User.class可以在編譯時就確定下來Class的泛型，而new User().getClass()實際上是運行時才能確定下來實際是什么泛型。舉個例子：

public class User{

}

public class Student extends User{

public static void main(String[] args) {

User user1 = new User();

User user2 = new Student();

//輸出User

System.out.println(user1.getClass());

//輸出Student

System.out.println(user2.getClass());

}

}

由此可見，Object.getClass()方法返回的Class泛型是運行時才能確定的,所以返回的類型是Class>,順帶貼下jdk源碼上的注釋

/**

* Returns the runtime class of this {@code Object}. The returned

* {@code Class} object is the object that is locked by {@code

* static synchronized} methods of the represented class.

*

*

The actual result type is {@code Class extends |X|>}

* where {@code |X|} is the erasure of the static type of the

* expression on which {@code getClass} is called. For

* example, no cast is required in this code fragment:

*

*

* {@code Number n = 0; }

* {@code Class extends Number> c = n.getClass(); }

*

*

* @return The {@code Class} object that represents the runtime

* class of this object.

* @see Class Literals, section 15.8.2 of

* The Java? Language Specification.

*/

public final native Class> getClass();

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