actually my point was that you can only get the generic type
information in the cases where it is there in compile time
but, for instance, imagine that you had this:
public class GenericTest<E> {
private List<E> list = new ArrayList<E>();
}
even if you create an instance of GenericTest with String, the generic
type of the list field would be ParameterizedType and theres
absolutely no way to get the String parameter you passed in the
constructor - thats because in java, generics are implemented with
erasure instead of reified types, in which case you could fetch the
type information
On 11/11/08, Tasos Zervos <[EMAIL PROTECTED]> wrote:
>
> Can you please give an example with some bits of code?
> I'm not sure what scenario you are trying to solve (or implying you
> can't solve).
> What are you going to use a Map<?,?> instance field for?
> (Would this be a "Generics" way of initialise a field?)
>
>
> On Nov 10, 10:27 pm, "Marcelo Fukushima" <[EMAIL PROTECTED]> wrote:
> > just a quick observation: it only works because the maps you're trying
> > to extract the generic info from, contains such information in the
> > containing class file (the GenericTest fields specifically) - try
> > changing the field types to Map<?, ?> and you wont the get info
> > anymore, even tough you created the maps with the type information
> >
> > ---------- Forwarded message ----------
>
> > From: Tasos Zervos <[EMAIL PROTECTED]>
> > Date: Mon, Nov 10, 2008 at 10:48 AM
> > Subject: [The Java Posse] Re: episode 215: reflection and generics
> > To: The Java Posse <javaposse@googlegroups.com>
> >
> > If you aren't keen to follow the article try this:
> >
> > import java.lang.reflect.*;
> > import java.util.*;
> >
> > public class GenericTest
> > {
> > public HashMap<String,String> map1 = new HashMap<String,String>();
> > public HashMap<Date,Date> map2 = new HashMap<Date,Date>();
> >
> > public static String fieldSignature(Object obj, String fieldName)
> > throws SecurityException,
> > NoSuchFieldException
> > {
> > StringBuffer signature = new StringBuffer(fieldName);
> > Field field = GenericTest.class.getField(fieldName);
> > Type genericFieldType = field.getGenericType();
> > if(genericFieldType instanceof ParameterizedType)
> > {
> > signature.append(" is of type
> > ").append(obj.getClass().getName()).append("<");
> > ParameterizedType aType = (ParameterizedType)
> > genericFieldType;
> > Type[] fieldArgTypes = aType.getActualTypeArguments();
> > for(Type fieldArgType : fieldArgTypes)
> > {
> > Class fieldArgClass = (Class) fieldArgType;
> > signature.append(fieldArgClass.getName()).append(",");
> > }
> > signature.append(">");
> > }
> > return signature.toString();
> > }
> >
> > public static void main(String[] args) throws SecurityException,
> > NoSuchFieldException
> > {
> > GenericTest test = new GenericTest();
> > System.out.println("map1.getClass() == map2.getClass() is "
> > + (test.map1.getClass() == test.map2.getClass()) );
> > System.out.println("map1.getClass() is " +
> > test.map1.getClass());
> > System.out.println("map2.getClass() is " +
> > test.map2.getClass());
> > System.out.println(GenericTest.fieldSignature(test.map1,
> > "map1"));
> > System.out.println(GenericTest.fieldSignature(test.map2,
> > "map2"));
> > }
> > }
> >
> > My quick test shows:
> > map1.getClass() == map2.getClass() is true
> > map1.getClass() is class java.util.HashMap
> > map2.getClass() is class java.util.HashMap
> > map1 is of type java.util.HashMap<java.lang.String,java.lang.String,>
> > map2 is of type java.util.HashMap<java.util.Date,java.util.Date,>
> >
> > :-D
> >
> > Tasos
> >
> > On Nov 10, 12:46 pm, Tasos Zervos <[EMAIL PROTECTED]> wrote:
> > > The equality test has to return true otherwise you would be breaking
> > > compatibility with older code.
> > > This doesn't mean that there aren't other ways to find the "signature"
> > > of map1 and map2.
> >
> > > The Reflection API does provide access to the "specific" types of
> > > generic signatures.
> >
> > > Have a look at this 2005
> article:http://www-128.ibm.com/developerworks/java/library/j-cwt11085.html#h2
> >
> > > The article series uses ASM (DRY) also later on.
> >
> > > On Nov 4, 11:05 pm, Christian Catchpole <[EMAIL PROTECTED]>
> > > wrote:
> >
> > > > Here is my analysis of the situation. I could be wrong. But here
> > > > goes..
> >
> > > > When I got my copy of Java 5 my first question was, do generics really
> > > > take the cast out of the equation? I disassembled the code to find
> > > > the cast still exists. This implies that when you compile this..
> >
> > > > HashMap<String,String> map = new HashMap<String,String>()
> > > > String string = map.get("");
> >
> > > > The generated code actually equates to this..
> >
> > > > HashMap map = new HashMap()
> > > > String string = (String)map.get("");
> >
> > > > The class returned by map.getClass() does not know the map only
> > > > contains Strings. It's actually the reference to the map which
> > > > marshals the types.
> >
> > > > I did a quick test...
> >
> > > > HashMap<String,String> map1 = new HashMap<String,String>();
> > > > HashMap<Date,Date> map2 = new HashMap<Date,Date>();
> >
> > > > System.out.println(map1.getClass() == map2.getClass());
> >
> > > > true
> >
> > > > They use the same class and can't therefore hold the type information
> > > > for both declarations.
> >
> > > > I can only assume this re-compiler the posse were talking about, scans
> > > > the code for the actual cast / type check to determine the types.
> >
> > --
>
> > []'s
> > Marcelo Takeshi Fukushima
>
> >
>
--
[]'s
Marcelo Takeshi Fukushima
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