Create instance of generic type in Java

Question

Is it possible to create an instance of a generic type in Java  I m thinking based on what I ve seen that the answer is no  due to type erasure   but I d be interested if anyone can see something I m missing   class SomeContainer lt E gt        E createContents                 return what              EDIT  It turns out that Super Type Tokens could be used to resolve my issue  but it requires a lot of reflection-based code  as some of the answers below have indicated   I ll leave this open for a little while to see if anyone comes up with anything dramatically different than Ian Robertson s Artima Article

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If you mean  new E    then it is impossible  And I would add that it is not always correct - how do you know if E has public no-args constructor  But you can always delegate creation to some other class that knows how to create an instance - it can be Class lt E gt  or your custom code like this  interface Factory lt E gt       E create           class IntegerFactory implements Factory lt Integer gt         private static int i   0     Integer create                 return i

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return    E   Class   ParameterizedType this getClass   getGenericSuperclass    getActualTypeArguments   0   newInstance

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package org foo com   import java lang reflect ParameterizedType  import java lang reflect Type          Basically the same answer as noah s      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

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If you want not to type class name twice during instantiation like in   new SomeContainer lt SomeType gt  SomeType class     You can use factory method    lt E gt  SomeContainer lt E gt  createContainer Class lt E gt  class      Like in   public class Container lt E gt         public static  lt E gt  Container lt E gt  create Class lt E gt  c            return new Container lt E gt  c              Class lt E gt  c       public Container Class lt E gt  c            super            this c   c             public E createInstance               throws InstantiationException              IllegalAccessException           return c newInstance

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Java unfortunatly does not allow what you want to do  See the official workaround       You cannot create an instance of a type parameter  For example  the following code causes a compile-time error    public static  lt E gt  void append List lt E gt  list        E elem   new E        compile-time error     list add elem          As a workaround  you can create an object of a type parameter through reflection    public static  lt E gt  void append List lt E gt  list  Class lt E gt  cls  throws Exception       E elem   cls newInstance         OK     list add elem          You can invoke the append method as follows    List lt String gt  ls   new ArrayList lt  gt     append ls  String class

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I don t know if this helps  but when you subclass  including anonymously  a generic type  the type information is available via reflection  e g   public abstract class Foo lt E gt       public E instance       public Foo   throws Exception       instance     Class   ParameterizedType this getClass           getGenericSuperclass    getActualTypeArguments   0   newInstance                    So  when you subclass Foo  you get an instance of Bar e g      notice that this in anonymous subclass of Foo assert  new Foo lt Bar gt       instance instanceof Bar     But it s a lot of work  and only works for subclasses  Can be handy though

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As you said  you can t really do it because of type erasure   You can sort of do it using reflection  but it requires a lot of code and lot of error handling

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You can use   Class forName String  getConstructor arguments types  newInstance arguments    But you need to supply the exact class name  including packages  eg  java io FileInputStream  I used this to create a math expressions parser

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Hope this s not too late to help    Java is type-safe  meaning that only Objects are able to create instances  In my case I cannot pass parameters to the createContents method  My solution is using extends unlike the answer below  private static class SomeContainer lt E extends Object gt        E e      E createContents   throws Exception          return  E  e getClass   getDeclaredConstructor   newInstance             This is my example case in which I can t pass parameters  public class SomeContainer lt E extends Object gt        E object       void resetObject throws Exception          object    E  object getClass   getDeclaredConstructor   newInstance             Using reflection create run time error  if you extends your generic class with none object type  To extends your generic type to object convert this error to compile time error

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You ll need some kind of abstract factory of one sort or another to pass the buck to   interface Factory lt E gt        E create       class SomeContainer lt E gt        private final Factory lt E gt  factory      SomeContainer Factory lt E gt  factory            this factory   factory            E createContents             return factory create

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I don t know if this helps  but when you subclass  including anonymously  a generic type  the type information is available via reflection  e g   public abstract class Foo lt E gt       public E instance       public Foo   throws Exception       instance     Class   ParameterizedType this getClass           getGenericSuperclass    getActualTypeArguments   0   newInstance                    So  when you subclass Foo  you get an instance of Bar e g      notice that this in anonymous subclass of Foo assert  new Foo lt Bar gt       instance instanceof Bar     But it s a lot of work  and only works for subclasses  Can be handy though

