Get type of a generic parameter in Java with reflection

Question

Is it possible to get the type of a generic parameter   An example   public final class Voodoo       public static void chill List lt   gt  aListWithTypeSpiderMan               Here I d like to get the Class-Object  SpiderMan          Class typeOfTheList                   public static void main String    args            chill new ArrayList lt SpiderMan gt

User · Answer

The quick answer the the Question is no you can t  because of Java generic type erasure   The longer answer would be that if you have created your list like this   new ArrayList lt SpideMan gt        Then in this case the generic type is preserved in the generic superclass of the new anonymous class above    Not that I recommend doing this with lists  but it is a listener implementation   new Listener lt Type gt      public void doSomething Type t          And since extrapolating the generic types of super classes and super interfaces change between JVMs  the generic solution is not as straight forward as some answers might suggest   Here is now I did it

User · Answer

Here is another trick  Use a generic vararg array  import java util ArrayList   class TypedArrayList lt E gt  extends ArrayList lt E gt         SafeVarargs     public TypedArrayList  E    typeInfo                   Get generic type at runtime             System out println  typeInfo getClass   getComponentType   getTypeName              public class GenericTest       public static void main  String   args                   No need to supply the dummy argument         ArrayList lt Integer gt  ar1   new TypedArrayList lt  gt              ArrayList lt String gt  ar2   new TypedArrayList lt  gt              ArrayList lt   gt  ar3   new TypedArrayList lt  gt

User · Answer

You can get the type of a generic parameter with reflection like in this example that I found here   import java lang reflect ParameterizedType  import java lang reflect Type   public class Home lt E gt         SuppressWarnings   unchecked       public Class lt E gt  getTypeParameterClass            Type type   getClass   getGenericSuperclass            ParameterizedType paramType    ParameterizedType  type          return  Class lt E gt   paramType getActualTypeArguments   0              private static class StringHome extends Home lt String gt        private static class StringBuilderHome extends Home lt StringBuilder gt        private static class StringBufferHome extends Home lt StringBuffer gt                       This prints  String    StringBuilder  and  StringBuffer              public static void main String   args  throws InstantiationException  IllegalAccessException           Object object0   new StringHome   getTypeParameterClass   newInstance            Object object1   new StringBuilderHome   getTypeParameterClass   newInstance            Object object2   new StringBufferHome   getTypeParameterClass   newInstance            System out println object0 getClass   getSimpleName             System out println object1 getClass   getSimpleName             System out println object2 getClass   getSimpleName

User · Answer

No it isn t possible   You can get a generic type of a field given a class is the only exception to that rule and even that s a bit of a hack   See Knowing type of generic in Java for an example of that

User · Answer

One construct  I once stumbled upon looked like  Class lt T gt  persistentClass    Class lt T gt        ParameterizedType getClass   getGenericSuperclass           getActualTypeArguments   0     So there seems to be some reflection-magic around that I unfortunetly don t fully understand    Sorry

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As pointed out by  bertolami  it s not possible to us a variable type and get its future value  the content of typeOfList variable    Nevertheless  you can pass the class as parameter on it like this   public final class voodoo       public static void chill List lt T gt  aListWithTypeSpiderMan  Class lt T gt  clazz               Here I d like to get the Class-Object  SpiderMan          Class typeOfTheList   clazz             public static void main String    args            chill new List lt SpiderMan gt     Spiderman class              That s more or less what Google does when you have to pass a class variable to the constructor of ActivityInstrumentationTestCase2

User · Answer

This is impossible because generics in Java are only considered at compile time  Thus  the Java generics are just some kind of pre-processor  However you can get the actual class of the members of the list

User · Answer

Actually I got this to work  Consider the following snippet   Method m  Type   genericParameterTypes   m getGenericParameterTypes    for  int i   0  i  lt  genericParameterTypes length  i           if  genericParameterTypes i  instanceof ParameterizedType                     Type   parameters     ParameterizedType genericParameterTypes i   getActualTypeArguments      parameters 0  contains java lang String for method like  method List lt String gt  value                I m using jdk 1 6

