Generics with static methods

Question

How this works without any Exceptoin ? Because T must be same in this case but one is String and another one is ArrayList<Integer> .

public static void main(String[] args) {
    Serializable s = pick("d", new ArrayList<Integer>());   
    System.out.println("s:"+s);
}
static <T> T pick(T a1, T a2) {
    return a2;
}

Answer 1

The compiler uses type inference to determine the type of T . It picks the most specific type that works for all types considered. Here, the type of s is Serializable , and you pass in a String and an ArrayList<Integer> . Both String and ArrayList are Serializable , with no other relation, so the inferred type for T is Serializable .

Answer 2

由于您返回类型为T的对象并将其存储到Serializable类型的变量中，我猜编译器会在您的调用中推断出T是Serializable，因此String和ArrayList都有资格成为pick的参数。