What is the difference between these two objects?

Why do they both exist?

When do I want to use one vs. the other?

解决方案

EDIT: Java Conversions got @deprecated in Scala 2.13.0. Use scala.jdk.CollectionConverters instead.

JavaConversions provide a series of implicit methods that convert between a Java collection and the closest corresponding Scala collection, and vice versa. This is done by creating wrappers that implement either the Scala interface and forward the calls to the underlying Java collection, or the Java interface, forwarding the calls to the underlying Scala collection.

JavaConverters uses the pimp-my-library pattern to “add” the asScala method to the Java collections and the asJava method to the Scala collections, which return the appropriate wrappers discussed above. It is newer (since version 2.8.1) than JavaConversions (since 2.8) and makes the conversion between Scala and Java collection explicit. Contrary to what David writes in his answer, I'd recommend you make it a habit to use JavaConverters as you'll be much less likely to write code that makes a lot of implicit conversions, as you can control the only spot where that will happen: where you write .asScala or .asJava.

Here's the conversion methods that JavaConverters provide:

Pimped Type | Conversion Method | Returned Type

=================================================================================================

scala.collection.Iterator | asJava | java.util.Iterator

scala.collection.Iterator | asJavaEnumeration | java.util.Enumeration

scala.collection.Iterable | asJava | java.lang.Iterable

scala.collection.Iterable | asJavaCollection | java.util.Collection

scala.collection.mutable.Buffer | asJava | java.util.List

scala.collection.mutable.Seq | asJava | java.util.List

scala.collection.Seq | asJava | java.util.List

scala.collection.mutable.Set | asJava | java.util.Set

scala.collection.Set | asJava | java.util.Set

scala.collection.mutable.Map | asJava | java.util.Map

scala.collection.Map | asJava | java.util.Map

scala.collection.mutable.Map | asJavaDictionary | java.util.Dictionary

scala.collection.mutable.ConcurrentMap | asJavaConcurrentMap | java.util.concurrent.ConcurrentMap

—————————————————————————————————————————————————————————————————————————————————————————————————

java.util.Iterator | asScala | scala.collection.Iterator

java.util.Enumeration | asScala | scala.collection.Iterator

java.lang.Iterable | asScala | scala.collection.Iterable

java.util.Collection | asScala | scala.collection.Iterable

java.util.List | asScala | scala.collection.mutable.Buffer

java.util.Set | asScala | scala.collection.mutable.Set

java.util.Map | asScala | scala.collection.mutable.Map

java.util.concurrent.ConcurrentMap | asScala | scala.collection.mutable.ConcurrentMap

java.util.Dictionary | asScala | scala.collection.mutable.Map

java.util.Properties | asScala | scala.collection.mutable.Map[String, String]

To use the conversions directly from Java, though, you're better off calling methods from JavaConversions directly; e.g.:

List javaList = new ArrayList(Arrays.asList("a", "b", "c"));

System.out.println(javaList); // [a, b, c]

Buffer scalaBuffer = JavaConversions.asScalaBuffer(javaList);

System.out.println(scalaBuffer); // Buffer(a, b, c)

List javaListAgain = JavaConversions.bufferAsJavaList(scalaBuffer);

System.out.println(javaList == javaListAgain); // true