User · Answer

I noticed that many people lean towards the getGenericSuperclass   solution  class RootGeneric lt T gt      public Class lt T gt  persistentClass    Class lt T gt         ParameterizedType getClass   getGenericSuperclass           getActualTypeArguments   0      However  this solution is error prone  It will not work properly if there are generics in the descendants  Consider this  class Foo lt S gt  extends RootGeneric lt Integer gt      class Bar extends Foo lt Double gt      Which type will Bar persistentClass have  Class lt Integer gt   Nope  it will be Class lt Double gt   This will happen due to getClass   always returns the top most class  which is Bar in this case  and its generic super class is Foo lt Double gt   Hence  the argument type will be Double  If you need a reliable solution which doesn t fail I can suggest two   Use Guava  It has a class that was made exactly for this purpose  com google common reflect TypeToken  It handles all the corner cases just fine and offers some more nice functionality  The downside is an extra dependency  Given you ve used this class  your code would look simple and clear  like this   class RootGeneric lt T gt       SuppressWarnings  quot unchecked quot     public final Class lt T gt  persistentClass    Class lt T gt    new TypeToken lt T gt  getClass       getType         Use the custom method below  It implements a significantly simplified logic similar to the Guava class  mentioned above  However  I d not guarantee it s error prone  It does solve the problem with the generic descendants though   abstract class RootGeneric lt T gt       SuppressWarnings  quot unchecked quot     private Class lt T gt  getTypeOfT         Class lt T gt  type   null      Class lt   gt  iter   getClass        while  iter getSuperclass      null          Class lt   gt  next   iter getSuperclass          if  next    null  amp  amp  next isAssignableFrom RootGeneric class             type                Class lt T gt                     ParameterizedType  iter getGenericSuperclass    getActualTypeArguments   0           break                iter   next            if  type    null          throw new ClassCastException  quot Cannot determine type of T quot              return type

User · Answer

Use   Class lt   gt  typeOfTheList   aListWithTypeSpiderMan toArray   getClass   getComponentType

User · Answer

You cannot get a generic parameter from a variable  But you can from a method or field declaration   Method method   getClass   getDeclaredMethod  chill   List class   Type   params   method getGenericParameterTypes    ParameterizedType firstParam    ParameterizedType  params 0   Type   paramsOfFirstGeneric   firstParam getActualTypeArguments

User · Answer

Nope   that is not possible  Due to downwards compatibility issues  Java s generics are based on type erasure  i a  at runtime  all you have is a non-generic List object  There is some information about type parameters at runtime  but it resides in class definitions  i e  you can ask  what generic type does this field s definition use     not in object instances

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There is a solution actually  by applying the  anonymous class  trick and the ideas from the Super Type Tokens   public final class Voodoo       public static void chill final List lt   gt  aListWithSomeType               Here I d like to get the Class-Object  SpiderMan          System out println aListWithSomeType getClass   getGenericSuperclass             System out println   ParameterizedType  aListWithSomeType              getClass                getGenericSuperclass    getActualTypeArguments   0              public static void main String    args            chill new ArrayList lt SpiderMan gt                 class SpiderMan       The trick lies in the creation of an anonymous class  new ArrayList lt SpiderMan gt        in the place of the original  simple  new ArrayList lt SpiderMan gt     The use of an anoymous class  if possible  ensures that the compiler retains information about the type argument SpiderMan given to the type parameter List lt   gt   Voil

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Because of type erasure the only way to know the type of the list would be to pass in the type as a parameter to the method   public class Main        public static void main String   args            doStuff new LinkedList lt String gt     String class               public static  lt E gt  void doStuff List lt E gt  list  Class lt E gt  clazz

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Just for me reading this snippet of code was hard  I just divided it into 2 readable lines       assuming that the Generic Type parameter is of type  T  ParameterizedType p    ParameterizedType  getClass   getGenericSuperclass    Class lt T gt  c   Class lt T gt  p getActualTypeArguments   0     I wanted to create an instance of the Type parameter without having any parameters to my method    publc T getNewTypeInstance        ParameterizedType p    ParameterizedType  getClass   getGenericSuperclass        Class lt T gt  c   Class lt T gt  p getActualTypeArguments   0           for me i wanted to get the type to create an instance        from the no-args default constructor     T t   null      try          t   c newInstance         catch Exception e              no default constructor available           return t

