Get generic type of java util List

Question

I have   List lt String gt  stringList   new ArrayList lt String gt     List lt Integer gt  integerList   new ArrayList lt Integer gt       Is there a  easy  way to retrieve the generic type of the list

User · Answer

At runtime  no  you can t   However via reflection the type parameters are accessible  Try  for Field field   this getDeclaredFields          System out println field getGenericType        The method getGenericType   returns a Type object  In this case  it will be an instance of ParametrizedType  which in turn has methods getRawType    which will contain List class  in this case  and getActualTypeArguments    which will return an array  in this case  of length one  containing either String class or Integer class

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You can do the same for method parameters as well  Method method   someClass getDeclaredMethod  quot someMethod quot    Type   types   method getGenericParameterTypes      Now assuming that the first parameter to the method is of type List lt Integer gt  ParameterizedType pType    ParameterizedType  types 0   Class lt   gt  clazz    Class lt   gt   pType getActualTypeArguments   0   System out println clazz     prints out java lang Integer

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For finding generic type of one field     Class   ParameterizedType field getGenericType    getActualTypeArguments   0   getSimpleName

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Generally impossible  because List lt String gt  and List lt Integer gt  share the same runtime class   You might be able to reflect on the declared type of the field holding the list  though  if the declared type does not itself refer to a type parameter whose value you don t know

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Expanding on Steve K s answer          Performs a forced cast      Returns null if the collection type does not match the items in the list     param data The list to cast     param listType The type of list to cast to     static  lt T gt  List lt   super T gt  castListSafe List lt   gt  data  Class lt T gt  listType       List lt T gt  retval   null        This test could be skipped if you trust the callers  but it wouldn t be safe then      if data  null  amp  amp   data isEmpty    amp  amp  listType isInstance data iterator   next   getClass                SuppressWarnings  unchecked    It s OK  we know List lt T gt  contains the expected type          List lt T gt  foo    List lt T gt  data          return retval            return retval    Usage   protected WhateverClass add List lt   gt  data     For fluant useage     if data  null     data isEmpty           throw new IllegalArgumentException  add       data  null  null   empty              collection              Class lt   gt  colType   data iterator   next   getClass     Something     aMethod castListSafe data  colType       aMethod List lt Foo gt  foo      for Foo foo  List         System out println Foo           aMethod List lt Bar gt  bar      for Bar bar  List         System out println Bar

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Use Reflection to get the Field for these then you can just do  field genericType to get the type that contains the information about generic as well

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The generic type of a collection should only matter if it actually has objects in it  right  So isn t it easier to just do   Collection lt   gt  myCollection   getUnknownCollectionFromSomewhere    Class genericClass   null  Iterator it   myCollection iterator    if  it hasNext         genericClass   it next   getClass      if  genericClass    null      do whatever we needed to know the type for   There s no such thing as a generic type in runtime  but the objects inside at runtime are guaranteed to be the same type as the declared generic  so it s easy enough just to test the item s class before we process it   Another thing you can do is simply process the list to get members that are the right type  ignoring others  or processing them differently    Map lt Class lt   gt   List lt Object gt  gt  classObjectMap   myCollection stream        filter Objects  nonNull       collect Collectors groupingBy Object  getClass        Process the list of the correct class  and or handle objects of incorrect    class  throw exceptions  etc   You may need to group subclasses by    filtering the keys  For instance   List lt Number gt  numbers   classObjectMap entrySet   stream            filter e- gt Number class isAssignableFrom e getKey              flatMap e- gt e getValue   stream             map Number class  cast           collect Collectors toList       This will give you a list of all items whose classes were subclasses of Number which you can then process as you need  The rest of the items were filtered out into other lists  Because they re in the map  you can process them as desired  or ignore them    If you want to ignore items of other classes altogether  it becomes much simpler   List lt Number gt  numbers   myCollection stream        filter Number class  isInstance       map Number class  cast       collect Collectors toList       You can even create a utility method to insure that a list contains ONLY those items matching a specific class   public  lt V gt  List lt V gt  getTypeSafeItemList Collection lt Object gt  input  Class lt V gt  cls        return input stream                filter cls  isInstance               map cls  cast               collect Collectors toList

