Java Generics中存储的类型信息在哪里？

Question

For a generic class, where is the type information stored and how is it enforced at run time. For example, if I a create a List instance as: List<String> strList = new List<String>(); , this means that 'strList' can be used only with String type. But where is that information stored or enforced? Is it at class metadata level or at the instance level? If I create another List instance of type: List<Long> longList = new List<Long>(); how does java make sure that 'strList' can only deal with String and 'longList' can only deal with Long. I have the same question for generic methods. Each time a generic method is invoked with a different type, how do the type checks work?

Answer 1

But where is that information stored or enforced?

It is stored in the source code, and enforced in the compiler.

The type checks are easy to break, resulting in runtime failures:

List<String> strList = new ArrayList<String>();

List<Integer> intList = (List<Integer>) (List<?>) strList;
intList.add(0);

String str = strList.get(0); // ClassCastException.

However, this is not silently broken: you will get an unchecked cast warning, telling you that the compiler can't be sure whether you are doing something unsafe. The burden is on you either to avoid such unchecked casts (best), or to reason about your code to ensure that the unsafe cast is actually safe.