User · Answer

I want to try to break down the answer from  DerMike to explain   First  type erasure does not mean that the JDK eliminates type information at runtime   It s a method for allowing compile-time type checking and runtime type compatibility to coexist in the same language   As this block of code implies  the JDK retains the erased type information--it s just not associated with checked casts and stuff   Second  this provides generic type information to a generic class exactly one level up the heirarchy from the concrete type being checked--i e  an abstract parent class with generic type parameters can find the concrete types corresponding to its type parameters for a concrete implementation of itself that directly inherits from it   If this class were non-abstract and instantiated  or the concrete implementation were two levels down  this wouldn t work  although a little bit of jimmying could make it apply to any predetermined number of levels beyond one  or up to the lowest class with X generic type parameters  et cetera    Anyway  on to the explanation   Here s the code again  separated into lines for ease of reference    1  Class genericParameter0OfThisClass    2       Class  3            ParameterizedType  4              getClass   5                  getGenericSuperclass    6                      getActualTypeArguments   0     Let  us  be the abstract class with generic types that contains this code   Reading this roughly inside out     Line 4 gets the current concrete class  Class instance   This identifies our immediate descendant s concrete type   Line 5 gets that class  supertype as a Type  this is us   Since we re a parametric type we can safely cast ourselves to ParameterizedType  line 3    The key is that when Java determines this Type object  it uses type information present in the child to associate type information with our type parameters in the new ParameterizedType instance   So now we can access concrete types for our generics   Line 6 gets the array of types mapped into our generics  in order as declared in the class code   For this example we pull out the first parameter   This comes back as a Type   Line 2 casts the final Type returned to a Class   This is safe because we know what types our generic type parameters are able to take and can confirm that they will all be classes  I m not sure how in Java one would go about getting a generic parameter that doesn t have a Class instance associated with it  actually        and that s pretty much it   So we push type info from our own concrete implementation back into ourselves  and use it to access a class handle   we could double up getGenericSuperclass   and go two levels  or eliminate getGenericSuperclass   and get values for ourselves as a concrete type  caveat  I haven t tested these scenarios  they haven t come up for me yet    It gets tricky if your concrete children are be an arbitrary number of hops away  or if you re concrete and not final  and especially tricky if you expect any of your  variably deep  children to have their own generics   But you can usually design around those considerations  so this gets you most of the way   Hope this helped someone   I recognize this post is ancient   I ll probably snip this explanation and keep it for other questions

User · Answer

Appendix to  DerMike s answer for getting the generic parameter of a parameterized interface  using  getGenericInterfaces   method inside a Java-8 default method to avoid duplication    import java lang reflect ParameterizedType    public class ParametrizedStuff     SuppressWarnings  unchecked   interface Awesomable lt T gt        default Class lt T gt  parameterizedType             return  Class lt T gt     ParameterizedType          this getClass   getGenericInterfaces   0                getActualTypeArguments   0            static class Beer     static class EstrellaGalicia implements Awesomable lt Beer gt       public static void main String   args        System out println  Type is      new EstrellaGalicia   parameterizedType            -- gt  Type is  ParameterizedStuff Beer

User · Answer

I ve coded this for methods which expect to accept or return Iterable lt      gt   Here is the code          Assuming the given method returns or takes an Iterable lt T gt   this determines the type T     T may or may not extend WindupVertexFrame      private static Class typeOfIterable Method method  boolean setter        Type type      if  setter            Type   types   method getGenericParameterTypes               The first parameter to the method expected to be Iterable lt     gt            if  types length    0              throw new IllegalArgumentException  Given method has 0 params      method           type   types 0             else           type   method getGenericReturnType                  Now get the parametrized type of the generic      if    type instanceof ParameterizedType           throw new IllegalArgumentException  Given method s 1st param type is not parametrized generic      method       ParameterizedType pType    ParameterizedType  type      final Type   actualArgs   pType getActualTypeArguments        if  actualArgs length    0          throw new IllegalArgumentException  Given method s 1st param type is not parametrized generic      method        Type t   actualArgs 0       if  t instanceof Class          return  Class lt   gt   t       if  t instanceof TypeVariable           TypeVariable tv     TypeVariable  actualArgs 0           AnnotatedType   annotatedBounds   tv getAnnotatedBounds               GenericDeclaration genericDeclaration   tv getGenericDeclaration               return  Class  tv getAnnotatedBounds   0  getType               throw new IllegalArgumentException  Unknown kind of type      t getTypeName

[java] Get type of a generic parameter in Java with reflection

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