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import org junit Assert  import org junit Test   import java lang reflect Field  import java lang reflect ParameterizedType  import java util ArrayList  import java util Collection  import java util List   public class GenericTypeOfCollectionTest       public class FormBean              public class MyClazz           private List lt FormBean gt  list   new ArrayList lt FormBean gt                 Test     public void testName   throws Exception           Field   fields   MyClazz class getFields            for  Field field   fields                  1  Check if field is of Collection Type             if  Collection class isAssignableFrom field getType                         2  Get Generic type of your field                 Class fieldGenericType   getFieldGenericType field                     3  Compare with  lt FromBean gt                  Assert assertTrue  List lt FormBean gt                      FormBean class isAssignableFrom fieldGenericType                                         Returns generic type of any field     public Class getFieldGenericType Field field            if  ParameterizedType class isAssignableFrom field getGenericType   getClass                   ParameterizedType genericType                 ParameterizedType  field getGenericType                return   Class                 genericType getActualTypeArguments   0    getSuperclass                        Returns dummy Boolean Class to compare with ValueObject  amp  FormBean         return new Boolean false  getClass

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As others have said  the only correct answer is no  the type has been erased   If the list has a non-zero number of elements  you could investigate the type of the first element   using it s getClass method  for instance    That won t tell you the generic type of the list  but it would be reasonable to assume that the generic type was some superclass of the types in the list   I wouldn t advocate the approach  but in a bind it might be useful

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If those are actually fields of a certain class  then you can get them with a little help of reflection   package test   import java lang reflect Field  import java lang reflect ParameterizedType  import java util ArrayList  import java util List   public class Test        List lt String gt  stringList   new ArrayList lt String gt         List lt Integer gt  integerList   new ArrayList lt Integer gt          public static void main String    args  throws Exception           Field stringListField   Test class getDeclaredField  stringList            ParameterizedType stringListType    ParameterizedType  stringListField getGenericType            Class lt   gt  stringListClass    Class lt   gt   stringListType getActualTypeArguments   0           System out println stringListClass      class java lang String           Field integerListField   Test class getDeclaredField  integerList            ParameterizedType integerListType    ParameterizedType  integerListField getGenericType            Class lt   gt  integerListClass    Class lt   gt   integerListType getActualTypeArguments   0           System out println integerListClass      class java lang Integer            You can also do that for parameter types and return type of methods   But if they re inside the same scope of the class method where you need to know about them  then there s no point of knowing them  because you already have declared them yourself

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Short answer  no   This is probably a duplicate  can t find an appropriate one right now   Java uses something called type erasure  which means at runtime both objects are equivalent  The compiler knows the lists contain integers or strings  and as such can maintain a type safe environment  This information is lost  on an object instance basis  at runtime  and the list only contain  Objects    You CAN find out a little about the class  what types it might be parametrized by  but normally this is just anything that extends  Object   i e  anything  If you define a type as  class  lt A extends MyClass gt  AClass          AClass class will only contain the fact that the parameter A is bounded by MyClass  but more than that  there s no way to tell

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If you need to get the generic type of a returned type  I used this approach when I needed to find methods in a class which returned a Collection and then access their generic types   import java lang reflect Method  import java lang reflect ParameterizedType  import java lang reflect Type  import java util Collection  import java util List   public class Test        public List lt String gt  test             return null             public static void main String   args  throws Exception            for  Method method   Test class getMethods                  Class returnClass   method getReturnType                if  Collection class isAssignableFrom returnClass                     Type returnType   method getGenericReturnType                    if  returnType instanceof ParameterizedType                        ParameterizedType paramType    ParameterizedType  returnType                      Type   argTypes   paramType getActualTypeArguments                        if  argTypes length  gt  0                            System out println  Generic type is     argTypes 0                                                                                This outputs      Generic type is class java lang String

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Had the same problem  but I used instanceof instead  Did it this way   List lt Object gt  listCheck    List lt Object gt   Object  stringList      if   listCheck isEmpty             if  listCheck get 0  instanceof String               System out println  List type is String                    if  listCheck get 0  instanceof Integer               System out println  List type is Integer                       This involves using unchecked casts so only do this when you know it is a list  and what type it can be

[java] Get generic type of java.util.List

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