Java 8 Method references called on a local variable

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I am in the process of learning Java 8 and I came across something that I find a bit strange. Imagine the following snippet:

private MyDaoClass myDao;

public void storeRelationships(Set<Relationship<ClassA, ClassB>> relationships) {

    RelationshipTransformer transformer = new RelationshipTransformerImpl();

    myDao.createRelationships(
            relationships.stream()
            .map((input) -> transformer.transformRelationship(input)
        ).collect(Collectors.toSet());
    );
  }

Basically, I need to map the input set called relationships to a different type in order to conform to the API of the DAO I'm using; For the conversion, I would like to use an existing RelationshipTransformerImpl class that I instantiate as a local variable.

Now, here's my question: If I was to modify the above code like so:

public void storeRelationships(Set<Relationship<ClassA, ClassB>> relationships) {

    RelationshipTransformer transformer = new RelationshipTransformerImpl();

    myDao.createRelationships(
            relationships.stream()
            .map((input) -> transformer.transformRelationship(input)
        ).collect(Collectors.toSet());
    );

    transformer = null;  //setting the value of an effectively final variable
  }

I would obviously get a compiler error, since the local variable "transformer" is no longer "effectively final". However, if I change the lambda so that it uses a method reference:

public void storeRelationships(Set> relationships) {

    RelationshipTransformer transformer = new RelationshipTransformerImpl();

    myDao.createRelationships(
            relationships.stream()
            .map(transformer::transformRelationship
        ).collect(Collectors.toSet());
    );

    transformer = null;  //setting the value of an effectively final variable
  }

Then I no longer get a compilation error ! Why does this happen? I thought the two ways to write the lambda expression should be equivalent, but there's clearly something more going on.

Thanks

asked 32 secs ago

Emil D

595

Java 8 Method references called on a local variable