ConvertToLambda hint should ignore default methods.

[mbien]

Default methods don't qualify for functional interfaces since they are not abstract and can therefore not be converted into lambdas.

The hint has to go through the interface hierarchy and check if the implemented method was a default method.

example:

    private interface NotFunctional1 {
        default void a(int i) {};
    }
    
    // can't be converted to lambda
    NotFunctional1 nf = new NotFunctional1() { 
        @Override
        public void a(int i) {
        }
    };

real world example: com.sun.source.util.TaskListener

[lahodaj]

Looks like a reasonable direction to me, with some minor comments inline.

[lahodaj]

So, two comments here:

• should we rather check the method implements an abstract interface method, rather than checking it is not default? (Meaning, if the method based is not found, we don't try to do the conversion?)
• should this be moved to be part of ensurePreconditionsAreChecked? (And then passesAllPreconditions and passesFatalPreconditions?)

[mbien]

should we rather check the method implements an abstract interface method, rather than checking it is not default? (Meaning, if the method based is not found, we don't try to do the conversion?)

changed it to an override-abstract-method check. More straight forward, I agree.

should this be moved to be part of ensurePreconditionsAreChecked? (And then passesAllPreconditions and passesFatalPreconditions?)

I haven't implemented this so far. since the method getMethodFromFunctionalInterface is wrong without the change - it would return a method even when the interface is not functional. The precondition checks are all working, since the existence of the method is part of the checks.

Please take another look if you agree, if not I can still make it a precondition - but we probably should also rename the method. Also please note that many methods are actually unused. E.g passesAllPreconditions isn't used anywhere which means the test coverage is also not ideal atm.

[mbien]

@jlahoda forgot to ping

[mbien]

would like to start to merge a few of my PRs this weekend since they start to pile up, this would be a potential candidate, cc @lahodaj

[mbien]

planning to merge this this weekend before feature freeze unless someone objects.

[matthiasblaesing]

Implementation looks sane to me and I was able to reproduce the problem and found it fixed after this.