Scala class

******* Scala Class  **********

By default all the arguments passed to class are constructor arguments
All the free flowing code with the class body is constructor behavior.
In case if you want to create class level variables, you need to use val or var
Class methods can be defined with in the class body


scala> class Order(orderId: Int,orderDate: String,orderCustomerId: Int,orderStatus: String){
     | println("I am inside order constructor")
     | }
defined class Order

If we define the arguments in class itself,this mean we are creating a defaulty constructor with those arguments.

We can visualize this class as a java class by running below command

scala> :javap -p Order
Compiled from "<console>"
public class Order {
  public Order(int, java.lang.String, int, java.lang.String);
}

## TO initalize a class or to create an object of the class Order, then,

scala> val order = new Order()
<console>:8: error: not enough arguments for constructor Order: (orderId: Int, orderDate: String, orderCustomerId: Int, orderStatus: String)Order.
Unspecified value parameters orderId, orderDate, orderCustomerId, ...
       val order = new Order()
 
The above command as thrown error because we dont have default constructor with no args.BUt below will work

scala> val order = new Order(1,"2014-10-10",100,"COMPLETED")
I am inside order constructor
order: Order = Order@2e185cd7

Suppose we want to create functions,
 then,                                                               ^

scala> class Order(orderId: Int,orderDate: String,orderCustomerId: Int,orderStatus: String){
     | println("I am inside order constructor")
     | override def toString = "Order ("+orderId+","+orderDate+","+orderStatus+")"
     | }
defined class Order

scala> val order = new Order(1,"2014-10-10",100,"COMPLETED")
I am inside order constructor
order: Order = Order (1,2014-10-10,COMPLETED)

scala> :javap -p Order
Compiled from "<console>"
public class Order {
  private final int orderId;
  private final java.lang.String orderDate;
  private final java.lang.String orderStatus;
  public java.lang.String toString();
  public Order(int, java.lang.String, int, java.lang.String);
}

#### suppose we want to access a variable say orderId,it is not possible because we dont not have getter and setter.To do that, we need to specify val for the constructor args during class creation.

scala> order.orderId
<console>:10: error: value orderId is not a member of Order
              order.orderId
                    

Now let us add val to the args

scala> :paste
// Entering paste mode (ctrl-D to finish)

class Order(val orderId: Int,val orderDate: String,val orderCustomerId: Int,val orderStatus: String){
println("I am inside order constructor")
override def toString = "Order ("+orderId+","+orderDate+","+orderStatus+")"
}

// Exiting paste mode, now interpreting.

defined class Order



scala> :javap -p Order
Compiled from "<console>"
public class Order {
  private final int orderId;
  private final java.lang.String orderDate;
  private final int orderCustomerId;
  private final java.lang.String orderStatus;
  public int orderId();
  public java.lang.String orderDate();
  public int orderCustomerId();
  public java.lang.String orderStatus();
  public java.lang.String toString();
  public Order(int, java.lang.String, int, java.lang.String);
}

scala> order.orderId()
<console>:10: error: value orderId is not a member of Order
              order.orderId()

Above commad did not work, bcoz scala doesnot accept () to invoke a method.Please refer below for more info

scala> :paste
// Entering paste mode (ctrl-D to finish)

class Order(val orderId: Int,val orderDate: String,val orderCustomerId: Int,val orderStatus: String){
println("I am inside order constructor")
override def toString = "Order ("+orderId+","+orderDate+","+orderStatus+")"
}


// Exiting paste mode, now interpreting.

defined class Order

scala> val order = new Order(1,"2014-10-10",100,"COMPLETED")
I am inside order constructor
order: Order = Order (1,2014-10-10,COMPLETED)

scala> order.orderId
res0: Int = 1

scala> order.orderDate
res1: String = 2014-10-10

scala> order.orderDate()
<console>:10: error: not enough arguments for method apply: (index: Int)Char in class StringOps.
Unspecified value parameter index.
              order.orderDate()
                             ^

Now if we want to change the value of orderID, we cant do that becoz we have used val.We need to change it to var,Which will internally create setters

Val will only create getter
var will create both setter and getter

scala> order.orderId = 2;
<console>:9: error: reassignment to val
       order.orderId = 2;
                     ^

scala> :paste
// Entering paste mode (ctrl-D to finish)

class Order(var orderId: Int,var orderDate: String,var orderCustomerId: Int,var orderStatus: String){
println("I am inside order constructor")
override def toString = "Order ("+orderId+","+orderDate+","+orderStatus+")"
}

// Exiting paste mode, now interpreting.

