How do I get a class instance of generic type T

Question

I have a generics class  Foo lt T gt   In a method of Foo  I want to get the class instance of type T  but I just can t call T class   What is the preferred way to get around it using T class

User · Answer

I have an  ugly but effective  solution for this problem  which I used recently   import java lang reflect TypeVariable    public static  lt T gt  Class lt T gt  getGenericClass            lt T gt  ins   new    lt T gt         TypeVariable lt   gt    cls   ins getClass   getTypeParameters          return  Class lt T gt  cls 0  getClass       private final class    lt T gt     generic helper class which does only provide type information       private

User · Answer

It s possible   class Foo lt T gt      Class lt T gt  clazz    Class lt T gt   DAOUtil getTypeArguments Foo class  this getClass    get 0       You need two functions from hibernate-generic-dao blob master dao src main java com googlecode genericdao dao DAOUtil java   For more explanations  see Reflecting generics

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I assume that  since you have a generic class  you would have a variable like that   private T t     this variable needs to take a value at the constructor   In that case you can simply create the following method   Class lt T gt  getClassOfInstance         return  Class lt T gt   t getClass        Hope it helps

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I found a generic and simple way to do that  In my class I created a method that returns the generic type according to it s position in the class definition  Let s assume a class definition like this   public class MyClass lt A  B  C gt         Now let s create some attributes to persist the types   public class MyClass lt A  B  C gt         private Class lt A gt  aType       private Class lt B gt  bType       private Class lt C gt  cType      Getters and setters  not necessary if you are going to use them internally            Then you can create a generic method that returns the type based on the index of the generic definition                 Returns a   link Type  object to identify generic types         return type             private Type getGenericClassType int index               To make it use generics without supplying the class type         Type type   getClass   getGenericSuperclass             while    type instanceof ParameterizedType                 if  type instanceof ParameterizedType                    type     Class lt   gt     ParameterizedType  type  getRawType    getGenericSuperclass                  else                   type     Class lt   gt   type  getGenericSuperclass                                     return   ParameterizedType  type  getActualTypeArguments   index           Finally  in the constructor just call the method and send the index for each type  The complete code should look like   public class MyClass lt A  B  C gt         private Class lt A gt  aType       private Class lt B gt  bType       private Class lt C gt  cType        public MyClass           this aType    Class lt A gt   getGenericClassType 0         this bType    Class lt B gt   getGenericClassType 1         this cType    Class lt C gt   getGenericClassType 2                        Returns a   link Type  object to identify generic types         return type             private Type getGenericClassType int index             Type type   getClass   getGenericSuperclass             while    type instanceof ParameterizedType                 if  type instanceof ParameterizedType                    type     Class lt   gt     ParameterizedType  type  getRawType    getGenericSuperclass                  else                   type     Class lt   gt   type  getGenericSuperclass                                     return   ParameterizedType  type  getActualTypeArguments   index

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I was looking for a way to do this myself without adding an extra dependency to the classpath  After some investigation I found that it is possible as long as you have a generic supertype  This was OK for me as I was working with a DAO layer with a generic layer supertype  If this fits your scenario then it s the neatest approach IMHO   Most generics use cases I ve come across have some kind of generic supertype e g  List lt T gt  for ArrayList lt T gt  or GenericDAO lt T gt  for DAO lt T gt   etc   Pure Java solution  The article Accessing generic types at runtime in Java explains how you can do it using pure Java    SuppressWarnings  unchecked   public GenericJpaDao       this entityBeanType     Class    ParameterizedType  getClass          getGenericSuperclass    getActualTypeArguments   0        Spring solution  My project was using Spring which is even better as Spring has a handy utility method for finding the type  This is the best approach for me as it looks neatest  I guess if you weren t using Spring you could write your own utility method   import org springframework core GenericTypeResolver   public abstract class AbstractHibernateDao lt T extends DomainObject gt  implements DataAccessObject lt T gt          Autowired     private SessionFactory sessionFactory       private final Class lt T gt  genericType       private final String RECORD COUNT HQL      private final String FIND ALL HQL        SuppressWarnings  unchecked       public AbstractHibernateDao                 this genericType    Class lt T gt   GenericTypeResolver resolveTypeArgument getClass    AbstractHibernateDao class           this RECORD COUNT HQL    select count    from     this genericType getName            this FIND ALL HQL    from     this genericType getName       t

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public  lt T gt  T yourMethodSignature Class lt T gt  type                get some object and check the type match the given type         Object result                            if  type isAssignableFrom result getClass                   return  T result            else                  handle the error

