Cannot override default methods of indirect superinterfaces in Java 25. #400

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1. wfouche commented on Mar 7, 2026

   

   wfouche

   on Mar 7, 2026

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   Java 25 test program.

   package org.example;
   
   interface A {
       default String getValue() {
           return "1";
       }
   } 
   
   interface B extends A {}
   
   class X implements A {
       public String getValue() {
           return "2";
       }
   }
   
   class Y implements B {
       public String getValue() {
           return "3";
       }
   }
   
   public class Java25 {
       public static void main(String[] args) {
           X x = new X();
           System.out.println(x.getValue());
           Y y = new Y();
           System.out.println(y.getValue());
       }
   }

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2. wfouche commented on Mar 7, 2026

   

   wfouche

   on Mar 7, 2026

   Last edited by wfouche

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   Flattening the interface graph also works (class Y implements A, B)

   package org.example;
   
   interface A {
       default String getValue() {
           return "1";
       }
   } 
   
   interface B extends A {}
   
   class X implements A {
       public String getValue() {
           return "2";
       }
   }
   
   class Y implements A, B {
       public String getValue() {
           return "3";
       }
   }
   
   public class Java25 {
       public static void main(String[] args) {
           X x = new X();
           System.out.println(x.getValue());
           Y y = new Y();
           System.out.println(y.getValue());
       }
   }

   2
   3
   
3. Stewori commented on Mar 8, 2026

   

   Stewori

   on Mar 8, 2026

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   Maybe a solution would be to insert the flattened list of inherited interfaces on the fly when bytecode for the class is generated. I cannot work on this myself, but I can mentor and support the solution and review process if someone wants to solve this issue. I think the starting point for this solution idea is org.python.compiler.ClassFile, it has a method addInterface. Next step would be to check from where ClassFileis called and how interfaces are supplied. That point can be adjusted to add the whole interface list.

   Or is there a better approach entirely?
4. jeff5 commented on Mar 25, 2026

   

   jeff5

   on Mar 25, 2026

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   I was surprised that Java would make such a change of semantics. I couldn't even find it in the release notes. But, if I have understood correctly, it is really the effect of a bug fix, so something that should never have been possible, and now isn't:
   https://bugs.openjdk.org/browse/JDK-8350029 and https://bugs.openjdk.org/browse/JDK-8026065 .

   We must be generating code for something that ought to be impossible. However, we're an implementation of Python, so I think the question is "what does this Python program mean", and how do we make it do that. Sorry, just scratching the surface of this but I thought the background might help.

   👍React with 👍1Reacted by Werner Fouché