将 MutableClassToInstanceMap 与泛型一起使用时出现编译错误
我在 putInstance 方法调用时收到以下编译错误“MutableClassToInstanceMap 类型中的方法 putInstance(Class, T) 不适用于参数 (Class, Number)”。有谁知道我做错了什么？谢谢！
public class TestMutableClassToInstanceMap {
public final MutableClassToInstanceMap<Number> identifiers = MutableClassToInstanceMap.create();
public static void main(String[] args) {
ArrayList<Number> numbers = new ArrayList<Number>();
numbers.add(new Integer(5));
TestMutableClassToInstanceMap test = new TestMutableClassToInstanceMap(numbers);
}
public TestMutableClassToInstanceMap(Collection<Number> numbers){
for (Number number : numbers) {
this.identifiers.putInstance(number.getClass(), number); //error here
}
this.identifiers.putInstance(Double.class, 5.0); // This works
}
}
I am getting the following compile error "The method putInstance(Class, T) in the type MutableClassToInstanceMap is not applicable for the arguments (Class, Number)" on the putInstance method call. Does anyone know what I am doing wrong?? Thanks!
public class TestMutableClassToInstanceMap {
public final MutableClassToInstanceMap<Number> identifiers = MutableClassToInstanceMap.create();
public static void main(String[] args) {
ArrayList<Number> numbers = new ArrayList<Number>();
numbers.add(new Integer(5));
TestMutableClassToInstanceMap test = new TestMutableClassToInstanceMap(numbers);
}
public TestMutableClassToInstanceMap(Collection<Number> numbers){
for (Number number : numbers) {
this.identifiers.putInstance(number.getClass(), number); //error here
}
this.identifiers.putInstance(Double.class, 5.0); // This works
}
}
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评论（2）
与常规的 put 方法相反，putInstance 方法的目的是提供编译时类型安全性。在这种情况下，你和我可以很容易地推断出
number必须是number.getClass()给出的类型，但对于编译器来说，该信息“丢失了”。 ”也就是说，据it所知，也许number是一个Integer，而number.getClass()是Long.class；它不够“聪明”，无法判断这是安全的。解决方案：只需使用
put()！您仍然可以获得运行时检查，并且之后仍然可以使用类型安全的getInstance()方法，例如Long l = this.identifiers.getInstance(Long.class);.（最后提示：请注意，因为
Long.class和long.class可以同时存在于ClassToInstanceMap中并映射到不同的值！）The purpose of the
putInstancemethod, as opposed to the regularput, is to provide compile-time type safety. In this case, you and I can easily reason thatnumbermust be of the type given bynumber.getClass(), but to the compiler, that information is "lost." That is, as far as it knows, maybenumberis an Integer andnumber.getClass()is Long.class; it's not "smart" enough to figure out this is safe.Solution: just use
put()! You still get the runtime checks, and you can still use the typesafegetInstance()method after that, such as inLong l = this.identifiers.getInstance(Long.class);.(Final tip: watch out, because
Long.classandlong.classcan both exist in aClassToInstanceMapand be mapped to different values!)凯文的回答很棒。此外，您可以从方法定义中看出编译器不会允许调用：
putInstance需要（在您的情况下）一个Class。number.getClass()提供一个Class这不是直接子类。一些方差和协方差的讨论在这里可能有用。
Kevin's answer is great. Besides, you can tell from the method definition that the compiler is not going to allow the call:
putInstancerequires (in your case) aClass<Number>.number.getClass()provides aClass<? extends Number>Which is not a direct subclass. Some variance and covariance talk could be useful here.