Java：汇总操作

Question

I'm studying generics, functional interfaces, lambda expressions, and aggregate operations. Although I understand these technologies individually, I'm having trouble conceptualizing them collectively, ie, how they work together.

Using an example taken from the Java Doc titled: interface Collector(T,A,R) :

Collector(Widget,?,TreeSet(Widget)) into = Collector.of(TreeSet::new, TreeSet::add, (left,right)->{left.addAll(right); return left;});

The above method, in the aforementioned javadoc, has the following behavioral parameters:

Supplier(A) supplier,
BiConsumer(A,T) accumulator,
BiConsumer(A) combiner

The supplier provided in the above example uses the method reference: TreeSet::new .

What is the relationship of the TreeSet::new , being used as the supplier of results, to the accumulator?..to that of the combiner?

How does this work, I don't understand how this is a supplier!?

Any documentation links concerning this broad subject matter would be worth much gold and silver. Thanks for your patience.

Answer 1

Quotes directly from the Java documentation :

The Supplier:

A function that creates and returns a new mutable result container.

The TreeSet::new is just a call to the constructor of TreeSet, which returns an instance of TreeSet. In this sense it is a supplier of TreeSets.

The BiConsumer, called the accumulator in this context, represents

A function that folds a value into a mutable result container.

It will offer objects of type T to the object of type A. Typically you will just call some add method of a collection, but you can also do some pre-processing here when defining a custom collector.

Lastly the BiConsumer is described as:

A function that accepts two partial results and merges them.

It is used when a process is executed in parallel and partial results (in this context, TreeSet objects with only a subset of the elements) need to be combined into the final TreeSet result. This would typically be an addAll method.

So by providing these necessary ingredients, you are able to construct your own collection mechanisms.