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You can use   Class forName String  getConstructor arguments types  newInstance arguments    But you need to supply the exact class name  including packages  eg  java io FileInputStream  I used this to create a math expressions parser

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As you said  you can t really do it because of type erasure   You can sort of do it using reflection  but it requires a lot of code and lot of error handling

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You can with a classloader and the class name  eventually some parameters   final ClassLoader classLoader       final Class lt   gt  aClass   classLoader loadClass  java lang Integer    final Constructor lt   gt  constructor   aClass getConstructor int class   final Object o   constructor newInstance 123   System out println  o       o

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In Java 8 you can use the Supplier functional interface to achieve this pretty easily   class SomeContainer lt E gt      private Supplier lt E gt  supplier     SomeContainer Supplier lt E gt  supplier        this supplier   supplier         E createContents         return supplier get            You would construct this class like this   SomeContainer lt String gt  stringContainer   new SomeContainer lt  gt  String  new     The syntax String  new on that line is a constructor reference   If your constructor takes arguments you can use a lambda expression instead   SomeContainer lt BigInteger gt  bigIntegerContainer       new SomeContainer lt  gt     - gt  new BigInteger 1

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Here s an implementation of createContents that uses TypeTools to resolve the raw class represented by E   E createContents   throws Exception     return TypeTools resolveRawArgument SomeContainer class  getClass    newInstance        This approach only works if SomeContainer is subclassed so the actual value of E is captured in a type definition   class SomeStringContainer extends SomeContainer lt String gt    Otherwise the value of E is erased at runtime and is not recoverable

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You can do this now and it doesn t require a bunch of reflection code   import com google common reflect TypeToken   public class Q26289147       public static void main final String   args  throws IllegalAccessException  InstantiationException               final StrawManParameterizedClass lt String gt  smpc   new StrawManParameterizedClass lt String gt                final String string    String  smpc type getRawType   newInstance            System out format  string      s    string              static abstract class StrawManParameterizedClass lt T gt                final TypeToken lt T gt  type   new TypeToken lt T gt  getClass                  Of course if you need to call the constructor that will require some reflection  but that is very well documented  this trick isn t   Here is the JavaDoc for TypeToken

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Think about a more functional approach  instead of creating some E  out of nothing  which is clearly a code smell   pass a function that knows how to create one  i e   E createContents Callable lt E gt  makeone         return makeone call       most simple case clearly not that useful

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Here is an option I came up with  it may help   public static class Container lt E gt        private Class lt E gt  clazz       public Container Class lt E gt  clazz            this clazz   clazz             public E createContents   throws Exception           return clazz newInstance              EDIT  Alternatively you can use this constructor  but it requires an instance of E     SuppressWarnings  unchecked   public Container E instance        this clazz    Class lt E gt   instance getClass

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You are correct   You can t do new E     But you can change it to  private static class SomeContainer lt E gt        E createContents Class lt E gt  clazz            return clazz newInstance              It s a pain   But it works   Wrapping it in the factory pattern makes it a little more tolerable

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I thought I could do that  but quite disappointed  it doesn t work  but I think it still worths sharing    Maybe someone can correct   import java lang reflect InvocationHandler  import java lang reflect Method  import java lang reflect Proxy   interface SomeContainer lt E gt        E createContents       public class Main         SuppressWarnings  unchecked       public static  lt E gt  SomeContainer lt E gt  createSomeContainer             return  SomeContainer lt E gt   Proxy newProxyInstance Main class getClassLoader                    new Class    SomeContainer class    new InvocationHandler                  Override             public Object invoke Object proxy  Method method  Object   args  throws Throwable                   Class lt   gt  returnType   method getReturnType                    return returnType newInstance                                         public static void main String   args            SomeContainer lt String gt  container   createSomeContainer             System out println  String created      container createContents                    It produces   Exception in thread  main  java lang ClassCastException  java lang Object cannot be cast to java lang String     at Main main Main java 26      at sun reflect NativeMethodAccessorImpl invoke0 Native Method      at sun reflect NativeMethodAccessorImpl invoke NativeMethodAccessorImpl java 57      at sun reflect DelegatingMethodAccessorImpl invoke DelegatingMethodAccessorImpl java 43      at java lang reflect Method invoke Method java 601      at com intellij rt execution application AppMain main AppMain java 120    Line 26 is the one with the       The only viable solution is the one by  JustinRudd

