How to use genericized Class class

[hiwa]

In the pre-generics era, we did:

Class claz = Class.forName("Foo");
Foo foo = (Foo)(claz.newInstance());

We anticipate we can be freed from casting by generics:

Class<Foo> claz = Class.forName("Foo"); // line 1
Foo foo = claz.newInstance();

But compiler emits 'incompatible types' error for the line 1 above.
We try another:

Class<Foo> claz = (Class<Foo>)(Class.forName("Foo")); // line 1
Foo foo = claz.newInstance();

Then, compiler emits 'unchecked cast' warning for the line 1 above.

What should be the correct syntax for getting a Class<Foo> object
from the forName() method without errors nor warnings?

Thanks in advance.

 

[Chris Smith]

What should be the correct syntax for getting a Class<Foo> object
from the forName() method without errors nor warnings?

You can't. Generics allow you to avoid casts in situations where the
compiler can check that type safety is preserved. In cases like that
above where no such compiler check is possible, the cast is just as
necessary as it always was.

On the other hand, a literal like:

Class<Foo> clz = Foo.class;
Foo f = clz.newInstance();

will work fine, since the Class<Foo> type is compiler-checked.

--
www.designacourse.com
The Easiest Way To Train Anyone... Anywhere.

Chris Smith - Lead Software Developer/Technical Trainer
MindIQ Corporation

 

[Tony Morris]

In the pre-generics era, we did:

Class claz = Class.forName("Foo");
Foo foo = (Foo)(claz.newInstance());

We anticipate we can be freed from casting by generics:

Class<Foo> claz = Class.forName("Foo"); // line 1
Foo foo = claz.newInstance();

But compiler emits 'incompatible types' error for the line 1 above.
We try another:

Class<Foo> claz = (Class<Foo>)(Class.forName("Foo")); // line 1
Foo foo = claz.newInstance();

Then, compiler emits 'unchecked cast' warning for the line 1 above.

What should be the correct syntax for getting a Class<Foo> object
from the forName() method without errors nor warnings?

Thanks in advance.

final Class<?> c = Class.forName(x);

if(Foo.class.isAssignableFrom(c))
{
Class<Foo> cf = c.asSubclass(Foo.class);
}


Take a look at the JTiger source code for more examples.
http://www.jtiger.org/

--
Tony Morris
Software Engineer, IBM Australia.
BInfTech, SCJP 1.4, SCJD

http://www.jtiger.org/ JTiger Unit Test Framework for Java
http://qa.jtiger.org/ Java Q&A (FAQ, Trivia)
http://xdweb.net/~dibblego/