java - 在三元中使用无界泛化静态函数时如何避免未经检查的强制转换？

Question

我想弄清楚是否有办法避免在此函数中进行未经检查的强制转换(使用 Java 7):

private static <O> ValueValidator<O> newForAlways(Always valid_orInvalid)  {
   @SuppressWarnings("unchecked")
   ValueValidator<O> vldtr = (ValueValidator<O>)(valid_orInvalid == Always.VALID
      ?  newForAlwaysValid()
      :  newForAlwaysInvalid());
   return  vldtr;
}

这是返回的两个函数的签名:

private static <O> ValueValidator<O> newForAlwaysValid()  {
private static <O> ValueValidator<O> newForAlwaysInvalid()  {

(这里是 Always 枚举，它只是一个 boolean 值替代:

enum Always {VALID, INVALID;};

)

所有三个函数都具有相同的返回类型并包含无界泛型。这两个问题解释了为什么会发生，尽管它们都是关于有界泛型的:

• Why doesn't the ternary operator like generic types with bounded wildcards?
• Generics compilation error with ternary operator in Java 8, but not in Java 7

所以即使这有效

ValueValidator<Integer> intVldtr = Test.<Integer>newForAlwaysValid();

这不是:

private static <O> ValueValidator<O> newForAlways(Always valid_orInvalid)  {
   return  (valid_orInvalid == Always.VALID
      ?  <O>newForAlwaysValid()
      :  <O>newForAlwaysInvalid());
}

C:\java_code\Test.java:15: error: illegal start of expression
      ?  <O>newForAlwaysValid()

...and 8 more similar errors...

这也不是:

private static <O> ValueValidator<O> newForAlways2(Always valid_orInvalid)  {
   return  (valid_orInvalid == Always.VALID
      ?  newForAlwaysValid()
      :  newForAlwaysInvalid());
}

C:\java_code\Test.java:15: error: incompatible types
          ?  newForAlwaysValid()
          ^
  required: ValueValidator<O>
  found:    ValueValidator<Object>
  where O is a type-variable:
    O extends Object declared in method <O>newForAlways2(Always)

那么，再重复一遍这个问题:除了 unchecked cast 之外，还有其他选择吗？ (我使用的是 Java 7。)

SSCCE :

public class Test  {
   public static void main(String[] ignored)  {
      ValueValidator<Integer> intVldtr = Test.<Integer>newForAlwaysValid();
      intVldtr = Test.<Integer>newForAlways(Always.VALID);
   }
   private static <O> ValueValidator<O> newForAlways(Always valid_orInvalid)  {
      @SuppressWarnings("unchecked")
      ValueValidator<O> vldtr = (ValueValidator<O>)(valid_orInvalid == Always.VALID
         ?  newForAlwaysValid()
         :  newForAlwaysInvalid());
      return  vldtr;
   }
   private static <O> ValueValidator<O> newForAlwaysValid()  {
      return  (new AlwaysValid<O>());
   }
   private static <O> ValueValidator<O> newForAlwaysInvalid()  {
      return  (new AlwaysInvalid<O>());
   }
}
enum Always {VALID, INVALID;};
abstract class ValueValidator<O>  {
   public abstract boolean isValid(O to_validate);
}
class AlwaysValid<O> extends ValueValidator<O>  {
   public boolean isValid(O to_validate)  {
      return  true;
   }
}
class AlwaysInvalid<O> extends ValueValidator<O>  {
   public boolean isValid(O to_validate)  {
      return  false;
   }
}

Answer 1

你可以这样做

ValueValidator<O> vldtr = valid_orInvalid == Always.VALID 
            ? Test.<O>newForAlwaysValid()
            : Test.<O>newForAlwaysInvalid();