vogella.de

You can use this update site http://www.scala-lang.org/scala-eclipse-plugin to install the Scala Plugin for Eclipse. See Eclipse Update Manager for using the update manager.

You need to activate the JDT Weaving. Select Windows -> Preferences -> JDT Weaving and press "Enable". A restart of Eclipse is required.

A class in Scala is per default defined as public. Everything which is included in the body of the class and which is field or method definition defines the primary constructor of this class.

Here is a very simple class

				
package de.vogella.scala.classexample

class Robot {
	val material = "steel"; 
}

			

The parameter of the primary constructor are directly defined in the class definition. Parameters are directly available within the class. To make them available outside of the class, either define a field and assign the parameter to it, or just add var or val in front of the variable in the primary constructor.

Here is an example for a class with parameters, one of them is automatically available via an accessor.

				
package de.vogella.scala.classexample

class Robot {
	val material = "steel"; 
}

			

				
package de.vogella.scala.classexample

object Test extends Application {
	  val  robot = new RobotWithConstructor (true, "steel")
	  robot.printStatus
	  // material can be accessed as it is defined via val in the primary constructor
	  println (robot.material)
      // Status on cannot be reached, the following line would result in syntax error
      // robot.on
}
			

Auxiliary constructors are defined via the keyword this. They have to call the primary construtor.

				
package de.vogella.scala.classexample

class RobotWithAuxiliaryConstructor  (on: Boolean, val material:String ) {
	 def printStatus() = {
	  println(on)
	  println(material)
    }
  def this() = this(true, "Steel");
}

			

				
package de.vogella.scala.classexample

class TestAuxiliary {
  val  robot = new RobotWithAuxiliaryConstructor()
	  robot.printStatus
	  // material can be accessed as it is defined via val in the primary constructor
	  println (robot.material)
      // Status on cannot be reached, the following line would result in syntax error
      // robot.on
}

			

A class can inherit from another class via the keyword "extends".

				
package de.vogella.scala.classexample;

class BetterRobot (material:String) extends RobotWithConstructor (true, material ) {
	
}
			

Scala provides a very powerful extention of inheritance, traits. Traits will be covered later.

Objects are Singletons. There are for example used to represent an application.

				
package de.vogella.scala.classexample

object Application {
	def main (args: Array[String]) {
	  println("The application is running")
	}
}

			

Objects which have the same name as a class and which are defined in the same file are called companion objects. They often provide fabric methods to create an object. See for example the List implementation in Scala.

Functions in Scala are similar to methods in Java. They are defined with the keyword "def" and have an optional parameter list and a return parameter after the ":" sign. The body of the functional is indicated by "=" and then curly braces {}

The return parameter is for a non-recursive function optional and will be determined by Scala automatically based on the function expressions. Also the curly braces for the method body are optional if the function statement has only one statement.

If a function has no parameters you can leave the () away for the call. Also the dot is not required for calling a function with only one parameter.

Create a scala project "de.vogella.scala.functions" and try the following code.

				
package de.vogella.scala.functions

object Function {
 def main(args: Array[String]) {
      println("Hello, world!")
      val f = Function;
      println(f.min(3,5))
      println(f.min2(3,5))
      // If you have no parameters you can leave the () away
      f.sayHello
      f.sayHello("Lars")
      // You can leave away the point and if you only have one paramter also the brackets
      f sayHello "Lars"
    }
 
  //Explicit return parameter
 def min(i:Int, j:Int):Int =  {
   if (i>j)
     return i
    else
     return j
 	}
 
 //Without explicit return parameter
 def min2(i:Int, j:Int) =  {
	  if (i>j)
		  i
	  else
		  j
 }
 
 // One one line a body and no parameters
 def sayHello = println("Hello")
 
 // One parameter
 def sayHello(s: String ) = println("Hello " + s)
}

			

For recursive functions you have to define the return type explicitly.

				
package de.vogella.scala.functions

object Factorial {
  
 def main(args: Array[String]) {
   val f = Factorial
   println( f.factorial(5) )
   println( f.factorial(6) )
   println( f.factorial(7) )
   println( f.factorial(8) )
 }
 
 def factorial (x:BigInt):BigInt= {
   if (x==0) 1
   else x* factorial (x-1)
 }
}

			

Scala permits almost very sign to define a method. For example you can define a method with the name +.

