BaseFoo cannot be inherited with different arguments: > and >

Question

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

Given the following code:

public enum X {
            


        

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.