[FEATURE] @Builder on an interface method

[juriad]

Use case:

I have this method defined in class DbConnection:

    @Builder(builderClassName = "MultiInsertBuilder", builderMethodName = "insertMultiple", buildMethodName = "insert")
    int insertMultipleValues(
            @Nullable Table table,
            @Singular Collection<? extends List<?>> values,
            Boolean ignoreDuplicates, /* ignore by default */
            Integer batchSize /* default 1000 */
    ) {...}

Now I have to extract an interface because there are different implementations for Oracle and Postgres.

Simply adding the annotation on a method in an interface leads to:

error: non-static variable this cannot be referenced from a static context

Implementation

There are a couple things that need to change in comparison to a builder on a method in a class or enum:

• the method returning a new builder instance must be marked as default.

• the class of the builder is static (that is how interfaces work), therefore it must capture the instance of the outer interface explicitly; instead of calling DbConnection.this.insertMultipleValues(...), it should create:

      private final DbConnection $that;
  
      MultiInsertBuilder(DbConnection $that) {
          this.$that = $that;
      }

  and change the call to: return $that.insertMultiValue(...)

Concerns

• This only works in Java 8+ since it relies on default methods. I don't think it harms not to support older versions. For older versions:
  • the builder method can simply be not generated (not forward-compatible solution),
  • alternatively this use of Builder will cause an error (forward-compatible solution). Or we may decide never to generate the builder method but that despite being forward-compatible would be limiting to many users.
• How the field should be called? $that, $enclosing, $outer?

Builder Support

I also found that @Builder is allowed on enum's constructor, which causes a compile error:

error: enum types may not be instantiated

It is not entirely related to this feature request, but can probably be fixed at the same time. I assume that we want to report an error to the user in such situation.

	type	method	constructor
class	Works	Works	Works
enum	Error	Works	Bug
interface	Error	Proposition	N/A

[juriad]

Ok, I started implementing it; it does not look hard; verbose but doable.

So far I only have one question: com.sun.tools.javac.code.Flags does not contain constant for DEFAULT because it was not a modifier in Java 6 (net.java.openjdk.custom-javac6.jar). How should I handle this?

---

I was fooled by error messages from Intellij plugin instead of Lombok itself. Actually the table should look like this (proposed improvement is highlighted):

	type	constructor	method	static method
class	Works	Works	Works	Works
enum	error: enum types may not be instantiated	error: enum types may not be instantiated	Works	Works
interface	error: $typename is abstract; cannot be instantiated	N/A	error: non-static variable this cannot be referenced from a static context	Works
annotation interface	error: modifier static not allowed here	N/A	error: non-static variable this cannot be referenced from a static context	N/A

[janrieke]

Just a hint: Before you invest too much time on a pull request, you should wait for some feedback from the maintainers. There may be reasons why this does not work (and should not work). One could be that this is not a real builder pattern any more, but a kind of method-argument collector.

I would simply not generate the builder() method. If you are on java8+, you could still add it manually to the interface or to the implementing class, as it's quite trivial.