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what you can do is -   First declare the variable of that generic  class   2 Then make a constructor of it and instantiate that object Then use it wherever you want to use it   example-  1      private Class lt E gt  entity    2   public xyzservice Class lt E gt  entity            this entity   entity           public E getEntity Class lt E gt  entity  throws InstantiationException  IllegalAccessException           return entity newInstance            3      E e   getEntity entity

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If you need a new instance of a type argument inside a generic class then make your constructors demand its class     public final class Foo lt T gt         private Class lt T gt  typeArgumentClass       public Foo Class lt T gt  typeArgumentClass             this typeArgumentClass   typeArgumentClass             public void doSomethingThatRequiresNewT   throws Exception            T myNewT   typeArgumentClass newInstance                          Usage   Foo lt Bar gt  barFoo   new Foo lt Bar gt  Bar class   Foo lt Etc gt  etcFoo   new Foo lt Etc gt  Etc class     Pros    Much simpler  and less problematic  than Robertson s Super Type Token  STT  approach  Much more efficient than the STT approach  which will eat your cellphone for breakfast     Cons    Can t pass Class to a default constructor  which is why Foo is final   If you really do need a default constructor you can always add a setter method but then you must remember to give her a call later  Robertson s objection    More Bars than a black sheep  although specifying the type argument class one more time won t exactly kill you   And contrary to Robertson s claims this does not violate the DRY principal anyway because the compiler will ensure type correctness  Not entirely Foo lt L gt proof  For starters    newInstance   will throw a wobbler if the type argument class does not have a default constructor  This does apply to all known solutions though anyway  Lacks the total encapsulation of the STT approach  Not a big deal though  considering the outrageous performance overhead of STT

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You ll need some kind of abstract factory of one sort or another to pass the buck to   interface Factory lt E gt        E create       class SomeContainer lt E gt        private final Factory lt E gt  factory      SomeContainer Factory lt E gt  factory            this factory   factory            E createContents             return factory create

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Here is an option I came up with  it may help   public static class Container lt E gt        private Class lt E gt  clazz       public Container Class lt E gt  clazz            this clazz   clazz             public E createContents   throws Exception           return clazz newInstance              EDIT  Alternatively you can use this constructor  but it requires an instance of E     SuppressWarnings  unchecked   public Container E instance        this clazz    Class lt E gt   instance getClass

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If you want not to type class name twice during instantiation like in   new SomeContainer lt SomeType gt  SomeType class     You can use factory method    lt E gt  SomeContainer lt E gt  createContainer Class lt E gt  class      Like in   public class Container lt E gt         public static  lt E gt  Container lt E gt  create Class lt E gt  c            return new Container lt E gt  c              Class lt E gt  c       public Container Class lt E gt  c            super            this c   c             public E createInstance               throws InstantiationException              IllegalAccessException           return c newInstance

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In Java 8 you can use the Supplier functional interface to achieve this pretty easily   class SomeContainer lt E gt      private Supplier lt E gt  supplier     SomeContainer Supplier lt E gt  supplier        this supplier   supplier         E createContents         return supplier get            You would construct this class like this   SomeContainer lt String gt  stringContainer   new SomeContainer lt  gt  String  new     The syntax String  new on that line is a constructor reference   If your constructor takes arguments you can use a lambda expression instead   SomeContainer lt BigInteger gt  bigIntegerContainer       new SomeContainer lt  gt     - gt  new BigInteger 1

