Object的
“public final Class<?> getClass()”方法的文档说：
The actual result type is Class<? extends |X|> where |X| is the
erasure of the static type of the expression on which getClass is
called. For example,no cast is required in this code fragment:
我不明白解释,特别是关于什么| X |被认为是 – “删除getClass被调用的表达式的静态类型”.
| X |表示什么形式？或者也许,其他地方| X |使用类型符号？
Java语言规范
mandates这种方法由编译器以特殊的方式处理：
The method getClass returns the Class object that represents the class of the object.
A Class object exists for each reference type. It can be used,for example,to discover the fully qualified name of a class,its members,its immediate superclass,and any interfaces that it implements.
The type of a method invocation expression of getClass is Class<? extends |T|> where T is the class or interface searched (§15.12.1) for getClass.
因此,getClass的返回类型是调用getClass()的表达式的静态(编译时)类型.例如：
String s = "";
Object o = s;
Class<? extends String> sc = s.getClass(); // ok
Class<? extends Object> oc = o.getClass(); // ok
oc = sc; // ok
sc = o.getClass(); // not ok
sc = oc; // not ok
符号| X |是defined的规格如下：
Type erasure is a mapping from types (possibly including parameterized types and type variables) to types (that are never parameterized types or type variables). We write |T| for the erasure of type T. The erasure mapping is defined as follows:
-
The erasure of a parameterized type (§4.5) G<T1,...,Tn> is |G|.
-
The erasure of a nested type T.C is |T|.C.
-
The erasure of an array type T[] is |T|[].
-
The erasure of a type variable (§4.4) is the erasure of its leftmost bound.
-
The erasure of every other type is the type itself.
例如,如果我们有：
List<String> list = ...;
表达式list.getClass()的类型为<？扩展列表>而不是Class扩展List< String>>.
