Generics' types at runtime (JVM)

Hi everybody
This is gonna be a tricky question I guess.
I'm using generics for an application of mine and I need to know the actual types of a class at runtime.
For example, let's say I have: "Hashtable<String, Integer> ht;"
Using Reflection I can easily find out that 'ht' is an 'Hashtable' as well as I can create an instance of Hashtable.
But if I use 'ht.getTypeParameters()' to find out the type of the generics, I get '<K,V>' instead of
'<String,Integer>', this because 'ht.getTypeParameters()' returns the formal types and not the actual types.
In the same way, "Class.forName("java.util.Hashtable");" works, while
"Class.forName("java.util.Hashtable<String,Integer>");" throws an exception because it can't find the class,
Does anybody of you know if and how it is possible to access actual generics types at runtime?
Thanks in advance
michele

Not possible. That's why we call it erasure ;-)
There is an RFE to reify generic type arguments at runtime:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5098163

Similar Messages

Getting generic type at runtime

Example:
public class GenericTest {
private List<String> list = new List<String>();
How can I get the type String at runtime?
Class c = GenericTest.class;
Field[] fiels = c.getFields();
for (Field f: fields) {
Class fc = f.getDeclaringClass();
// What are the next steps to to get String.class from fc?
}

Generic types (ie. the ones inside the < >) get Erased at compile-time. All this type information is therefore not available at run-time
Google "Java Generics Erasure" to get some more info. (There's a Generics forum by the way!)

How to create an array with Generic type?

Hi,
I need to create a typed array T[] from an object array Object[]. This is due to legacy code integration with older collections.
The method signature is simple:public static <T> T[] toTypedArray(Object[] objects)I tried using multiple implementations and go over them in the debugger. None of them create a typed collection as far as I can tell. The type is always Object[].
A simple implementation is just to cast the array and return, however this is not so safe.
What is interesting is that if I create ArrayList<String>, the debugger shows the type of the array in the list as String[].
If I create ArrayList<T>, the class contains Object[] and not T[].
I also triedT[] array = (T[]) Array.newInstance(T[].class.getComponentType(), objects.length);And a few other combinations. All work at runtime, create multiple compilation warnings, and none actually creates T[] array at runtime.
Maybe I am missing something, but Array.newInstace(...) is supposed to create a typed array, and I cannot see any clean way to pass Class<T> into it.T[].class.getComponentType()Returns something based on object and not on T, and T.class is not possible.
So is there anything really wrong here, or should I simply cast the array and live with the warnings?
Any help appreciated!

Ok. May be you could keep information about generic type in the your class:
public class Util {
    public static <T> T[] toTypedArray(Class<T> cls, Object[] objects){
        int size = objects.length;
        T[] t = (T[]) java.lang.reflect.Array.newInstance(cls, size);
        System.arraycopy(objects, 0, t, 0, size);
        return t;
public class Sample<T> {
    Class<T> cls;
    T[] array;
    public Sample(Class<T> cls) {
        this.cls = cls;
    public void setArray(Object[] objects){
        array = Util.toTypedArray(cls, objects);
    public T[] getArray(){
        return array;
    public static void main(String[] args) {
        Object[] objects = new Object[] { new LinkedList(), new ArrayList()};
        Sample<List> myClass = new Sample<List>(List.class);
        myClass.setArray(objects);
        for(List elem: myClass.getArray()){
            System.out.println(elem.getClass().getName());
}

