Instantiating a generic class in Java

Question

I know Java s generics are somewhat inferior to  Net s   I have a generic class Foo lt T gt   and I really need to instantiate a T in Foo using a parameter-less constructor  How can one work around Java s limitation

User · Answer

Quick solution that worked for me  I see there is already an answer for this and this may not even be the best way to go about it  Also  for my solution you ll need Gson   However  I ran into a situation where I needed to create an instance of a generic class of type java lang reflect Type   The following code will create an instance of the class you want with null instance variables   T object   new Gson   fromJson       myKnownType     Where myKnownType is known before hand and obtained via TypeToken getType     You can now set appropriate properties on this object  Again  this may not be the best way to do this but it works as a quick solution if that s what you need

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From https   stackoverflow com a 2434094 848072  You need a default constructor for T class       import java lang reflect ParameterizedType   class Foo      public bar         ParameterizedType superClass    ParameterizedType  getClass   getGenericSuperclass        Class type    Class  superClass getActualTypeArguments   0       try         T t   type newInstance            Do whatever with t       catch  Exception e             Oops  no default constructor       throw new RuntimeException e

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I really need to instantiate a T in Foo using a parameter-less    constructor   Simple answer is  you cant do that  java uses type erasure to implment generics which would prevent you from doing this      How can one work around Java s limitation    One way  there could be others  is to pass the object that you would pass the instance of T to the constructor of Foo lt T gt   Or you could have a method setBar T theInstanceofT   to get your T instead of instantiating in the class it self

User · Answer

One option is to pass in Bar class  or whatever type you re interested in - any way of specifying the appropriate Class lt T gt  reference  and keep that value as a field   public class Test       public static void main String   args  throws IllegalAccessException              InstantiationException           Generic lt Bar gt  x   new Generic lt  gt  Bar class           Bar y   x buildOne             public class Generic lt T gt        private Class lt T gt  clazz       public Generic Class lt T gt  clazz            this clazz   clazz             public T buildOne   throws InstantiationException  IllegalAccessException           return clazz newInstance             public class Bar       public Bar             System out println  Constructing              Another option is to have a  factory  interface  and you pass a factory to the constructor of the generic class  That s more flexible  and you don t need to worry about the reflection exceptions

User · Answer

Generics in Java are generally more powerful than in C    If you want to construct an object but without hardwiring a constructor static method  use an abstract factory  You should be able to find detailed information and tutorials on the Abstract Factory Pattern in any basic design patterns book  introduction to OOP or all over the interwebs  It s not worth duplicating code here  other than to mention that Java s closure syntax sucks   IIRC  C  has a special case for specifying a generic type has a no-args constructor  This irregularity  by definition  presupposes that client code wants to use this particular form of construction and encourages mutability   Using reflection for this is just wrongheaded  Generics in Java are a compile-time  static-typing feature  Attempts to use them at runtime are a clear indication of something going wrong  Reflection causes verbose code  runtime failures  unchecked dependencies and security vulnerabilities   Class forName is particularly evil

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Here s a rather contrived way to do it without explicitly using an constructor argument   You need to extend a parameterized abstract class   public class Test          public static void main String    args  throws Exception           Generic g   new Generic            g initParameter             import java lang reflect ParameterizedType  public abstract class GenericAbstract lt T extends Foo gt        protected T parameter        SuppressWarnings  unchecked       void initParameter   throws Exception  ClassNotFoundException           InstantiationException              Get the class name of this instance s type          ParameterizedType pt                ParameterizedType  getClass   getGenericSuperclass               You may need this split or not  use logging to check         String parameterClassName               pt getActualTypeArguments   0  toString   split    s   1              Instantiate the Parameter and initialize it          parameter    T  Class forName parameterClassName  newInstance             public class Generic extends GenericAbstract lt Foo gt       public class Foo       public Foo             System out println  Foo constructor

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I could do this in a JUnit Test Setup   I wanted to test a Hibernate facade so I was looking for a generic way to do it  Note that the facade also implements a generic interface  Here T is the database class and U the primary key   Ifacade lt T U gt  is a facade to access the database object T with the primary key U   public abstract class GenericJPAController lt T  U  C extends IFacade lt T U gt  gt         protected static EntityManagerFactory emf          The properties definition is straightforward       protected T testObject      protected C facadeManager        BeforeClass     public static void setUpClass               try               emf   Persistence createEntityManagerFactory  my entity manager factory               catch  Throwable ex                System err println  Failed to create sessionFactory object     ex               throw new ExceptionInInitializerError ex                          AfterClass     public static void tearDownClass                 Before     public void setUp            Get the class name           String className     ParameterizedType  getClass   getGenericSuperclass    getActualTypeArguments   2  getTypeName                Create the instance            try               facadeManager    C  Class forName className  newInstance              catch  ClassNotFoundException   InstantiationException   IllegalAccessException ex                Logger getLogger GenericJPAController class getName    log Level SEVERE  null  ex                     createTestObject                After     public void tearDown                           Test of testFindTEntities 0args method  of class        GenericJPAController lt T  U  C extends IFacade lt T U gt  gt           throws java lang ClassNotFoundException         throws java lang NoSuchMethodException         throws java lang InstantiationException         throws java lang IllegalAccessException              Test     public void  testFindTEntities 0args   throws ClassNotFoundException  NoSuchMethodException  InstantiationException  IllegalAccessException               Example of instance usage  Even intellisense  NetBeans  works here            try               List lt T gt  lista    List lt T gt   facadeManager findAllEntities                lista stream   forEach  ct  - gt                    System out println  Find all      stringReport                               catch  Throwable ex                System err println  Failed to access object     ex               throw new ExceptionInInitializerError ex                                            return             public abstract String stringReport         protected abstract T createTestObject        protected abstract T editTestObject        protected abstract U getTextObjectIndex

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For Java 8       There is a good solution at https   stackoverflow com a 36315051 2648077 post   This uses Java 8 Supplier functional interface

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And this is the Factory implementation  as Jon Skeet suggested   interface Factory lt T gt        T factory       class Araba         static inner class for Factory lt T gt  implementation     public static class ArabaFactory implements Factory lt Araba gt            public Araba factory                 return new Araba                        public String toString     return  Abubeee        class Generic lt T gt        private T var       Generic Factory lt T gt  fact            System out println  Constructor with Factory lt T gt  parameter            var   fact factory              Generic T var            System out println  Constructor with T parameter            this var   var            T get     return var       public class Main       public static void main String   string            Generic lt Araba gt  gen   new Generic lt Araba gt  new Araba ArabaFactory             System out print gen get               Output   Constructor with Factory lt T gt  parameter Abubeee

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Use The Constructor newInstance method  The Class newInstance method has been deprecated since Java 9 to enhance compiler recognition of instantiation exceptions   public class Foo lt T gt           public Foo                 Class lt T gt  newT   null           instantiateNew newT              T instantiateNew Class lt   gt  clsT                T newT          try               newT    T  clsT getDeclaredConstructor   newInstance              catch  InstantiationException   IllegalAccessException   IllegalArgumentException               InvocationTargetException   NoSuchMethodException   SecurityException e                   TODO Auto-generated catch block             e printStackTrace                return null                    return newT

[java] Instantiating a generic class in Java

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