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Here is an improved solution  based on ParameterizedType getActualTypeArguments  already mentioned by  noah   Lars Bohl  and some others    First small improvement in the implementation  Factory should not return instance  but a type  As soon as you return instance using Class newInstance   you reduce a scope of usage  Because only no-arguments constructors can be invoke like this  A better way is to return a type  and allow a client to choose  which constructor he wants to invoke   public class TypeReference lt T gt      public Class lt T gt  type        try         ParameterizedType pt    ParameterizedType  this getClass   getGenericSuperclass          if  pt getActualTypeArguments      null    pt getActualTypeArguments   length    0           throw new IllegalStateException  Could not define type                  if  pt getActualTypeArguments   length    1           throw new IllegalStateException  More than one type has been found                  Type type   pt getActualTypeArguments   0         String typeAsString   type getTypeName          return  Class lt T gt   Class forName typeAsString          catch  Exception e         throw new IllegalStateException  Could not identify type   e                  Here is a usage examples   Lars Bohl has shown only a signe way to get reified geneneric via extension   noah only via creating an instance with     Here are tests to demonstrate both cases   import java lang reflect Constructor   public class TypeReferenceTest      private static final String NAME    Peter      private static class Person      final String name       Person String name          this name   name                Test   public void erased         TypeReference lt Person gt  p   new TypeReference lt  gt         Assert assertNotNull p       try         p type          Assert fail          catch  Exception e         Assert assertEquals  Could not identify type   e getMessage                   Test   public void reified   throws Exception       TypeReference lt Person gt  p   new TypeReference lt Person gt           Assert assertNotNull p       Assert assertEquals Person class getName    p type   getName         Constructor ctor   p type   getDeclaredConstructor NAME getClass         Assert assertNotNull ctor       Person person    Person  ctor newInstance NAME       Assert assertEquals NAME  person name          static class TypeReferencePerson extends TypeReference lt Person gt        Test   public void reifiedExtenension   throws Exception       TypeReference lt Person gt  p   new TypeReferencePerson        Assert assertNotNull p       Assert assertEquals Person class getName    p type   getName         Constructor ctor   p type   getDeclaredConstructor NAME getClass         Assert assertNotNull ctor       Person person    Person  ctor newInstance NAME       Assert assertEquals NAME  person name           Note  you can force the clients of TypeReference always use    when instance is created by making this class abstract  public abstract class TypeReference lt T gt   I ve not done it  only to show erased test case

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Think about a more functional approach  instead of creating some E  out of nothing  which is clearly a code smell   pass a function that knows how to create one  i e   E createContents Callable lt E gt  makeone         return makeone call       most simple case clearly not that useful

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I thought I could do that  but quite disappointed  it doesn t work  but I think it still worths sharing    Maybe someone can correct   import java lang reflect InvocationHandler  import java lang reflect Method  import java lang reflect Proxy   interface SomeContainer lt E gt        E createContents       public class Main         SuppressWarnings  unchecked       public static  lt E gt  SomeContainer lt E gt  createSomeContainer             return  SomeContainer lt E gt   Proxy newProxyInstance Main class getClassLoader                    new Class    SomeContainer class    new InvocationHandler                  Override             public Object invoke Object proxy  Method method  Object   args  throws Throwable                   Class lt   gt  returnType   method getReturnType                    return returnType newInstance                                         public static void main String   args            SomeContainer lt String gt  container   createSomeContainer             System out println  String created      container createContents                    It produces   Exception in thread  main  java lang ClassCastException  java lang Object cannot be cast to java lang String     at Main main Main java 26      at sun reflect NativeMethodAccessorImpl invoke0 Native Method      at sun reflect NativeMethodAccessorImpl invoke NativeMethodAccessorImpl java 57      at sun reflect DelegatingMethodAccessorImpl invoke DelegatingMethodAccessorImpl java 43      at java lang reflect Method invoke Method java 601      at com intellij rt execution application AppMain main AppMain java 120    Line 26 is the one with the       The only viable solution is the one by  JustinRudd

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Hope this s not too late to help    Java is type-safe  meaning that only Objects are able to create instances  In my case I cannot pass parameters to the createContents method  My solution is using extends unlike the answer below  private static class SomeContainer lt E extends Object gt        E e      E createContents   throws Exception          return  E  e getClass   getDeclaredConstructor   newInstance             This is my example case in which I can t pass parameters  public class SomeContainer lt E extends Object gt        E object       void resetObject throws Exception          object    E  object getClass   getDeclaredConstructor   newInstance             Using reflection create run time error  if you extends your generic class with none object type  To extends your generic type to object convert this error to compile time error

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Use the TypeToken lt T gt  class   public class MyClass lt T gt        public T doSomething             return  T  new TypeToken lt T gt      getRawType   newInstance