Instantiation of generic type

Hi.
I have an issue with instantiating of generic type. My issue is similar to this Re: creating instance of generic type post.
So I have the following class
public class VehicleForm<V extends Vehicle>{
private V vehicle;
private Double price;
private Class<V>vItemClass;
public V getVehicle(){
return this.vehicle;
public void setVehicle(V vehicle){
this.vehicle=vehicle;
public Double getPrice(){
return this.price;
public void setPrice(Double price){
this.price=price;
private final Class<V> getGenericClassInstance() {
Class<V> persistentClass = null;
Type genericType = getClass().getGenericSuperclass();
if (genericType instanceof ParameterizedType) {
ParameterizedType pType = ((ParameterizedType) genericType);
// obtaining first generic type class
persistentClass = (Class<V>) pType.getActualTypeArguments()[0];
return persistentClass;
public VehicleForm(){
this.vItemClass=getGenericClassInstance();//vItemClass is null
this.vehicle=this.vItemClass.newInstance();//null poiner exception
I cannot write in default constructor
public VehicleForm(){
this.vehicle=new V();//runtime error will occure
because of generic type
For obtaining generics we can use java reflection functionality. Generic type list in runtime is obtaining through ParameterizedType. The code in getGenericClassInstance is standard to instantiate generic type var.
But unfortunately in my case it doesn't work. getGenericClassInstance returns null. Line Type genericType = getClass().getGenericSuperclass(); doesnt get generic type which is instance of ParameterizedType, so the condition if (genericType instanceof ParameterizedType) is false. My genericType is Object instance, but not ParameterizedType. So I cannot create Class<V> instance from null.
Can anybody help me? Thanks in advance.
Edited by: 877736 on 06.08.2011 12:50
Edited by: 877736 on 06.08.2011 12:51

877736 wrote:
My issue is similar to this Re: creating instance of generic type post...Did you look at the answers given there? Pretty much the same as ttjacobs is telling you here: This is NOT what generics was created for.
Even if you can do it, the code is likely to be messy, complex and brittle.
Also: this thread is in the wrong category, as the other poster (you?) was already told. There is a "Generics" section under Java APIs.
My suggestion: rethink your solution.
Winston

Retrieving generic type and letter

Hi,
I have a generic class -
public class AAA <T extends BBB>implements Serializable From which I want to get its generic type (the easy part),
and the generic type letter representation (In this case - T).
Can I do this by reflection? If so how,
Otherwise - is there any way I can do that which doesn't involve parsing the class as text?
Thanks,
Sharon.

Class.getGenericDeclaration() should lead you to the letter T.
The actual runtime type cannot be obtained because of erasure.
If the class is under your control, you can play a trick:
add to the constructor(s) a Class parameter and pass the actual type
used.

Creating instance of generic type

Hi.
Here is my class
public class ManagerForm<M extends Stuff>{
private Class<M>clazz;
private M manager;
public void setWorker(M manager){
this.manager=manager;
public M getWorker(){
return this.manager;
private final Class<M> getGenericClass() {
Class<M> persistentClass = null;
Type genericType = getClass().getGenericSuperclass();
if (genericType instanceof ParameterizedType) {
ParameterizedType pType = ((ParameterizedType) genericType);
// obtaining first generic type class
persistentClass = (Class<M>) pType.getActualTypeArguments()[0];
return persistentClass;
public ManagerForm(){
this.clazz=getGenericClass();
this.manager=this.clazz.newInstance();
In default constructor I need to inicialize U instance this.user.
I have an nullPointerException in line this.manager=this.clazz.newInstance();
persistantClass is returned nullable because genericType is not instance of ParameterizedType.
Could anybody say me what is wrong or propose other methods to create generic instance. I need help. Will be very appreciable.
Thanks in advance.

YoungWinston wrote:
877715 wrote:
Could anybody say me what is wrong or propose other methods to create generic instance. I need help. Will be very appreciable. I think you need to back up and explain exactly what you're trying to do here.
It would appear that you're attempting to divine the actual type of a specific ManagerForm, but I'm not absolutely certain. Also, since you already specify the type as a field, why can't you just usemanager.getClass()? I suspect that all you'll get from getClass().getGenericSuperclass() is 'Stuff', but I have to admit I'm no expert on this.
Far better to describe what you want than an implementation that plainly doesn't work.
WinstonWell. I have class ManagerForm<M extends Stuff>. there is private field manager that have generic type M. And I need to instantinate manager in default constructor.
I cant write
public ManagerForm(){
this.manager=new M();//will be compiler error
Thats why exists a mechanism to get current generic type in runtyme through ParameterType.
I have method getGenericClass that gets current class in runtime and then I create an instance of this class through <class>.newInstance(); And this is the standard way to instantinate generic type.
I hoped line Type genericType = getClass().getGenericSuperclass(); of method returned ParameterizedType. But variable genericType cannot be casted to ParameterizedType. I don't know why. So I cannot get current generic param M and instantiate its.

