An interesting thing about java generics-do not laugh at me if u think it too basic - 未名空间MITBBS历史存档

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An interesting thing about java generics-do not laugh at me if u think it too basic

An interesting thing about java generics-do not laugh at me if u think it too basic# Java - 爪哇娇娃

w*d2008-06-18 07:06

1 楼

我常重复做梦自己在一栋老家的房子里狂跑，后面一直有人在追我，具体谁不知道，梦
里觉得很害怕，我就一直跑，告诉自己只要离开这个屋子就安全了，可是跑到天亮也没
有跑出去，这能说明什么吗?这个梦重复作了好几年，这里能给解释解释吗?

h*o2008-06-18 07:06

2 楼

public class Base {
private Class persistentClass;

public Base() {
Class clazz = getClass();
this.persistentClass = (Class) ((ParameterizedType)
clazz.getGenericSuperclass()).getActualTypeArguments()[0];

}
}
It seems that there is no way you can use it as follows:
Base b = new Base();
to use it you have to define a subclass specifying the type parameter just
as you do in C++ template.
BaseDerative extends Base....
NOT:

i*n2008-06-18 07:06

3 楼

你的前世在那个屋子里练长跑来着。

m*t2008-06-18 07:06

4 楼

Interesting... note though that the "String" you get is not the "String"
from "BaseDeritive", but the one from "Base".
In other words, it'll work even with "BaseDeritive extends Base".

w*d2008-06-18 07:06

5 楼

假半仙。。。。。

【在 i*******n 的大作中提到】

: 你的前世在那个屋子里练长跑来着。

h*o2008-06-18 07:06

6 楼

rite.
That is exactly how Hibernate does.
Do not know under hood how jvm handles this thing.

i*n2008-06-18 07:06

7 楼

嘿嘿，被你看出来了。
不过说到梦，我想起了原来看过的一部小说，好像名字叫“梦境”，倪匡写的，比较诡
异。

【在 w********d 的大作中提到】

: 假半仙。。。。。

F*n2008-06-18 07:06

8 楼

It's not a JVM implementation issue. It's simply the way Generics should be.
Remeber Java Generics is a compile time thing rather than runtime thing. In
other words, if you declare Base, then the parameter of this class is a
TypeVariable.
Even if you define a new Base variable, its type parameter is still
a TypeVariable because Base only help you check type at compile type
. It will not changed the parameter of Base into a String.
Consequently, your original code will throw

【在 h*********o 的大作中提到】

: public class Base {
: private Class persistentClass;
:
: public Base() {
: Class clazz = getClass();
: this.persistentClass = (Class) ((ParameterizedType)
: clazz.getGenericSuperclass()).getActualTypeArguments()[0];
:
: }
: }