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Here is an improved solution  based on ParameterizedType getActualTypeArguments  already mentioned by  noah   Lars Bohl  and some others    First small improvement in the implementation  Factory should not return instance  but a type  As soon as you return instance using Class newInstance   you reduce a scope of usage  Because only no-arguments constructors can be invoke like this  A better way is to return a type  and allow a client to choose  which constructor he wants to invoke   public class TypeReference lt T gt      public Class lt T gt  type        try         ParameterizedType pt    ParameterizedType  this getClass   getGenericSuperclass          if  pt getActualTypeArguments      null    pt getActualTypeArguments   length    0           throw new IllegalStateException  Could not define type                  if  pt getActualTypeArguments   length    1           throw new IllegalStateException  More than one type has been found                  Type type   pt getActualTypeArguments   0         String typeAsString   type getTypeName          return  Class lt T gt   Class forName typeAsString          catch  Exception e         throw new IllegalStateException  Could not identify type   e                  Here is a usage examples   Lars Bohl has shown only a signe way to get reified geneneric via extension   noah only via creating an instance with     Here are tests to demonstrate both cases   import java lang reflect Constructor   public class TypeReferenceTest      private static final String NAME    Peter      private static class Person      final String name       Person String name          this name   name                Test   public void erased         TypeReference lt Person gt  p   new TypeReference lt  gt         Assert assertNotNull p       try         p type          Assert fail          catch  Exception e         Assert assertEquals  Could not identify type   e getMessage                   Test   public void reified   throws Exception       TypeReference lt Person gt  p   new TypeReference lt Person gt           Assert assertNotNull p       Assert assertEquals Person class getName    p type   getName         Constructor ctor   p type   getDeclaredConstructor NAME getClass         Assert assertNotNull ctor       Person person    Person  ctor newInstance NAME       Assert assertEquals NAME  person name          static class TypeReferencePerson extends TypeReference lt Person gt        Test   public void reifiedExtenension   throws Exception       TypeReference lt Person gt  p   new TypeReferencePerson        Assert assertNotNull p       Assert assertEquals Person class getName    p type   getName         Constructor ctor   p type   getDeclaredConstructor NAME getClass         Assert assertNotNull ctor       Person person    Person  ctor newInstance NAME       Assert assertEquals NAME  person name           Note  you can force the clients of TypeReference always use    when instance is created by making this class abstract  public abstract class TypeReference lt T gt   I ve not done it  only to show erased test case

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return    E   Class   ParameterizedType this getClass   getGenericSuperclass    getActualTypeArguments   0   newInstance

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You can achieve this with the following snippet   import java lang reflect ParameterizedType   public class SomeContainer lt E gt       E createContents   throws InstantiationException  IllegalAccessException         ParameterizedType genericSuperclass    ParameterizedType           getClass   getGenericSuperclass           SuppressWarnings  unchecked         Class lt E gt  clazz    Class lt E gt            genericSuperclass getActualTypeArguments   0         return clazz newInstance            public static void main  String   args   throws Throwable         SomeContainer lt  Long  gt  scl   new SomeContainer lt  gt           Long l   scl createContents          System out println  l

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When you are working with E at compile time you don t really care the actual generic type  quot E quot   either you use reflection or work with base class of generic type  so let the subclass provide instance of E  abstract class SomeContainer lt E gt        abstract protected E createContents        public void doWork            E obj   createContents               Do the work with E            class BlackContainer extends SomeContainer lt Black gt       protected Black createContents             return new Black

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If you want not to type class name twice during instantiation like in   new SomeContainer lt SomeType gt  SomeType class     You can use factory method    lt E gt  SomeContainer lt E gt  createContainer Class lt E gt  class      Like in   public class Container lt E gt         public static  lt E gt  Container lt E gt  create Class lt E gt  c            return new Container lt E gt  c              Class lt E gt  c       public Container Class lt E gt  c            super            this c   c             public E createInstance               throws InstantiationException              IllegalAccessException           return c newInstance

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Here is an option I came up with  it may help   public static class Container lt E gt        private Class lt E gt  clazz       public Container Class lt E gt  clazz            this clazz   clazz             public E createContents   throws Exception           return clazz newInstance              EDIT  Alternatively you can use this constructor  but it requires an instance of E     SuppressWarnings  unchecked   public Container E instance        this clazz    Class lt E gt   instance getClass

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You can achieve this with the following snippet   import java lang reflect ParameterizedType   public class SomeContainer lt E gt       E createContents   throws InstantiationException  IllegalAccessException         ParameterizedType genericSuperclass    ParameterizedType           getClass   getGenericSuperclass           SuppressWarnings  unchecked         Class lt E gt  clazz    Class lt E gt            genericSuperclass getActualTypeArguments   0         return clazz newInstance            public static void main  String   args   throws Throwable         SomeContainer lt  Long  gt  scl   new SomeContainer lt  gt           Long l   scl createContents          System out println  l

