问题
The sample method is given below:
static <T> void doSomething(List<? super T> list1, List<? extends T> list2) { }
I am wondering what type will be inferred in this situation and by what logic. Let's say I am calling this method like this:
doSomething(new ArrayList<Object>(), new ArrayList<String>());
Would T type evaluate as Object or String?
回答1:
This call does not bind T to a specific class. Java does not need to know the exact T because of type erasure implementation of the generics. As long as the types that you pass are consistent with the declaration, the code should compile; in your case, lists of Object and String are consistent with the declaration.
Let's expand your code a little so that we could force binding of T to a specific type. Perhaps the easiest way to do it is to pass Class<T>, like this:
static <T> void doSomething(List<? super T> list1, List<? extends T> list2, Class<T> cl) {
System.out.println(cl);
}
Now let us try calling doSomething with String.class and with Object.class:
doSomething(new ArrayList<Object>(), new ArrayList<String>(), Object.class);
doSomething(new ArrayList<Object>(), new ArrayList<String>(), String.class);
Both calls successfully compile, producing the output
class java.lang.Object
class java.lang.String
Demo on ideone.
来源：https://stackoverflow.com/questions/24350273/confusion-over-java-generic-method-type-inference