Tuesday, 18 October 2016

Introduction to SCALA : Hello World Program in Scala : Day 1 Learnings

Your first lines of code in Scala

















The “Hello, world!” Program

As a first example, we use the standard “Hello, world!” program to demonstrate the use of the Scala tools without knowing too much about the language.
  1. object HelloWorld {
  2. def main(args: Array[String]): Unit = {
  3. println("Hello, world!")
  4. }
  5. }
The structure of this program should be familiar to Java programmers: it consists of the method main which prints out a friendly greeting to the standard output.
We assume that both the Scala software and the user environment are set up correctly. For example:
EnvironmentVariableValue (example)
Unix$SCALA_HOME/usr/local/share/scala
$PATH$PATH:$SCALA_HOME/bin
Windows%SCALA_HOME%c:\Progra~1\Scala
%PATH%%PATH%;%SCALA_HOME%\bin

Run it interactively!

The scala command starts an interactive shell where Scala expressions are interpreted interactively.
  1. > scala
  2. This is a Scala shell.
  3. Type in expressions to have them evaluated.
  4. Type :help for more information.
  5. scala> object HelloWorld {
  6. | def main(args: Array[String]): Unit = {
  7. | println("Hello, world!")
  8. | }
  9. | }
  10. defined module HelloWorld
  11. scala> HelloWorld.main(Array())
  12. Hello, world!
  13. scala>:q
  14. >
The shortcut :q stands for the internal shell command :quit used to exit the interpreter.

Compile it!

The scalac command compiles one (or more) Scala source file(s) and generates Java bytecode which can be executed on any standard JVM. The Scala compiler works similarly to javac, the Java compiler of the Java SDK.
  1. > scalac HelloWorld.scala
By default scalac generates the class files into the current working directory. You may specify a different output directory using the -d option.
  1. > scalac -d classes HelloWorld.scala

Execute it!

The scala command executes the generated bytecode with the appropriate options:
  1. > scala HelloWorld
scala allows us to specify command options, such as the -classpath (alias -cp) option:
  1. > scala -cp classes HelloWorld
The argument of the scala command has to be a top-level object. If that object extends trait App, then all statements contained in that object will be executed; otherwise you have to add a method main which will act as the entry point of your program.
Here is how the “Hello, world!” example looks like using the App trait:
  1. object HelloWorld extends App {
  2. println("Hello, world!")
  3. }

Script it!

We may also run our example as a shell script or batch command (see the examples in the man pages of the scala command).
The bash shell script script.sh containing the following Scala code (and shell preamble):
  1. #!/usr/bin/env scala
  2. object HelloWorld extends App {
  3. println("Hello, world!")
  4. }
  5. HelloWorld.main(args)
can be run directly from the command shell:
  1. > ./script.sh
Note: We assume here that the file script.sh has execute permission and the search path for the scala command is specified in the PATH environment variable.

Introduction to SCALA: Installing Scala Day 1 Learnings

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