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I don t know if this helps  but when you subclass  including anonymously  a generic type  the type information is available via reflection  e g   public abstract class Foo lt E gt       public E instance       public Foo   throws Exception       instance     Class   ParameterizedType this getClass           getGenericSuperclass    getActualTypeArguments   0   newInstance                    So  when you subclass Foo  you get an instance of Bar e g      notice that this in anonymous subclass of Foo assert  new Foo lt Bar gt       instance instanceof Bar     But it s a lot of work  and only works for subclasses  Can be handy though

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Here is an option I came up with  it may help   public static class Container lt E gt        private Class lt E gt  clazz       public Container Class lt E gt  clazz            this clazz   clazz             public E createContents   throws Exception           return clazz newInstance              EDIT  Alternatively you can use this constructor  but it requires an instance of E     SuppressWarnings  unchecked   public Container E instance        this clazz    Class lt E gt   instance getClass

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When you are working with E at compile time you don t really care the actual generic type  quot E quot   either you use reflection or work with base class of generic type  so let the subclass provide instance of E  abstract class SomeContainer lt E gt        abstract protected E createContents        public void doWork            E obj   createContents               Do the work with E            class BlackContainer extends SomeContainer lt Black gt       protected Black createContents             return new Black

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There are various libraries that can resolve E for you using techniques similar to what the Robertson article discussed  Here s an implemenation of createContents that uses TypeTools to resolve the raw class represented by E   E createContents   throws Exception     return TypeTools resolveRawArgument SomeContainer class  getClass    newInstance        This assumes that getClass   resolves to a subclass of SomeContainer and will fail otherwise since the actual parameterized value of E will have been erased at runtime if it s not captured in a subclass

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You can with a classloader and the class name  eventually some parameters   final ClassLoader classLoader       final Class lt   gt  aClass   classLoader loadClass  java lang Integer    final Constructor lt   gt  constructor   aClass getConstructor int class   final Object o   constructor newInstance 123   System out println  o       o

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An imporovement of  Noah s answer    Reason for Change  a  Is safer if more then 1 generic type is used in case you changed the order   b  A class generic type signature changes from time to time so that you will not be surprised by unexplained exceptions in the runtime   Robust Code  public abstract class Clazz lt P extends Params  M extends Model gt         protected M model       protected void createModel         Type   typeArguments     ParameterizedType  this getClass   getGenericSuperclass    getActualTypeArguments        for  Type type   typeArguments            if   type instanceof Class   amp  amp   Model class isAssignableFrom  Class  type                  try                   model     Class lt M gt   type  newInstance                  catch  InstantiationException   IllegalAccessException e                    throw new RuntimeException e                                     Or use the one liner  One Line Code  model     Class lt M gt     ParameterizedType  this getClass   getGenericSuperclass    getActualTypeArguments   1   newInstance

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Use the TypeToken lt T gt  class   public class MyClass lt T gt        public T doSomething             return  T  new TypeToken lt T gt      getRawType   newInstance

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If you mean  new E    then it is impossible  And I would add that it is not always correct - how do you know if E has public no-args constructor  But you can always delegate creation to some other class that knows how to create an instance - it can be Class lt E gt  or your custom code like this  interface Factory lt E gt       E create           class IntegerFactory implements Factory lt Integer gt         private static int i   0     Integer create                 return i

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Here s an implementation of createContents that uses TypeTools to resolve the raw class represented by E   E createContents   throws Exception     return TypeTools resolveRawArgument SomeContainer class  getClass    newInstance        This approach only works if SomeContainer is subclassed so the actual value of E is captured in a type definition   class SomeStringContainer extends SomeContainer lt String gt    Otherwise the value of E is erased at runtime and is not recoverable

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As you said  you can t really do it because of type erasure   You can sort of do it using reflection  but it requires a lot of code and lot of error handling

User · Answer

If you mean  new E    then it is impossible  And I would add that it is not always correct - how do you know if E has public no-args constructor  But you can always delegate creation to some other class that knows how to create an instance - it can be Class lt E gt  or your custom code like this  interface Factory lt E gt       E create           class IntegerFactory implements Factory lt Integer gt         private static int i   0     Integer create                 return i

