why can scala support this: new PrintWriter("filename") { write("file contents"); close }?

Question

I have a question regarding a trick

new PrintWriter("/tmp/some.file") { write("file contents"); close }

that I learned from this interesting post: Scala: write string to file in one statement

The technique works great (for test code), however I am a little puzzled about what is going on syntactically in Scala.

PrintWriter is a java class... but it looks like a code block is being passed to the print writer instance, and implicitly method calls are being executed on that instance. The PrintWriter java class definition has no apply() method that takes a function block.

So .. I'm puzzled as to what is going on visa vi Scala syntax

thanks !

rogue-one · Accepted Answer · 2017-05-21T23:16:34.900

6

This works because you are effectively creating an anonymous object that extends PrintWriter class. the code write("file contents"); close is simply part of the constructor of this anonymous object.

scala> val writer = new PrintWriter("/tmp/some.file") { write("file contents"); close }
writer: java.io.PrintWriter = $anon$1@6c07add6

scala> writer.getClass.getName
res4: String = $anon$1

edited May 21 '17 at 23:16

answered May 21 '17 at 23:11

rogue-one

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s/anonymous object/anonymous class. the `new SomeClass { ... }` is also in Java, except in Java the code for constructor needs to be explicit. https://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html – pedrofurla May 21 '17 at 23:14
great answer ! embarrassed I didn't realize that ;^) – Chris Bedford May 21 '17 at 23:42
@ChrisBedford happens to the best of us :) – rogue-one May 22 '17 at 00:15

why can scala support this: new PrintWriter("filename") { write("file contents"); close }?

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