defined class Order

scala> val order = new Order(1,"2014-10-10",100,"COMPLETED")
I am inside order constructor
order: Order = Order (1,2014-10-10,COMPLETED)

scala> :javap -i order
usage       :javap [opts] [path or class or -]...
-help       Prints this help message
-raw        Don't unmangle REPL names
-app        Show the DelayedInit body of Apps
-fun        Show anonfuns for class or Class#method
-verbose/-v Stack size, number of locals, method args
-private/-p Private classes and members
-package    Package-private classes and members
-protected  Protected classes and members
-public     Public classes and members
-l          Line and local variable tables
-c          Disassembled code
-s          Internal type signatures
-sysinfo    System info of class
-constants  Static final constants

scala> :javap -p order
Compiled from "<console>"
public class  {
  public static final  MODULE$;
  private final Order order;
  public static {};
  public Order order();
  public ();
}

scala> :javap -p Order
Compiled from "<console>"
public class Order {
  private int orderId;
  private java.lang.String orderDate;
  private int orderCustomerId;
  private java.lang.String orderStatus;
  public int orderId();
  public void orderId_$eq(int);
  public java.lang.String orderDate();
  public void orderDate_$eq(java.lang.String);
  public int orderCustomerId();
  public void orderCustomerId_$eq(int);
  public java.lang.String orderStatus();
  public void orderStatus_$eq(java.lang.String);
  public java.lang.String toString();
  public Order(int, java.lang.String, int, java.lang.String);
}

scala> order.OrderId
<console>:10: error: value OrderId is not a member of Order
              order.OrderId
                    ^

scala> oerder.orderId
<console>:8: error: not found: value oerder
              oerder.orderId
              ^

scala> order.orderId
res2: Int = 1

scala> order.orderId = 2
order.orderId: Int = 2

scala> order
res3: Order = Order (2,2014-10-10,COMPLETED)

scala> order.toString
res4: String = Order (2,2014-10-10,COMPLETED)

scala> order.orderId_=(3)

scala> order
res6: Order = Order (3,2014-10-10,COMPLETED)

scala> order.orderId=(4)
order.orderId: Int = 4

scala> order
res7: Order = Order (4,2014-10-10,COMPLETED)

scala> order.orderId=5
order.orderId: Int = 5

scala> order
res8: Order = Order (5,2014-10-10,COMPLETED)


*******Object

Objects are special types of classes.

It is singleton class
main function is defined in object
If the class name and object name are same then they are called as companions
We cannot instantiate object


******* there is no static method in scala *******

In Scala class, if you write a main method ,after compile,scala class will be converted to java class which in turns converts static main method to normal main method.

---------
Sample object,

scala> object HellowWorld {
     |  def main(args: Array[String]):Unit = {
     |   println("Hello world")
     | }
     | }
defined object HellowWorld

scala> HelloWorld.main(Array(""))
<console>:8: error: not found: value HelloWorld
              HelloWorld.main(Array(""))
              ^

scala> HellowWorld.main(Array(""))
Hello world
----------


Companions

This mean both class and object have same name in single program.Let us see an example.

scala> :paste
// Entering paste mode (ctrl-D to finish)


class Order(var orderId: Int,var orderDate: String,var orderCustomerId: Int,var orderStatus: String){
println("I am inside order constructor")
override def toString = "Order ("+orderId+","+orderDate+","+orderStatus+")"
}

object Order {
 def apply(orderId:Int,orderDate:String,orderCustomerId:Int,orderStatus:String): Order = {
  new Order(orderId,orderDate,orderCustomerId,orderStatus)
 }
}


// Exiting paste mode, now interpreting.

defined class Order
defined object Order

scala> val order = Order.apply(1,"2014-01-01",123,"Completed")
I am inside order constructor
order: Order = Order (1,2014-01-01,Completed)

*** Note : Although we have a class and an object in same program,scala will create two different classes.
1.Order - represents class
2.Order$ - represents object

scala> :javap -p Order
Compiled from "<console>"
public class Order {
  private int orderId;
  private java.lang.String orderDate;
  private int orderCustomerId;
  private java.lang.String orderStatus;
  public int orderId();
  public void orderId_$eq(int);
  public java.lang.String orderDate();
  public void orderDate_$eq(java.lang.String);
  public int orderCustomerId();
  public void orderCustomerId_$eq(int);
  public java.lang.String orderStatus();
  public void orderStatus_$eq(java.lang.String);
  public java.lang.String toString();
  public Order(int, java.lang.String, int, java.lang.String);
}

scala> :javap -p Order$
Compiled from "<console>"
public class Order$ {
  public static final Order$ MODULE$;
  public static {};
  public Order apply(int, java.lang.String, int, java.lang.String);
  public Order$();
}


