为什么这个通用的Java方法接受两个不同类型的对象? [英] Why does this generic java method accept two objects of different type?

问题描述

此方法应获取两个相同类型的对象，并随机返回其中一个对象:

This method shall take two objects of the same type and return one of those objects by random:

public static <T> T random(T o1, T o2)
{
   return Math.random() < 0.5 ? o1 : o2;
}

现在，为什么编译器会接受两个具有不同类型的参数?

Now, why does the compiler accept two parameters with distinctive types?

random("string1", new Integer(10)); // Compiles without errors

既然我知道两个参数都被隐式地转换了，我想知道为什么编译器在调用以下方法时会确实抱怨:

Now that I know that both parameters are getting implicitly upcasted, I wonder why the compiler does complain when calling the following method:

public static <T> List<T> randomList(List<T> l1, List<T> l2) {
        return Math.random() < 0.5 ? l1 : l2;
    }

致电:

randomList(new ArrayList<String>(), new ArrayList<Integer>()); // Does not Compile

如果这些ArrayList参数也被转换为对象，为什么这次会给我一个错误?

If those ArrayList Parameters are also getting upcasted to Object, why does it give me an error this time?

推荐答案

T推断为Object，并且两个参数都隐含地被抛弃.

T is inferred to be Object, and both arguments are getting implicitly upcast.

因此，代码等同于:

Main.<Object>random((Object)"string1", (Object)new Integer(10));

更令人惊讶的是，以下编译:

What may be even more surprising is that the following compiles:

random("string1", 10);

第二个参数将自动装箱到Integer中，然后两个参数都被转换为Object.

The second argument is getting auto-boxed into an Integer, and then both arguments are getting upcast to Object.

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