How can we give the Data Format (File Type ) in Runtime

Hi all,
How can we give the Data Format (File Type ) in Runtime for the following method,
cl_gui_frontend_services=>gui_download.
Thanks in advance
Sri

There is a filetype parameter which you can set
CALL METHOD cl_gui_frontend_services=>gui_download
EXPORTING
*    BIN_FILESIZE              =
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*    FILETYPE                  = 'ASC'
*    APPEND                    = SPACE
*    WRITE_FIELD_SEPARATOR     = SPACE
*    HEADER                    = '00'
*    TRUNC_TRAILING_BLANKS     = SPACE
*    WRITE_LF                  = 'X'
*    COL_SELECT                = SPACE
*    COL_SELECT_MASK           = SPACE
*    DAT_MODE                  = SPACE
*    CONFIRM_OVERWRITE         = SPACE
*    NO_AUTH_CHECK             = SPACE
*    CODEPAGE                  = SPACE
*    IGNORE_CERR               = ABAP_TRUE
*    REPLACEMENT               = '#'
*    WRITE_BOM                 = SPACE
*    TRUNC_TRAILING_BLANKS_EOL = 'X'
* IMPORTING
*    FILELENGTH                =
changing
    data_tab                  =
* EXCEPTIONS
*    FILE_WRITE_ERROR          = 1
*    NO_BATCH                  = 2
*    GUI_REFUSE_FILETRANSFER   = 3
*    INVALID_TYPE              = 4
*    NO_AUTHORITY              = 5
*    UNKNOWN_ERROR             = 6
*    HEADER_NOT_ALLOWED        = 7
*    SEPARATOR_NOT_ALLOWED     = 8
*    FILESIZE_NOT_ALLOWED      = 9
*    HEADER_TOO_LONG           = 10
*    DP_ERROR_CREATE           = 11
*    DP_ERROR_SEND             = 12
*    DP_ERROR_WRITE            = 13
*    UNKNOWN_DP_ERROR          = 14
*    ACCESS_DENIED             = 15
*    DP_OUT_OF_MEMORY          = 16
*    DISK_FULL                 = 17
*    DP_TIMEOUT                = 18
*    FILE_NOT_FOUND            = 19
*    DATAPROVIDER_EXCEPTION    = 20
*    CONTROL_FLUSH_ERROR       = 21
*    NOT_SUPPORTED_BY_GUI      = 22
*    ERROR_NO_GUI              = 23
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Reference to generic type Set E should be parmeterized

I have the following code in a class:
1:Event theEvent = (Event) session.load(Event.class, eventId);
            2:Set<Event> eventSet;
            3:eventSet = user.getFavouriteEvents();
            4:eventSet.add(theEvent);At line 3, I get the following warning message:
Type safety: The expresson of raw type Set is converted to Set<Event>. References to generic type Set<E> should be parameterized.
What does this mean? What do I need to do to get past this warning.
Thanks
Sharma

Found the problem. I forgot to parameterize the Set in the getFavouriteEvents() method.
Thanks
Sharma

Import from database an internal table with generic Type : Web Dynpro ABAP

Hi everyone,
i have a requirement in which i'm asked to transfer data flow between two frameworks, from WD Component to another. The problem is that i have to transfer internal tables with generic types. i used the import/ export from database approache but in that way i get an error message saying "Object references and data references not yet supported".
Here is my code to extract a generic internal table from memory.
    DATA l_table_f4 TYPE TABLE OF REF TO data.
FIELD-SYMBOLS: <l_table_f4> TYPE STANDARD TABLE.
DATA lo_componentcontroller TYPE REF TO ig_componentcontroller .
DATA: ls_indx TYPE indx.
lo_componentcontroller =   wd_this->get_componentcontroller_ctr( ).
lo_componentcontroller->fire_vh_search_action_evt( ).
ASSIGN l_table_f4 TO <l_table_f4>.
*-- Import table for Help F4
IMPORT l_table_f4 TO <l_table_f4> FROM DATABASE indx(v1) TO ls_indx ID 'table_help_f4_ID'.
The error message is desplayed when last instruction is executed " IMPORT l_table_f4...".
I saw another post facing the same problem but never solved "Generic Type for import Database".
Please can anyone help ?
Thanks & Kind regards.