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I don t know if this helps  but when you subclass  including anonymously  a generic type  the type information is available via reflection  e g   public abstract class Foo lt E gt       public E instance       public Foo   throws Exception       instance     Class   ParameterizedType this getClass           getGenericSuperclass    getActualTypeArguments   0   newInstance                    So  when you subclass Foo  you get an instance of Bar e g      notice that this in anonymous subclass of Foo assert  new Foo lt Bar gt       instance instanceof Bar     But it s a lot of work  and only works for subclasses  Can be handy though

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You are correct   You can t do new E     But you can change it to  private static class SomeContainer lt E gt        E createContents Class lt E gt  clazz            return clazz newInstance              It s a pain   But it works   Wrapping it in the factory pattern makes it a little more tolerable

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You ll need some kind of abstract factory of one sort or another to pass the buck to   interface Factory lt E gt        E create       class SomeContainer lt E gt        private final Factory lt E gt  factory      SomeContainer Factory lt E gt  factory            this factory   factory            E createContents             return factory create

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From Java Tutorial - Restrictions on Generics   Cannot Create Instances of Type Parameters  You cannot create an instance of a type parameter  For example  the following code causes a compile-time error   public static  lt E gt  void append List lt E gt  list        E elem   new E        compile-time error     list add elem       As a workaround  you can create an object of a type parameter through reflection   public static  lt E gt  void append List lt E gt  list  Class lt E gt  cls  throws Exception       E elem   cls newInstance         OK     list add elem       You can invoke the append method as follows   List lt String gt  ls   new ArrayList lt  gt     append ls  String class

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There are various libraries that can resolve E for you using techniques similar to what the Robertson article discussed  Here s an implemenation of createContents that uses TypeTools to resolve the raw class represented by E   E createContents   throws Exception     return TypeTools resolveRawArgument SomeContainer class  getClass    newInstance        This assumes that getClass   resolves to a subclass of SomeContainer and will fail otherwise since the actual parameterized value of E will have been erased at runtime if it s not captured in a subclass

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You can use   Class forName String  getConstructor arguments types  newInstance arguments    But you need to supply the exact class name  including packages  eg  java io FileInputStream  I used this to create a math expressions parser

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If you mean  new E    then it is impossible  And I would add that it is not always correct - how do you know if E has public no-args constructor  But you can always delegate creation to some other class that knows how to create an instance - it can be Class lt E gt  or your custom code like this  interface Factory lt E gt       E create           class IntegerFactory implements Factory lt Integer gt         private static int i   0     Integer create                 return i

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You can use   Class forName String  getConstructor arguments types  newInstance arguments    But you need to supply the exact class name  including packages  eg  java io FileInputStream  I used this to create a math expressions parser

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package org foo com   import java lang reflect ParameterizedType  import java lang reflect Type          Basically the same answer as noah s      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

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Java unfortunatly does not allow what you want to do  See the official workaround       You cannot create an instance of a type parameter  For example  the following code causes a compile-time error    public static  lt E gt  void append List lt E gt  list        E elem   new E        compile-time error     list add elem          As a workaround  you can create an object of a type parameter through reflection    public static  lt E gt  void append List lt E gt  list  Class lt E gt  cls  throws Exception       E elem   cls newInstance         OK     list add elem          You can invoke the append method as follows    List lt String gt  ls   new ArrayList lt  gt     append ls  String class

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You are correct   You can t do new E     But you can change it to  private static class SomeContainer lt E gt        E createContents Class lt E gt  clazz            return clazz newInstance              It s a pain   But it works   Wrapping it in the factory pattern makes it a little more tolerable

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You ll need some kind of abstract factory of one sort or another to pass the buck to   interface Factory lt E gt        E create       class SomeContainer lt E gt        private final Factory lt E gt  factory      SomeContainer Factory lt E gt  factory            this factory   factory            E createContents             return factory create

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From Java Tutorial - Restrictions on Generics   Cannot Create Instances of Type Parameters  You cannot create an instance of a type parameter  For example  the following code causes a compile-time error   public static  lt E gt  void append List lt E gt  list        E elem   new E        compile-time error     list add elem       As a workaround  you can create an object of a type parameter through reflection   public static  lt E gt  void append List lt E gt  list  Class lt E gt  cls  throws Exception       E elem   cls newInstance         OK     list add elem       You can invoke the append method as follows   List lt String gt  ls   new ArrayList lt  gt     append ls  String class

