JPA Abstract Inheritance with List<AbstractClass>

My code:

@MappedSuperclass
public abstract class Header {
    public abstract List<Line> getLines();
}

@Entity
public class ChildHeader extends Header{
    @OneToMany
    public List<ChildLine> getLines();
}
 
@MappedSuperclass
public abstract class Line {
    public abstract Header getHeader();
}

@Entity
public class ChildLine extends Line {
    @ManyToOne
    public ChildHeader getHeader();
}

Problem: I get the error message "Error: 'getLines()' in 'ChildHeader' clashes with 'getLines()' in 'Header'; attempting to use incompatible return type"

Attempted solution: Change List<Line> to <T extends Line> List<T> and List<ChildLine> to <T extends ChildLine> List<T>

New Error: 'getLines()' in 'ChildHeader' clashes with 'getLines()' in 'Header'; both methods have same erasure, yet neither overrides the other

Attempted solution: Tried using Line[] and ChildLine[] instead of lists

New Error: "List/array has to be annotated with an @OrderColumn (or @IndexColumn)", but there is no ordering column. The order of the array doesn't matter, but seems like @OneToMany or JPA is requiring one.

This is an impossible situation. What can I do?

Since ChildHeader's "getLines" isn't overriding Header's declaration (which, given it's an abstract method, will eventually have to be done), for all intents and purposes, you're attempting to overload the method by different return types.

In Java, that's not allowed.

Is there any reason you can't just override Header's "getLines" method like so:

@Entity
public class ChildHeader extends Header{
    @OneToMany
    public List<Line> getLines(); 
          // since this isn't an abstract class, there
          // should be an implementation here
}

Since "ChildLine" extends "Line", you could still return a list that has nothing but "ChildLine's". You'll just need to ensure that when such a method is called, those "ChildLines" from the Line list are transferred to a ChildLine List.

Similar logic applies to the line class. ChildLine is attempting to overload "getHeader()" from Line when it would be easier to override the method by making sure their return types match.

> „…This is an impossible situation…“

Not if you follow the JLS' guidance on overriding instance methods…

> ### 8.4.8.1. Overriding (by Instance Methods)
>
> An instance method m_C declared in or inherited by class C, overrides from C another method m_A declared in class A, iff all of the following are true:
>
> …
>
> * The signature of m_C is a subsignature (&sect;8.4.2) of the signature of m_A.
>
> …
>

Pay particular attention to the section on method signatures…

> ### 8.4.2. Method Signature
>
> Two methods or constructors, M and N, have the same signature if they have the same name, the same type parameters (if any) (&sect;8.4.4), and, after adapting the formal parameter types of N to the the type parameters of M, the same formal parameter types…

The crux of the errors you got is because your methods don't meet the JLS's criteria.

> „…What can I do?…“

If you refactored your Header class…

public abstract class Header {
    
    public abstract < L extends Line > List< L > getLines( );
}

And if you refactored your ChildHeader class…

public class ChildHeader extends Header {
    
    @Override
    @SuppressWarnings("unchecked")
    public < C extends Line > List< C > getLines(){ … }
}

Then you could do what I demonstrate in this working example…

…
Header header = new ChildHeader( );
  
List< ChildLine > lines = header.getLines( );
  
ChildHeader childHeader = lines.get( 0 ).getHeader( );
…