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I wanted to pass T class to a method which make use of Generics  The method readFile reads a  csv file specified by the fileName with fullpath  There can be csv files with different contents hence i need to pass the model file class so that i can get the appropriate objects  Since this is reading csv file i wanted to do in a generic way  For some reason or other none of the above solutions worked for me  I need to use  Class lt   extends T gt  type to make it work  I use opencsv library for parsing the CSV files   private  lt T gt List lt T gt  readFile String fileName  Class lt   extends T gt  type         List lt T gt  dataList   new ArrayList lt T gt         try           File file   new File fileName            Reader reader   new BufferedReader new InputStreamReader new FileInputStream file             Reader headerReader   new BufferedReader new InputStreamReader new FileInputStream file              CSVReader csvReader   new CSVReader headerReader              create csv bean reader         CsvToBean lt T gt  csvToBean   new CsvToBeanBuilder reader                   withType type                   withIgnoreLeadingWhiteSpace true                   build             dataList   csvToBean parse              catch  Exception ex            logger error  Error     ex              return dataList      This is how the readFile method is called  List lt RigSurfaceCSV gt  rigSurfaceCSVDataList   readSurfaceFile surfaceFileName  RigSurfaceCSV class

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As explained in other answers  to use this ParameterizedType approach  you need to extend the class  but that seems like extra work to make a whole new class that extends it     So  making the class abstract it forces you to extend it  thus satisfying the subclassing requirement   using lombok s  Getter     Getter public abstract class ConfigurationDefinition lt T gt         private Class lt T gt  type               public ConfigurationDefinition                this type    Class lt T gt     ParameterizedType  this getClass   getGenericSuperclass    getActualTypeArguments   0                         Now to extend it without defining a new class   Note the    on the end    extended  but don t overwrite anything - unless you want to    private ConfigurationDefinition lt String gt  myConfigA   new ConfigurationDefinition lt String gt          private ConfigurationDefinition lt File gt  myConfigB   new ConfigurationDefinition lt File gt              Class stringType   myConfigA getType    Class fileType   myConfigB getType

User · Answer

That is pretty straight forward  If you need from within the same class   Class clazz   this getClass    ParameterizedType parameterizedType    ParameterizedType  clazz getGenericSuperclass    try           Class typeClass   Class forName  parameterizedType getActualTypeArguments   0  getTypeName                 You have the instance of type  T  in typeClass variable          System out println   Class instance name      typeClass getName            catch  ClassNotFoundException e            System out println   ClassNotFound   Something wrong     e getMessage

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If you are extending or implementing any class interface that are using generics   you may get the Generic Type of parent class interface  without modifying any existing class interface at all   There could be three possibilities    Case 1 When your class is extending a class that is using Generics  public class TestGenerics       public static void main String   args            Type type   TestMySuperGenericType class getGenericSuperclass            Type   gTypes     ParameterizedType type  getActualTypeArguments            for Type gType   gTypes               System out println  Generic type   gType toString                        class GenericClass lt T gt        public void print T obj        class TestMySuperGenericType extends GenericClass lt Integer gt        Case 2 When your class is implementing an interface that is using Generics  public class TestGenerics       public static void main String   args            Type   interfaces   TestMySuperGenericType class getGenericInterfaces            for Type type   interfaces               Type   gTypes     ParameterizedType type  getActualTypeArguments                for Type gType   gTypes                   System out println  Generic type   gType toString                                      interface GenericClass lt T gt        public void print T obj      class TestMySuperGenericType implements GenericClass lt Integer gt        public void print Integer obj        Case 3 When your interface is extending an interface that is using Generics  public class TestGenerics       public static void main String   args            Type   interfaces   TestMySuperGenericType class getGenericInterfaces            for Type type   interfaces               Type   gTypes     ParameterizedType type  getActualTypeArguments                for Type gType   gTypes                   System out println  Generic type   gType toString                                      interface GenericClass lt T gt        public void print T obj      interface TestMySuperGenericType extends GenericClass lt Integer gt

User · Answer

You can t do it because of type erasure  See also Stack Overflow question Java generics - type erasure - when and what happens

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Here is a working solution    SuppressWarnings  unchecked   private  Class lt T gt  getGenericTypeClass             try                    String className     ParameterizedType  getClass   getGenericSuperclass    getActualTypeArguments   0  getTypeName                    Class lt   gt  clazz   Class forName className                   return   Class lt T gt   clazz             catch   Exception e                    throw new  IllegalStateException  Class is not parametrized with generic type    Please use extends  lt  gt                     NOTES  Can be used only as superclass   Has to be extended with typed class   Child extends Generic lt Integer gt     OR   Has to be created as anonymous implementation  new Generic lt Integer gt

User · Answer

A better route than the Class the others suggested is to pass in an object that can do what you would have done with the Class  e g   create a new instance   interface Factory lt T gt      T apply        lt T gt  void List lt T gt  make10 Factory lt T gt  factory      List lt T gt  result   new ArrayList lt T gt       for  int a   0  a  lt  10  a        result add factory apply       return result     class FooFactory lt T gt  implements Factory lt Foo lt T gt  gt      public Foo lt T gt  apply         return new Foo lt T gt            List lt Foo lt Integer gt  gt  foos   make10 new FooFactory lt Integer gt

