Unexpected behavior using generics with Java Collections

Question

Something weird i've just realized:

Java documentation states that List collection has a method T get(int index) ... as you see the method returns T

However i can do:

List<Integer> l1 = new ArrayList<>();
l1.add(1);
List l2 = l1;
l2.add("Hello my friend");
Object o2 = l1.get(1);
System.out.println(o2);

And the result is "Hello my friend" !! ... this does not comply with stated in documentation since the result shall be Integer!

Is there any other possible explanaition?

Answer 1

Generics are compile-time checks. All bets are off once you start using raw types...which you are. This is why raw types are dangerous, and should never be used in new code.

Answer 2

List l2不是通用的，因此它基本上是List<Object> ，当您使用非通用列表时，添加到列表中的所有内容都将成为Object ，由您来正确使用它们，否则您将得到ClassCastException 。

Answer 3

如果您将l2声明为泛型，那么您将得到一个编译时错误（如果您的编译器配置为抱怨这些事情）。

final List<Integer> l2 = l1;