class java.lang.Class cannot be cast to class java.lang.reflect.Parameterized

Question

Java 11

I need to get generic type on runtime.

Here client code:

     var dataTemplateClient = new DataTemplateJdbc<Client>(dbExecutor, entitySQLMetaDataClient); 

    public class Client {
        @Id
        private Long id;
        private String name;
    }

Here code:

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.sql.Connection;
import java.util.Collections;
import java.util.List;
import java.util.Optional;

    public class DataTemplateJdbc<T> implements DataTemplate<T> {
        private final DbExecutor dbExecutor;
        private final EntitySQLMetaData entitySQLMetaData;
    
        public DataTemplateJdbc(DbExecutor dbExecutor, EntitySQLMetaData entitySQLMetaData) {
            this.dbExecutor = dbExecutor;
            this.entitySQLMetaData = entitySQLMetaData;
            ParameterizedType genericSuperclass = (ParameterizedType) getClass().getGenericSuperclass();
            Type type = genericSuperclass.getActualTypeArguments()[0];
        }

But I get error in this line:

ParameterizedType genericSuperclass = (ParameterizedType) getClass().getGenericSuperclass();

Error:

Exception in thread "main" java.lang.ClassCastException: class java.lang.Class cannot be cast to class java.lang.reflect.ParameterizedType (java.lang.Class and java.lang.reflect.ParameterizedType are in module java.base of loader 'bootstrap')
    at mypackage.DataTemplateJdbc.<init>(DataTemplateJdbc.java:26)

Answer 1

Basically, what you are trying to do won't work. You cannot get the actual type T at runtime.

When you call getClass() on a DataTemplateJdbc , it will give you the Class object that represents the erased type. That is all that is available.

The reason that the typecast fails is that a Class can never represent a parameterized type.