User · Answer

A standard approach workaround solution is to add a class object to the constructor s   like    public class Foo lt T gt         private Class lt T gt  type      public Foo Class lt T gt  type          this type   type             public Class lt T gt  getType           return type             public T newInstance           return type newInstance

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There is a small loophole however  if you define your Foo class as abstract  That would mean you have to instantiate you class as   Foo lt MyType gt  myFoo   new Foo lt MyType gt          Note the double braces at the end    Now you can retrieve the type of T at runtime   Type mySuperclass   myFoo getClass   getGenericSuperclass    Type tType     ParameterizedType mySuperclass  getActualTypeArguments   0     Note however that mySuperclass has to be the superclass of the class definition actually defining the final type for T   It is also not very elegant  but you have to decide whether you  prefer new Foo lt MyType gt      or new Foo lt MyType gt  MyType class   in your code     For example   import java lang reflect ParameterizedType  import java lang reflect Type   import java util ArrayDeque  import java util Deque  import java util NoSuchElementException          Captures and silently ignores stack exceptions upon popping      public abstract class SilentStack lt E gt  extends ArrayDeque lt E gt      public E pop         try         return super pop              catch  NoSuchElementException nsee           return create                 public E create         try         Type sooper   getClass   getGenericSuperclass          Type t     ParameterizedType sooper  getActualTypeArguments    0           return  E  Class forName  t toString     newInstance               catch  Exception e           return null                Then   public class Main          Note the braces        private Deque lt String gt  stack   new SilentStack lt String gt            public static void main  String args                Returns a new instance of String        String s   stack pop          System out printf   s     s  n   s

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Many people don t know this trick  Actually  I just found it today  It works like a dream  Just check this example out  public static void main String   args        Date d new Date       Or anything you want      printMethods i      public static  lt T gt  void printMethods T t       Class lt T gt  clazz   Class lt T gt   t getClass       There you go      for   Method m   clazz getMethods             System out println  m getName

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I had this problem in an abstract generic class  In this particular case  the solution is simpler   abstract class Foo lt T gt        abstract Class lt T gt  getTClass                  and later on the derived class   class Bar extends Foo lt Whatever gt         Override     Class lt T gt  getTClass             return Whatever class

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Imagine you have an abstract superclass that is generic   public abstract class Foo lt   extends T gt       And then you have a second class that extends Foo with a generic Bar that extends T   public class Second extends Foo lt Bar gt       You can get the class Bar class in the Foo class by selecting the Type  from bert bruynooghe answer  and infering it using Class instance   Type mySuperclass   myFoo getClass   getGenericSuperclass    Type tType     ParameterizedType mySuperclass  getActualTypeArguments   0     Parse it as String String className   tType toString   split      1   Class clazz   Class forName className     You have to note this operation is not ideal  so it is a good idea to cache the computed value to avoid multiple calculations on this  One of the typical uses is in generic DAO implementation    The final implementation   public abstract class Foo lt T gt         private Class lt T gt  inferedClass       public Class lt T gt  getGenericClass            if inferedClass    null               Type mySuperclass   getClass   getGenericSuperclass                Type tType     ParameterizedType mySuperclass  getActualTypeArguments   0               String className   tType toString   split      1               inferedClass   Class forName className                     return inferedClass            The value returned is Bar class when invoked from Foo class in other function or from Bar class

User · Answer

I m using workaround for this   class MyClass extends Foo lt T gt            MyClass myClassInstance   MyClass class newInstance

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This question is old  but now the best is use google Gson   An example to get custom viewModel   Class lt CustomViewModel lt String gt  gt  clazz   new GenericClass lt CustomViewModel lt String gt  gt    getRawType    CustomViewModel lt String gt  viewModel   viewModelProvider get clazz     Generic type class  class GenericClass lt T gt  private val rawType  Class lt   gt          constructor   this   Gson Types  getRawType object   TypeToken lt T gt       getType          fun getRawType    Class lt T gt            return rawType as Class lt T gt

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Actually  I suppose you have a field in your class of type T  If there s no field of type T  what s the point of having a generic Type  So  you can simply do an instanceof on that field   In my case  I have a List lt T gt  items  in my class  and I check if the class type is  Locality  by  if  items get 0  instanceof Locality        Of course  this only works if the total number of possible classes is limited

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The short answer is  that there is no way to find out the runtime type of generic type parameters in Java  I suggest reading the chapter about type erasure in the Java Tutorial for more details   A popular solution to this is to pass the Class of the type parameter into the constructor of the generic type  e g   class Foo lt T gt        final Class lt T gt  typeParameterClass       public Foo Class lt T gt  typeParameterClass            this typeParameterClass   typeParameterClass             public void bar                you can access the typeParameterClass here and do whatever you like

[java] How do I get a class instance of generic type T?

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