Marcelo,
If instead of GenericTest<E> you replace with e.g. Map<E>
what I interpret you are saying is that you can't tell
from the (runtime) Map object what type <E> it is meant to be used
with.
However, the client of such an object *will* know the type, like this:
public Map<String,String> map1 = new HashMap<String,String>();
Wouldn't a client using GenericTest<E> not include GenericTest<String>
at compile time?
Am I still missing something here?
On Nov 11, 4:42 pm, "Marcelo Fukushima" <[EMAIL PROTECTED]> wrote:
> 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 <[email protected]>
>
> > > 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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