hi KIan,
go:
general type
TYPE : BEGIN OF ty_itab,
           field1 TYPE ztab-field1,
           field2 TYPE ztab-field2,
*your own fields here:
           field TYPE i,
           field(30) TYPE c,
           END OF ty_itab.
work area
DATA : gw_itab TYPE ty_itab.
internal table
DATA : gt_itab TYPE TABLE OF ty_itab.
hope this helps
ec

Java5: Can't find a way to get rid of warnings about generic types

Hi all,
i have this method:
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        list.add(obj);
        int j = list.size() - 1;
        for (; j > index; j--) {
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Type safety: The method add(Object) belongs to the raw type List. References to generic type List<E> should be parameterized
I can't find a way to rewrite this method (Java5), that i get no warnings compiling it and calling it with:
List<AnyClass> list = new ArrayList<AnyClass>();
list.add(new AnyClass());
AnyClass ac = new AnyClass();
insert(list, 0, ac);May be you know another way to insert an element to a List?
Thanks

Never tried it and do not have 5.0 installed to test
it, but what if you define the ArrayList as follows?
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'ResourceDictionary' root element is a generic type and requires a x:Class attribute to support the x:TypeArguments attribute sp

Error : 'ResourceDictionary' root element is a generic type and requires a x:Class attribute to support the x:TypeArguments attribute specified on the root element tag.
Hi,
I get this error when i include some namespaces in my ResourceDictionary to specify a Style for a custom control.
Can anyone help me?
Thx
Stardusty

Hi,
That's the whole point. I don't want to use x:TypeArguments on a ResourceDictionary but the compiler says it needs it.
And i don't know why.
This code give no error:
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:sys="clr-namespaceystem;assembly=mscorlib">
</ResourceDictionary>
And by adding 3 namespaces it gives that weard error:
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:controls="clr-namespace:MyTime.View.Controls"
xmlns:converters="clr-namespace:MyTime.View.Converters"
xmlns:validationrules="clr-namespace:MyTime.View.ValidationRules"
xmlns:sys="clr-namespaceystem;assembly=mscorlib">
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Generic type doesn't support raw dataypes (int, boolean, etc.)?

It seems that Java 5 generics always autoboxes raw datatypes (int, double, boolean, ...) to the wrapper classes (Integer, Double, Boolean, ...), because when I use e.g. Integer.TYPE (instead of Integer.class), I got ClassCastExceptions. Is this non-support somewhere written down or is there a way to handle raw types without autoboxing them to their wrapper class?

I'm having some difficulty understanding your question, but my first suggestion is to see section 4.4 Type Variables of the Java Language Specification, which states: "If no bound is given for a type variable, Object is assumed."
Also, I think you mean "primivite" instead of "raw" types. Raw types are the "name of a generic type declaration used without any accompanying
actual type parameters" (sectoin 4.8 Raw Types).
Regards,
Nick

How To: Use reflection to create instance of generic type?

I would like to be able to use reflection to instantiate an instance of a generic type, but can't seem to avoid getting type safety warnings from the compiler. (I'm using Eclipse 3.1.1) Here is a trivial example: suppose I want to create an instance of a list of strings using reflection.
My first guess was to write the following:
Class cls = Class.forName("java.util.ArrayList<String>");
List<String> myList = cls.newInstance();The call to Class.forName throws a ClassNotFoundException. OK, fine, so I tried this:
Class cls = Class.forName("java.util.ArrayList");
List<String> myList = cls.newInstance();Now the second line generates the warning "Type safety: The expression of type List needs unchecked conversion to conform to List<String>".
If I change the second line to
List<String> myList = (List<String>)cls.newInstance();then I get the compiler warning "Type safety: The cast from Object to List<String> is actually checking against the erased type List".
This is a trivial example that illustrates my problem. What I am trying to do is to persist type-safe lists to an XML file, and then read them back in from XML into type-safe lists. When reading them back in, I don't know the type of the elements in the list until run time, so I need to use reflection to create an instance of a type-safe list.
Is this erasure business prohibiting me from doing this? Or does the reflection API provide a way for me to specify at run time the type of the elements in the list? If so, I don't see it. Is my only recourse to simply ignore the type safety warnings?

