How would the compiler know that String is involved?  That would only be known
at run time.

In this case, all the type-resolution mechanism can tell is that T is some
Object type.  T might not have a no-arg constructor, so the compiler cannot
tell that 'new T()' is a valid constructor.  You can pass in a Class <?
extends T> object to provide run-time type information (RTTI):

<example>
package testit;

import java.util.logging.Level;
import java.util.logging.Logger;

public class General <T>
{
  private final Class <? extends T> clazz;
  private final T ob;

  public General( Class<? extends T> clazz )
  {
    try
    {
      ob = clazz.newInstance();
    }
    catch ( InstantiationException ex )
    {
      Logger.getLogger( getClass().getName() )
            .log( Level.SEVERE, null, ex );
      throw new RuntimeException( "Illegal constructor", ex );
    }
    catch ( IllegalAccessException ex )
    {
      Logger.getLogger( getClass().getName() )
            .log( Level.SEVERE, null, ex );
      throw new RuntimeException( ex );
    }
    this.clazz = clazz;
  }
}
</example>

http://java.sun.com/docs/books/tutorial/java/generics/erasure.html

T is a placeholder that ONLY exists to automatically cast things, and
to check that it's safe to do so.  It inserts casts around the uses, but the
underlying type in the code is Object (or the supertype specified in something
like <T extends CharSequence>).

Let's also give it a method
 T getObj() { return ob; }

The Gen class is one set of code, and it hast to work correctly with all the
following:

 Gen<Object> obgGen = new Gen<Object>();
 Object o = objGen.getObj();

 Gen<String> strGen = new Gen<String>();
 String s = strGen.getObj();

 Gen<Integer> intGen = new Gen<Integer>();
 Integer i = intGen.getObj();

To do so, it cannot keep a member with a specific type, or it would work with
one and not others.  It keeps an Object, and casts it as necessary.  Generic
use that can't be accomplished with a cast isn't allowed.

This means you can't new up things of a specific type, because that exact
non-type-specific code has to work with multiple type specifiers.  

The common workarounds are to use a template or factory in the Gen class, that
is specified as an argument to the thing that needs to construct objects.

class Gen<T extends Clonable> {
   private T ob;
   public Gen(T templateObject) {
       ob = templateObject.clone()
   }
   ...
}

You can do similar stuff by reflecting on the templateObject, or by passing a
factory, as in:

public class Foo {
   public interface Factory<U> {
       public U getInstance();
   }

   public static class Gen<T> {
       private T obj;
       Gen(Factory<T> factory) {
           obj = factory.getInstance();
       }

       T getObj() { return obj; };
   }

   public static void main(String[] args) {
       Gen<String> stringGen = new Gen<String>(new Factory<String>() {
           public String getInstance() { return "foofoo"; }
       });

       System.out.println(stringGen.getObj());
   }
}
--
Mark Rafn    dagon@dagon.net    <http://www.dagon.net/>