In Java, the error “could not create query for public abstract java.util.List” typically occurs when trying to create a query using a list as a parameter, but not specifying the type of the list elements. This error is commonly encountered when using technologies such as JPA or Hibernate for database queries.
To understand this error better, let’s consider an example. Suppose we have an entity class called “User” and we want to fetch a list of users from the database based on some criteria. We can use a JPA repository and define a method like this:
public interface UserRepository extends JpaRepository<User, Long> {
List<User> findByAgeGreaterThan(int age);
}
In this example, we have defined a method called “findByAgeGreaterThan” in the UserRepository interface. It takes an integer parameter “age” and is expected to return a list of User objects whose age is greater than the specified value.
Now, let’s say we mistakenly forget to specify the type of the list elements, like this:
public interface UserRepository extends JpaRepository<User, Long> {
List findByAgeGreaterThan(int age);
}
In this case, we have omitted the type parameter <User> in the List declaration. As a result, the compiler will generate the error message “could not create query for public abstract java.util.List” because it is unable to determine the type of objects that should be returned by the query.
To fix this error, make sure to include the type parameter in the List declaration. In our example, the correct method declaration should be:
public interface UserRepository extends JpaRepository<User, Long> {
List<User> findByAgeGreaterThan(int age);
}
By specifying the type parameter, the compiler will be able to create the query correctly and return a list of User objects as expected.