Re: How should I re-write this?

From:

Lew <lew@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 26 Mar 2010 12:29:18 -0700 (PDT)

Message-ID:

<d892a754-db9c-4434-b00e-358a17810b28@30g2000yqi.googlegroups.com>

Fencer wrote:

Hello, consider the following program:

package action;

public class MyTest {

    public static void main(String[] args) {
       try {
          ClassLoader cl = MyTest.class.getClassLoader();
          Class<?> c = cl.loadClass("action.SomeClass");
          SomeClass inst = (SomeClass)c.newInstance();
          System.out.println(inst);
          SomeClass inst2 = MyTest.loadClass("action.SomeClas=

s");

          System.out.println(inst2);
       } catch (ClassNotFoundException e) {
          e.printStackTrace();
       } catch (InstantiationException e) {
          e.printStackTrace();
       } catch (IllegalAccessException e) {
          e.printStackTrace();
       }
    }

    public static <T> T loadClass(String className) {
       T inst = null;
       try {
          ClassLoader cl = MyTest.class.getClassLoader();
          Class<?> c = cl.loadClass(className);
          inst = (T)c.newInstance();
       } catch (ClassNotFoundException e) {
          e.printStackTrace();
       } catch (InstantiationException e) {
          e.printStackTrace();
       } catch (IllegalAccessException e) {
          e.printStackTrace();
       }

       return inst;
    }

}

class SomeClass {
    @Override public String toString() {
       return "1337";
    }

}

I'm getting a warning on line 27 for the following statement:
inst = (T)c.newInstance();
The warning reads:
Type safety: Unchecked cast from capture#4-of ? to T

Ok, I think I get it. The compiler cannot know at compile time that the
cast will work. But if you look at the code in the main method, I don't
get a warning on line 11:
SomeClass inst2 = MyTest.loadClass("action.SomeClass");
I thought the above line has the same problem as the one that is being
warned about?

No, because here you are (unsafely) telling the compiler what the 'T'
of the generic method is.

I have two questions:
1. Why don't I get a warning for line 11
SomeClass inst2 = MyTest.loadClass("action.SomeClass"); ?

Because the compiler infers 'T' from the declaration.
'ClassLoader#loadClass()' is not generically parametrized so cannot do
that.

2. How should I rewrite the generic method loadClass?

Well, you really don't give the method enough information to prevent a
'ClassCastException', so the generic parameter 'T' is essentially a
lie. I'm not sure that you can reliably generify it without catching
the 'ClassCastException'.

--
Lew