Java：从具有指定类型的泛型类扩展

Question

I'm trying to extend a generic class, like:

public class GenericClass<T> implements GenericClassInterface<T>
{
     public T myMethod(String typeID) {
        T test = _get_test_value_; // here I use jackson to retrieve T de-serialized from JSON
        return T;
     }
}

Then I extend that class:

public class SpecificClass extends GenericClass<CustomType> implements SpecificClassInterface
{
     public CustomType getIstance(String typeID) {
          return super.myMethod(typeID);
     }
}

The code compiles correctly, but when I debug I find out that type T , when myMethod is called from SpecificClass, is Object , and not CustomType , even if I specified CustomType when I extended GenericClass in SpecificClass .

How come? Is this a limitation of generics in Java?

Answer 1

The type parameter T is not available in runtime (during debugging). This is indeed a limitation of generics in Java and referred to as type erasure. (All type parameters are transformed to Object as you have observed.)

See for instance

• Java generics - type erasure - when and what happens , or
• What is the concept of erasure in generics in Java?