Question

In Eclipse plugin, I could create scala Object, APP and class. But what’s the difference among these 3. Basically, for object, I implement def main(…), then I could run it. But in class, it seems it is similar to normal Java class, but what’s the meaning scala APP, it creates a object which extends APP. What’s this meant for?

Answer 1

In the JVM, the starting point of the application is a static method main(String[] args), in some class given to the JVM.

In scala there is no static methods, the equivalent is to put a method in an object (as opposed to class). Putting your main method in a class will not work, it is the same as not marking the method static in java.

App is a helper that allows not to write the main method, and put the code directly in the object body.

object MyApp extends App {
  doStuff
}

does the same thing as

object MyApp {
  def main(args: Array[String]) {doStuff}
}

(arguments can be used in doStuff under the name args)