class MongoDBClinet(val host: String, val port: Int)
val means these values are immutable
Primary constructor - It is called when object is created or called from overloaded constructors
If use var then scala also creates getters and setters
If use val then scala creates only getters , remember val are not changable so no setters
When both val and var are missing the instance values are treated as private and not accessible to anyone outside the class
Section 3.1
Download mongodb from http://www.mongodb.org/downloads
Extract it to some location
Start the mongodb
cd C:\Jagat\tools\mongodb-win32-i386-2.4.9\
mkdir db_data
bin\mongod.exe --db_path=db_data
:http://127.0.0.1:28017/
This will show admin path
The client waits at port 27017
Section 3.2
Classes and Objects
Construct the class with default values for host and port
class MongoClient ( val host : String , val port : Int ) {
def this() = this("127.0.0.1",27017)
}
The first statement in overloaded constructor has to be either other overloaded consturcotr or the primaty constructor.
To do some otherration before you invoke construcotr we use companion objects
Add the mongodb jar driver to class path of REPL session
Add jar to scala REPL classpath
scala> :cp C:\Jagat\tools\scala-eclipse\workspace\scalaination\lib\mongo-java-dr
iver-2.12.0.jar
Added 'C:\Jagat\tools\scala-eclipse\workspace\scalaination\lib\mongo-java-driver
-2.12.0.jar'. Your new classpath is:
Section 3.3
Packaging
In scala you can have nested packages
package A {
package B {
}
}
You can also use java style packaging with package declared at the top of the file
Scala packaging structure does not have to be matching with the folder structure in file system like Java. But when we compile the classes the required folder structure is auto generated as JVM needs that for working. Remember under the hood scala is running on the top of JVM
To add the jar in scala classpath via command line
scalac -classpath my.jar MyNewClass.sclaa
Section 3.4
Scala imports
You can add import statement at any point in the code.
The import will be visible lexically in the code.
* equivalent of java is _ in scala
To import all use
my.package._
If you declare classes or objects without any package , then they all belong to empty package. The cannot be imported to any other package. But members of empty package can see each other
Remap package scala class to avoid conflicts
import java.sql.{Data => SqlDate }
Hide a class , the Date class below cannot be used
import java.sql.{Data => _ }
Section 3.5
Objects and companion objects
There is no static variables in scala
Scala supports concept of having companion object and classes.
bstract class Role {
def canAccess(page: String): Boolean
}
class Root extends Role {
override def canAccess(page: String) = page != "Admin"
}
object Role {
def apply(roleName: String) = roleName match {
case "root" => new Root
case "analyst" => new Root
}
}
Note the object Role and class Role in above code , they are companion object and companion class
Package objects
Package objects allow you to define something at central place which can be used by all the memebers of the package
They are generally defined in file named
package.scala , in the package that corresponds to it
package object ch3 {
val minAge = 18
}
This above variable can be used anywhere in this package named ch3
