Java: Passing generic type to an another generic type class

Question

Main.Java : Main Class which creating the object of Test which of generic type Test.Java: Its Generic class which is creating object of Wrapper class of generic type Wrapper.java: Its a Generic Class of Type We are passing String,Long as type for creating Test.java object . And passing long as a type for creating the object of Wrapper class. Main.Java:

    public class MainClass {
   Test<String,Long> test;

   MainClass()
   {
       test = new Test<String,Long>(){};   
   }
   public static void main(String[] arr)
   {
       MainClass m = new MainClass();
   }
  }

Test.Java

     public class Test<T,U> 
    {
     Wrapper<U> mywrapper;
    Test()
    {
      Type type = this.getClass().getGenericSuperclass();
      System.out.println("TYPE: " + type.toString());

      Type type1 = ((ParameterizedType)this.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
      System.out.println("TYPE1: " + type1.getClass().getName());


      Type[] types = ((ParameterizedType)type).getActualTypeArguments();

      for (int i = 0; i < types.length; i++) {
      System.out.println("TYPES[" + i + "]: " + types[i].toString());
      }
      mywrapper = new Wrapper<U>(){};
  }
}

Wrapper.Java

 public class Wrapper<U> {
  Wrapper()
  {
      Type type = this.getClass().getGenericSuperclass();
      System.out.println("TYPE: " + type.toString());

      Type type1 = ((ParameterizedType)this.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
      System.out.println("TYPE1: " + type1.getClass().getName());


      Type[] types = ((ParameterizedType)type).getActualTypeArguments();

      for (int i = 0; i < types.length; i++) {
      System.out.println("TYPES[" + i + "]: " + types[i].toString());
      }
  }
}

And after executing the program we are getting output as
TYPE: Test TYPE1: java.lang.Class TYPES[0]: class java.lang.String TYPES[1]: class java.lang.Long TYPE: Wrapper *TYPE1: sun.reflect.generics.reflectiveObjects.TypeVariableImpl TYPES[0]: U*

So from the output you can see for the Wrapper class we are getting Type as U instead of Long as you can see in Test.Java.

thangamanikasi · Accepted Answer · 2014-04-07 16:55:37Z

2

Hope the following program will clarify your doubt.

public class VMGeneric<A>
{
    private A objVariable;

    public VMGeneric(A localVariable)
    {
        objVariable = localVariable;
    }

    public String getNameOfGenericType()
    {
        return objVariable.getClass().getCanonicalName();
    }

    public static void main(String[] args)
    {
        VMGeneric<String> o1 = new VMGeneric<>("");
        VMGeneric<Object> o2 = new VMGeneric<>(new Object());

        System.out.println(o1.getNameOfGenericType());
        System.out.println(o2.getNameOfGenericType());
    }
}

answered Apr 7, 2014 at 16:55

thangamanikasi

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Comments

newacct · Accepted Answer · 2014-04-08 05:20:35Z

0

So from the output you can see for the Wrapper class we are getting Type as U instead of Long as you can see in Test.Java.

Right. Class metadata reflects what is written at compile time. So in order to get the type it must be hard-coded at compile-time.

answered Apr 8, 2014 at 5:20

newacct

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Comments

debabrata · Accepted Answer · 2014-04-08 05:52:44Z

0

Is it possible to get the type of the Parameter in Wrapper.java . Actually my doubt is why its working in first level indirection and not working for Wrapper.java. On problem I found in my code is that mywrapper = new Wrapper(){}; will create an anonymous class of type U and thats the reason its printing U as datatype. But if I remove the {} then I am getting classclassException as Wrapper.java

 java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType

answered Apr 8, 2014 at 5:52

debabrata

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Comments

thangamanikasi · Accepted Answer · 2014-04-09 05:36:31Z

0

FYI: In Wrapper, U is not a parameter. Instead You are saying Your class can accept any type of object instead of U. No one will know until you pass the real object. Again U is not a parameter.

answered Apr 9, 2014 at 5:36

thangamanikasi

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