具有其他类型的Collection getter的泛型类 [英] Generic classes with Collection getter of other types

问题描述

我有类似这样的东西：

  public class GenericClass< T> {
 T doSomething（）{...} 
 
集合< String> getCollection（）{...} 
} 
  

现在我实例化该类的一个对象没有类型参数，因为我没有在这方面进行讨论，或者我不知道它。

  GenericClass obj = new GenericClass （）; 
 for（String str：obj.getCollection（）{// does not work 
 ... 
} 
  

问题是，编译器不会简单地抛弃有关类型参数的信息，而且还会抛出Collection（String）的类型，尽管这与参数无关。

我做错了什么，或者是Java的限制？如果是这样，为什么会有这个限制？

解决方案

当你不知道 T时，你做错了什么。 $ b < 类型，你应该使用通配符： GenericClass<> 。当你实例化一个新的 GenericClass ，所以只需使用 Object ：

  GenericClass<？> obj = new GenericClass< Object>（）; 
  

出于向后兼容性的原因，故意在一个没有任何通用信息的类上所有（ GenericClass 没有任何<> ）都会失去所有泛型类型的安全性，可以与预先通用的代码一起使用。

currenty I'm facing a problem with generic classes in Java.

I have something like this:

public class GenericClass<T> {
    T doSomething() {...}

    Collection<String> getCollection() {...}
}

Now I instantiate an object of that class without the type parameter, since I'm not intereseted in that or I don't know it.

GenericClass obj = new GenericClass();
for (String str : obj.getCollection() { // won't work
    ...
}

The problem is, that the compiler does not simply throw away the information about the type-parameter, but also about the type of the Collection (String), although that's independent from the parameter.

Am I doing something wrong, or is that a restriction of Java? If so, why is that a restriction?

解决方案

You're doing something wrong.

When you don't know the T type, you should just use a wildcard: GenericClass<?>. You can't do this when you're instantiating a new GenericClass, though, so just use Object:

GenericClass<?> obj = new GenericClass<Object>();

For backwards-compatibility reasons, it is deliberate that a class without any generic information at all (GenericClass without any <>) loses all generic type safety so it can be used safely with pre-generic code.

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