Using forEach on Documents in MongoDB

Question

I am following this. I am unable to understand why the following would mean print each Document in JSON format. That is how forEach works and why the method name apply is run for each Document

Block<Document> printBlock = new Block<Document>() {
       @Override
       public void apply(final Document document) {
           System.out.println(document.toJson());
       }
};

collection.find(new Document()).forEach(printBlock);

Answer 1

This is because the find operation return a MongoIterable which has declared a forEach method for java-7 Iterable which has no forEach method at all.

So you can call the forEach method on a MongoIterable as in your question. In java8, the Iterable includes the forEach(Consumer) operation. if you use forEach with inlined Lambda Expression or Method Reference Exception in java8, you must to cast the lambda to the explicit target type, for example:

collection.find(new Document())
          .forEach( it-> System.out.println(it.toJson())); // compile-error

// worked with inlined lambda expression
collection.find(new Document())
          .forEach((Consumer<Document>) it -> System.out.println(it.toJson()));


collection.find(new Document())
          .forEach((Block<Document>) it -> System.out.println(it.toJson()));

// worked with method reference expression
collection.find(new Document())
          .forEach((Consumer<Document>) printBlock::apply);

collection.find(new Document())
          .forEach((Block<Document>) printBlock::apply);

For more details about lambda expression you can see here.