What is the groovy << operator mean in this context?

In a groovy tutorial, I encountered the following code:

class DateTagLib {
  def thisYear = {
    out << Calendar.getInstance().get(Calendar.YEAR)
  }
}

I don't know what the << means, and I'm having no luck with google.

开发者_Python百科

Edit: I now know that << sometimes is a bit shift. But what does it mean here?

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In groovy, the bitwise operators can be overridden with the leftShift (<<) and rightShift (>>) methods defined on the class. It's idiomatic groovy to use the leftShift method for append actions on strings, buffers, streams, arrays, etc and thats what you're seeing here.

For example:

• The overloaded leftShift methods on OutputStream which are used to append bytes, an InputStream, or an Object to the stream.
• List, which also uses it as an append

You are looking at a grails tag lib, so out represents the page that's being rendered. The results of this taglib will be added to the output buffer that will be rendered to the client.

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Assuming that out is System.out the << operator writes to out in this case. The idiom to overload << for output (writing to a stream) and >> for input (reading from a stream) comes from C++. When the stdio libraries (istream ostream etc.) got defined, the idea was born to use left shift and right shift for output and input.