BaseFoo cannot be inherited with different arguments: <T,X.Bar<T>> and <T,X.Foo<T>>

Question

This is a simplified version of Java inherited Fluent method return type in multiple level hierarchies.

Given the following code:

public enum X {
    ;
    static interface BaseFoo<T, S extends BaseFoo<T, S>> {
        S foo();
    }

    static interface Foo<T> extends BaseFoo<T, Foo<T>> {
        void foo1();
    }

    static interface BaseBar<T, S extends BaseBar<T, S>> extends BaseFoo<T, S> {
        S bar();
    }

    static interface Bar<T> extends BaseBar<T, Bar<T>>, Foo<T> {
        void bar1();
    }

}

run javac X.java I get the error message:

X.java:15: error: BaseFoo cannot be inherited with different arguments: <T,X.Bar<T>> and <T,X.Foo<T>>
    static interface Bar<T> extends BaseBar<T, Bar<T>>, Foo<T> {
           ^

Anyone has any solution?

Disclaim: I am trying to use the pattern to implement the fluent interface across a container class inheritance hierarchy.

Background: to make it easier for people to understand why I need this, here is the story. I want to create a container family: Traversal <- Sequence <- List. So Traversal has a method Traveral<T> accept(Visitor<T>) (no PECS for short), this method should always return this after iterating the visitor through the elements. When I have a List type, I want the method return List<T> instead of Traversal<T> because I want to make it possible to call something like myList.accept(v).head(15), where head(int) is a method of List not Traversal

Answer 1

A class or interface cannot implement or extend from different instantiation of a generic interface. Your Bar interface is breaking this rule. Let's examine the interface declaration:

static interface Bar<T> extends BaseBar<T, Bar<T>>, Foo<T>

So, Bar<T> extends two interfaces:

• BaseBar<T, Bar<T>>
• Foo<T>

In addition to that, those two interfaces extend from different instantiation of the same interface BaseFoo.

• BaseBar<T, S extends BaseBar<T, S>> extends BaseFoo<T, S>
• Foo<T> extends BaseFoo<T, Foo<T>>

Those inherited interfaces are eventually also the super interfaces of Bar interface. Thus your Bar interface tries to extend from 2 different instantiation of BaseFoo, which is illegal. Let's understand the reason using a simple example:

// Suppose this was allowed
class Demo implements Comparable<Demo> , Comparable<String> {
    public int compareTo(Demo arg)     { ... } 
    public int compareTo(String arg) { ... } 
}

then after type erasure, compiler would generate 2 bridge methods, for both the generic method. The class is translated to:

class Demo implements Comparable<Demo> , Comparable<String> {
    public int compareTo(Demo arg)     { ... } 
    public int compareTo(String arg) { ... } 

    // Bridge method added by compiler
    public int compareTo(Object arg)     { ... } 
    public int compareTo(Object arg) { ... } 
}

So, that results in creation of duplicates bridge method in the class. That is why it is not allowed.