Harald,
I appreciate all your help on this topic. I think we are close to putting this thing to rest, but I'd like to run one more thing by you.
I tried something similar to your suggestion:public static <T> List<T> loadFromStorage(Class<T> clazz) {
    List<T> list = new ArrayList<T>();
    for ( ...whatever ...) {
       T obj = clazz.newInstance();
       // code to load from storage ...
       list.add(obj);
    return list;
}And everything is fine except for one small gotcha. The argument to this method is a Class<T>, and what I read from my XML storage is the fully qualified name of my class(es). As you pointed out earlier, the Class.forName("Foo") method will return a Class<?> rather than a Class<Foo>. Therefore, I am still getting a compiler warning when attempting to produce the argument to pass to the loadFromStorage method.
I was able to get around this problem and eliminate the compiler warning, but I'm not sure I like the way I did it. All of my persistent classes extend a common base class. So, I added a static Map to my base class:class Base
   private static Map<String, Class<? extends Base>> classMap = null;
   static
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      classMap = Collections.synchronizedMap(map);
   public static Class<? extends Base> getClass(String name)
      return classMap.get(name);
   protected static void putClass(Class<? extends Base> cls)
      classMap.put(cls.getName(), cls);
}And added a static initializer to each of my persistent classes:class Foo extends Base
   static
      Base.putClass(Foo.class);
}So now my persistence code can replace Class.forName("my.package.Foo") with Base.getClass("my.package.Foo"). Since Foo.class is of type Class<Foo>, this will give me the Class<Foo> I want instead of a Class<?>.
Basically, it works and I have no compiler warnings, but it is unfortunate that I had to come up with my own mechanism to obtain a Class<Foo> object when my starting point was the string "my.package.Foo". I think that the JDK, in order to fully support reflection with generic types, should provide a standard API for doing this. I should not have to invent my own.
Maybe it is there and I'm just not seeing it. Do you know of another way, using reflection, to get from a string "my.package.Foo" to a Class<Foo> object?
Thanks again for your help,
Gary

New instance of a generic type?

Hello!
I'm quite new to writing generics and I have this (possibly lame) question: how can I create a new instance of a generic type?
Let's say that inside a class MyClass<T> I have a method with following interface, whose only purpose would be to return new instance of the generic type T: public T getInstance(); Inside this method, I tried the obvious way of getting that instance, that is return new T(); and I also understood why that cannot be done.
However, Googling didn't really reveal any other ways of doing this, so I'm a little stuck. Could anybody please show me the light here?

triceo wrote:
However, Googling didn't really reveal any other ways of doing this.Because there aren't any.
triceo wrote:
Could anybody please show me the light here?Search this forum. I doubt you'd have to go back more than a few hours to find the last time this question was asked.

Rules for casting generics type

I know that there is a section in jsr14 spec about rules of casting generic types, but it seems outdated with
JLS3.
It is obvious that e.g. Class<String> cannot be casted to Class<Integer>.
However in
<T,V> void foo (Class<T> ct) {
Class<V> cv = (Class<V>)ct;
The cast is permitted by the compiler though of course marked as unchecked (The cast should be rejected according to jsr14 spec since none of the types is a subtype of the other).
So the question is: what are the rules, compiler uses to check casts?

1. There is an (unsafe) explicit cast from l1 to l2.
2. There is an implicit cast in the last line, because
l2 is a List<String>, the line is compiled to the
equivalent of
System.out.println(" result:" + (String) l2.get(0));
// ClassCastException: java.lang.String

Generics' types at runtime (JVM)

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