A DESCRIPTION OF THE PROBLEM :
JVMS §5.4.3.3 Method Resolution says:

>[When] method resolution attempts to locate the referenced method in the superinterfaces of the specified class C:
>
>If the maximally-specific superinterface methods of C for the name and descriptor specified by the method reference include exactly one method that does not have its ACC_ABSTRACT flag set, then this method is chosen and method lookup succeeds.
>
>Otherwise, if any superinterface of C declares a method with the name and descriptor specified by the method reference that has neither its ACC_PRIVATE flag nor its ACC_STATIC flag set, one of these is arbitrarily chosen and method lookup succeeds."

I.e., if we have a class which inherits from its interfaces two equally-specific competing definitions of a default method, one of these is chosen arbitrarily.

In reality, as shown by the attached test case, the VM throws an error:

java.lang.IncompatibleClassChangeError: Conflicting default methods: I1.foo I2.foo

In trying to determine if it is the VM or the spec that is wrong here, I notice that JLS §13.5.7, which gives an equivalent example involving "Cowboy" and "CowboyArtist", says that an IncompatibleClassChangeError will be thrown.


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import java.lang.classfile.ClassFile;
import java.lang.constant.*;
import java.lang.reflect.Method;

class MaxSpecificBug {
    public static void main(String... args) throws Throwable {
        // public class C implements I1, I2 {}
        byte[] cBytes = ClassFile.of().build(ClassDesc.of("C"), cb -> {
            cb.withFlags(ClassFile.ACC_PUBLIC | ClassFile.ACC_SUPER);
            cb.withInterfaceSymbols(ClassDesc.of("I1"), ClassDesc.of("I2"));
            cb.withMethodBody("<init>", ConstantDescs.MTD_void, ClassFile.ACC_PUBLIC, cdb -> {
                cdb.aload(0);
                cdb.invokespecial(ConstantDescs.CD_Object, "<init>", ConstantDescs.MTD_void);
                cdb.return_();
            });
        });
        
        // public interface I1 { default void foo() { System.out.println("Hello from I1"); } }
        byte[] i1Bytes = ClassFile.of().build(ClassDesc.of("I1"), cb -> {
            cb.withFlags(ClassFile.ACC_PUBLIC | ClassFile.ACC_INTERFACE | ClassFile.ACC_ABSTRACT);
            cb.withMethodBody("foo", ConstantDescs.MTD_void, ClassFile.ACC_PUBLIC, cdb -> {
                cdb.getstatic(ClassDesc.of("java.lang.System"), "out", ClassDesc.of("java.io.PrintStream"));
                cdb.ldc("Hello from I1");
                cdb.invokevirtual(ClassDesc.of("java.io.PrintStream"), "println",
                    MethodTypeDesc.of(ConstantDescs.CD_void, ConstantDescs.CD_Object));
                cdb.return_();
            });
        });
        
        // public interface I2 { default void foo() { System.out.println("Hello from I2"); } }
        byte[] i2Bytes = ClassFile.of().build(ClassDesc.of("I2"), cb -> {
            cb.withFlags(ClassFile.ACC_PUBLIC | ClassFile.ACC_INTERFACE | ClassFile.ACC_ABSTRACT);
            cb.withMethodBody("foo", ConstantDescs.MTD_void, ClassFile.ACC_PUBLIC, cdb -> {
                cdb.getstatic(ClassDesc.of("java.lang.System"), "out", ClassDesc.of("java.io.PrintStream"));
                cdb.ldc("Hello from I2");
                cdb.invokevirtual(ClassDesc.of("java.io.PrintStream"), "println",
                    MethodTypeDesc.of(ConstantDescs.CD_void, ConstantDescs.CD_Object));
                cdb.return_();
            });
        });
        
        DirectClassLoader dcl = new DirectClassLoader();
        dcl.load(i1Bytes);
        dcl.load(i2Bytes);
        Class<?> clss = dcl.load(cBytes);
        
        Object instance = clss.getConstructor().newInstance();
        Method foo = clss.getMethod("foo");
        
        // expected: "Hello from I1" or "Hello from I2"; no error
        // reality: IncompatibleClassChangeError
        foo.invoke(instance);
    }
    
    
    static final class DirectClassLoader extends ClassLoader {
        public Class<?> load(byte[] classBytes) {
            return super.defineClass(null, classBytes, 0, classBytes.length);
        }
    }
}
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