I was addicted to using Java's Stream API in Scala

Introduction

Overview

I'm new to Scala, and I'm addicted to mixing Java and Scala Lists. I think that people who usually write Java and are new to Scala may be addicted to it, so I will share the cause and solution.

environment

• Scala2.12.6
• Java10.0.2

Thing you want to do

I want to extract only the necessary information from the list of tweets returned by the API of Twitter4J (ResponseList [Status]) and get a List of my own class Tweet type. In Java, if you use Stream API, you can kill it instantly.

Original code

code

The ResponseList returned from the Twitter4J API can use the Stream API, so it looks like this when written in Java style.

• It does not move.

val statuses: ResponseList[Status] = twitter.getUserTimeline(paging)
      val tweets: List[Tweet] = statuses
        .stream()
        .map(status => new Tweet(status.getId, status.getText, status.getUser.getName))
        .collect(Collectors.toList) //For some reason there is a compile error here ...

Cause

The Stream API termination operation .collect (Collectors.toList) returns java.util.List, but the type of the variable to be stored is Scala List type.

Scala can use both Scala API and Java API, but they are different. Both offer types with the same name, but don't mix them up.

Policy to rewrite

1. Since there is no help for it, adopt java.util.List as the variable type

The easiest.

val statuses: ResponseList[Status] = twitter.getUserTimeline(paging)
val tweets: java.util.List[Tweet] = statuses
    .stream()
    .map(status => new Tweet(status.getId, status.getText, status.getUser.getName))
    .collect(Collectors.toList)

However, if the method returns the variable tweets, Scala's List is good! Because it's Scala.

2. Convert Java collection class to Scala collection class

According to the Scala documentation (https://docs.scala-lang.org/en/overviews/collections/conversions-between-java-and-scala-collections.html), Scala and Java collections are converted to each other. It seems that it can be done.

//JavaConverters class is responsible for converting collections
import scala.collection.JavaConverters._

val statuses: ResponseList[Status] = twitter.getUserTimeline(paging)
val tweets: List[Tweet] = statuses
      //ResponseList(Java) => mutable.Buffer(Scala)
      .asScala 
      //Then use the Scala collection operation API
      .map(status => new Tweet(status.getId, status.getText, status.getUser.getName))
      .toList

Supplement about Java Converters

Comparison table between Scala collections and Java collections

How to use

It is possible to convert by declaring ʻimport collection.JavaConverters._ and then using ʻasJava method and ʻasScala` method. However, it seems that it is not all-purpose, so please use it with care. For more information, see Official Documentation

Reference URL

Convert collections between Scala Documentation JAVA and SCALA