As you can leave out the dot and the braces for methods with only one parameter you can write in Scala a + b and "+" is a call to your method on a with the parameter b.

override is a keyword in Scala. Everytime you override a function you have to indicate this via this keyword.

				
package de.vogella.scala.functions

class OverrideExample {
	// Without override this would give an error
	override def toString() = "test"
}

			

Scala supports the uniform access principle which states that it should not make a difference if the programmer access a variable directly or via a method.

The following demonstrates this. Create the Scala project "de.vogella.scala.uniformaccess".

From the client the usage of both versions of Square is the same.

				
package de.vogella.scala.uniformaccess

class Square {
        private  var l = 1 ;
        var wide = 1;

        def length   = l;

        def length_=(s : Int)= {
        	require (s > 0 )
        	this.l = s
    }
}

			

				
package de.vogella.scala.uniformaccess

class Square {
		var length = 1 ;
        var wide = 1;
}

			

				
package de.vogella.scala.uniformaccess

object Main {
	def main(args: Array[String]) {
	  var s = new Square();
	  println(s.length);
      s.length = 2;
      println(s.length);
    }

}

			

A List in Scala is immutatable, e.g. it is not possible to add and remove entries from a list. If you add or remove an element from a list you always create a modified copy of the original list.

				
package de.vogella.scala.collections;

  object Lists {
    def main(args: Array[String]) {
      val languages = List("Java", ".NET", "C++")
      println (languages)
      println (languages(2))
      println (languages.contains("Java"))
      // + should be replaced with ::: but the Eclipse does currenlty not support this
      val additionalLanguages = languages + ("COBOL") 
      println (additionalLanguages)
    }
   
  }
  
  


			

Scala also supports Arrays. In an array you can change elements. It is not possible to add elements to an Array. This operation always created a modified copy of the existing array.

				
package de.vogella.scala.collections

object ArrayTest {
    def main(args: Array[String]) {
	val languages = Array("Java", ".NET")
	println (languages)
    }
}

			

The following shows how to create Maps.

				
package de.vogella.scala.collections

object MapTest {
    def main(args: Array[String]) {
	   val languages = Map("Scala"-> "JVM", "Java" -> "JVM", ".NET"-> "CLR");
	   println (languages("Scala"))
       
 }
}

			

The following demonstrates several methods for collections. The count, filter and exists method allow to define a closure which identifies the correct element(s).

				
package de.vogella.scala.collections

 object CollectionsMethods {
	def main(args: Array[String]) {
      val list= List(1, 2, 3, 4)
      // Prints number of all elements
       println (list.size )
      // Count only the elements which can be divided by 2
      println (list.count(i=> i %2==0))
      // Check if a elements in included in list
      println(list.exists(i=> i ==4));
      println(list.exists(i=> i ==5));
      // Using filter to get elements
      val newList = list.filter(i=> i>=3)
      println (newList)
      // Map does allow to transform a list
       val newList2 = list.map(i=> i*3)
       println (newList2)
    }
}

			

Traits are similar to abstract classes you can also use them to define methods and variables. The difference is that every class in Scala can only extends one class (abstract or normal) but and can can extend / implement as many traits as it wants. By this Scala allows "multiple inheritance".

Scala multiple inheritance is safe, e.g. it doesn't have the so-called diomond problem. Scala solve this problem by a linearisation of the call path for the methods. So even if two traits implement the same method it is clean which one is called if a class implements both.

To use the trait the class extends the first trait and the other are used with the keyword "with". If the class already extends another class you use directly the keyword with.

Create a Scala project "de.vogella.scala.traits".

Create the following coding.

				package de.vogella.scala.traits

trait MyTrait {
	def sayHello() = println("Hello from MyTrait")
}

			

				package de.vogella.scala.traits

trait MyTrait2 {
    def sayHello() = println("Hello from MyTrait2")
	def sayHello2() = println("Hello2 from MyTrait2")
}

			

				package de.vogella.scala.traits

object TraitTest extends MyTrait with MyTrait2 {

  def main(args: Array[String]) = {
	  sayHello
	  sayHello2
  }
}

			

Scala provides several pre-defined traits which can be used. For example the "Ordered" Trait defines the method compare as abstract which the client needs to implement. The "Ordered" Trait pre-define the compare operations which can then be used directly.

Source Code of Examples