new BuilderMethod<>(getter);
}
...
}

MyInterface z = Builder.of(MyInterface.class).with(MyInterface::getLength).returning(1L).with(MyInterface::getNullLength).returning(null).build();
System.out.println("length:" + z.getLength());

// YIPPIE COMPILATION ERRROR:
// The method returning(Long) in the type BuilderExample.Builder<BuilderExample.MyInterface>.BuilderMethod<Long> is not applicable for the arguments (String)
MyInterface zz = Builder.of(MyInterface.class).with(MyInterface::getLength).returning("NOT A NUMBER").build();
System.out.println("length:" + zz.getLength());

(agak asing)