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You are correct   You can t do new E     But you can change it to  private static class SomeContainer lt E gt        E createContents Class lt E gt  clazz            return clazz newInstance              It s a pain   But it works   Wrapping it in the factory pattern makes it a little more tolerable

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You can do this now and it doesn t require a bunch of reflection code   import com google common reflect TypeToken   public class Q26289147       public static void main final String   args  throws IllegalAccessException  InstantiationException               final StrawManParameterizedClass lt String gt  smpc   new StrawManParameterizedClass lt String gt                final String string    String  smpc type getRawType   newInstance            System out format  string      s    string              static abstract class StrawManParameterizedClass lt T gt                final TypeToken lt T gt  type   new TypeToken lt T gt  getClass                  Of course if you need to call the constructor that will require some reflection  but that is very well documented  this trick isn t   Here is the JavaDoc for TypeToken

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If you need a new instance of a type argument inside a generic class then make your constructors demand its class     public final class Foo lt T gt         private Class lt T gt  typeArgumentClass       public Foo Class lt T gt  typeArgumentClass             this typeArgumentClass   typeArgumentClass             public void doSomethingThatRequiresNewT   throws Exception            T myNewT   typeArgumentClass newInstance                          Usage   Foo lt Bar gt  barFoo   new Foo lt Bar gt  Bar class   Foo lt Etc gt  etcFoo   new Foo lt Etc gt  Etc class     Pros    Much simpler  and less problematic  than Robertson s Super Type Token  STT  approach  Much more efficient than the STT approach  which will eat your cellphone for breakfast     Cons    Can t pass Class to a default constructor  which is why Foo is final   If you really do need a default constructor you can always add a setter method but then you must remember to give her a call later  Robertson s objection    More Bars than a black sheep  although specifying the type argument class one more time won t exactly kill you   And contrary to Robertson s claims this does not violate the DRY principal anyway because the compiler will ensure type correctness  Not entirely Foo lt L gt proof  For starters    newInstance   will throw a wobbler if the type argument class does not have a default constructor  This does apply to all known solutions though anyway  Lacks the total encapsulation of the STT approach  Not a big deal though  considering the outrageous performance overhead of STT

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An imporovement of  Noah s answer    Reason for Change  a  Is safer if more then 1 generic type is used in case you changed the order   b  A class generic type signature changes from time to time so that you will not be surprised by unexplained exceptions in the runtime   Robust Code  public abstract class Clazz lt P extends Params  M extends Model gt         protected M model       protected void createModel         Type   typeArguments     ParameterizedType  this getClass   getGenericSuperclass    getActualTypeArguments        for  Type type   typeArguments            if   type instanceof Class   amp  amp   Model class isAssignableFrom  Class  type                  try                   model     Class lt M gt   type  newInstance                  catch  InstantiationException   IllegalAccessException e                    throw new RuntimeException e                                     Or use the one liner  One Line Code  model     Class lt M gt     ParameterizedType  this getClass   getGenericSuperclass    getActualTypeArguments   1   newInstance

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what you can do is -   First declare the variable of that generic  class   2 Then make a constructor of it and instantiate that object Then use it wherever you want to use it   example-  1      private Class lt E gt  entity    2   public xyzservice Class lt E gt  entity            this entity   entity           public E getEntity Class lt E gt  entity  throws InstantiationException  IllegalAccessException           return entity newInstance            3      E e   getEntity entity

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If you want not to type class name twice during instantiation like in   new SomeContainer lt SomeType gt  SomeType class     You can use factory method    lt E gt  SomeContainer lt E gt  createContainer Class lt E gt  class      Like in   public class Container lt E gt         public static  lt E gt  Container lt E gt  create Class lt E gt  c            return new Container lt E gt  c              Class lt E gt  c       public Container Class lt E gt  c            super            this c   c             public E createInstance               throws InstantiationException              IllegalAccessException           return c newInstance

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As you said  you can t really do it because of type erasure   You can sort of do it using reflection  but it requires a lot of code and lot of error handling

[java] Create instance of generic type in Java?

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