Java中的通用方法未返回预期的类型

Question

I have created a generic method toList():

public static <T> List<T> toList(T... args){
        List<T> list = new ArrayList<T>();
        for (T t : args) {
            list.add(t);
        }

        if(list instanceof Comparable<?>){
            System.out.println("The list was considered a list of Comparable objects");
        }
        if(list instanceof Serializable){
            System.out.println("The list was considered a list of Serializable objects");
        }
        if(list instanceof Object){
            System.out.println("The list was considered a list of Object class' objects");
        }
        return list;
    }

I am calling this method like :

    public static void main(String[] args) {

        System.out.println(Generics.<Comparable>toList(1,2,"3"));
  }

I was expecting that since I have supplied the type Comparable, the first if condition would hold true and o/p would be first Comparable, then Object. But to my surprise, control didn't go in first if and the o/p was Serializable and then Object.

Can anyone throw some light on the reason behind this? Also I am not clear with the concept of applying T in the method signature after the access modifier.

We have mentioned the access specifier, modifier and the return type. What is the need of supplying T ?

Answer 1

You are performing your assertions on the list object and not on the elements of the list.

list instanceof Comparable<?>

This will only return true if the list itself would implement the Comparable interface. As ArrayList is serializable and of course is an object the other two conditions will evaluate to true .

Answer 2

It seems to me what you are trying to do is test the element type, T , inside your method. Because of type erasure (follow the link) you cannot do this. The actual type of T is only known at compile time, not on runtime. On runtime, inside the method, Java will just think T is Object .