EffectivelyFinalAndMemberVariables.java

@FunctionalInterface
public interface SimpleFunctionalInterface {
    public int returnAnswerToUltimateQuestion();
}

    @Test
    public void normalInterfaceOnlyOneAbstractMethod() {

        int x = 1;

        SimpleFunctionalInterface myInterfaceImpl = () -> {
//            x = 2; // impossible
            instanz = 3; // possible because member variables are copied when creating lambdas. (http://stackoverflow.com/questions/25055392/lambdas-local-variables-need-final-instance-variables-dont)
            
          // TODO: Why is the class, in which the lambda is defined, copied? I thought that lambdas are "closed" definitions of functionality that can be passed around.
            return 0;
        };
    }

I do not like the term copied and I don't like the answers on stackoverflow either ... might want to provide my own.

If a lambda accesses a field, this is what happens (I'm making this stuff up as I go. I did not read the Java specification for this, nor did I look at the source code of javac:

The lambda gets translated into a class, including a constructor.

If the lambda accesses a field of the enclosing class a reference to the instance gets passed into the constructor of the lambda.

=> it can read and write fields
=> it can read the instance, i.e. this but it can't change where this is pointing to. Read up about this in JS why this limitation is a good thing.

If the lambda accesses a local variable (or parameter), that parameter gets passed in as another constructor argument.

=> it can read and write fields of that variable
=> it can read the variable, but it can't change where the pointer to the variable points to.

Thanks a lot! Changed my workshop accordingly. Could you please have a look at this commit? It should contain everything important, but a review would be nice. :)