**** apply method name is optional while creating object

val order = Order.apply(1,"2014-01-01",123,"Completed")

or

val order = Order(1,"2014-01-01",123,"Completed")

we can have many apply methods with diff args


********

Case Class
Case class is special type of class, where we get boiler plate code

It contains companion object with apply method
Default constructor will be provided for case class
It implements Product and Serializable, which gives us functionality for methods such as productArity, productElement etc
By default, arguments passed to case class will be immutable variables (val). We will get getter methods for all the variables
If we define arguments as mutable variables (var), we will get functionality for both getter and setter methods.
There are few other methods for which boiler plate code is generated
toString
equals
hashCode
canEqual
apply
and more



case class Order(orderId: Int,orderDate: String,orderCustomerId: Int,orderStatus: String){
 println("I am inside order constructor")
}


scala> case class Order(orderId: Int,orderDate: String,orderCustomerId: Int,orderStatus: String){
     |  println("I am inside order constructor")
     | }
defined class Order

scala> :javap -p Order
Compiled from "<console>"
public class Order implements scala.Product,scala.Serializable {
  private final int orderId;
  private final java.lang.String orderDate;
  private final int orderCustomerId;
  private final java.lang.String orderStatus;
  public int orderId();
  public java.lang.String orderDate();
  public int orderCustomerId();
  public java.lang.String orderStatus();
  public Order copy(int, java.lang.String, int, java.lang.String);
  public int copy$default$1();
  public java.lang.String copy$default$2();
  public int copy$default$3();
  public java.lang.String copy$default$4();
  public java.lang.String productPrefix();
  public int productArity();
  public java.lang.Object productElement(int);
  public scala.collection.Iterator<java.lang.Object> productIterator();
  public boolean canEqual(java.lang.Object);
  public int hashCode();
  public java.lang.String toString();
  public boolean equals(java.lang.Object);
  public Order(int, java.lang.String, int, java.lang.String);
}

Even Order$ will be seen as below

scala> :javap -p Order$
Compiled from "<console>"
public class Order$ extends scala.runtime.AbstractFunction4<java.lang.Object, java.lang.String, java.lang.Object, java.lang.String, Order> implements scala.Serializable {
  public static final Order$ MODULE$;
  public static {};
  public final java.lang.String toString();
  public Order apply(int, java.lang.String, int, java.lang.String);
  public scala.Option<scala.Tuple4<java.lang.Object, java.lang.String, java.lang.Object, java.lang.String>> unapply(Order);
  private java.lang.Object readResolve();
  public java.lang.Object apply(java.lang.Object, java.lang.Object, java.lang.Object, java.lang.Object);
  public Order$();
}


Now let us create var type args for case class

case class Order(var orderId: Int,var orderDate: String,var orderCustomerId: Int,var orderStatus: String){
 println("I am inside order constructor")
}


scala> case class Order(var orderId: Int,var orderDate: String,var orderCustomerId: Int,var orderStatus: String){
     |  println("I am inside order constructor")
     | }
defined class Order

scala> :javap -p Order
Compiled from "<console>"
public class Order implements scala.Product,scala.Serializable {
  private int orderId;
  private java.lang.String orderDate;
  private int orderCustomerId;
  private java.lang.String orderStatus;
  public int orderId();
  public void orderId_$eq(int);
  public java.lang.String orderDate();
  public void orderDate_$eq(java.lang.String);
  public int orderCustomerId();
  public void orderCustomerId_$eq(int);
  public java.lang.String orderStatus();
  public void orderStatus_$eq(java.lang.String);
  public Order copy(int, java.lang.String, int, java.lang.String);
  public int copy$default$1();
  public java.lang.String copy$default$2();
  public int copy$default$3();
  public java.lang.String copy$default$4();
  public java.lang.String productPrefix();
  public int productArity();
  public java.lang.Object productElement(int);
  public scala.collection.Iterator<java.lang.Object> productIterator();
  public boolean canEqual(java.lang.Object);
  public int hashCode();
  public java.lang.String toString();
  public boolean equals(java.lang.Object);
  public Order(int, java.lang.String, int, java.lang.String);
}

scala> :javap -p Order$
Compiled from "<console>"
public class Order$ extends scala.runtime.AbstractFunction4<java.lang.Object, java.lang.String, java.lang.Object, java.lang.String, Order> implements scala.Serializable {
  public static final Order$ MODULE$;
  public static {};
  public final java.lang.String toString();
  public Order apply(int, java.lang.String, int, java.lang.String);
  public scala.Option<scala.Tuple4<java.lang.Object, java.lang.String, java.lang.Object, java.lang.String>> unapply(Order);
  private java.lang.Object readResolve();
  public java.lang.Object apply(java.lang.Object, java.lang.Object, java.lang.Object, java.lang.Object);
  public Order$();
}

We will now can see getter and setter method for var args

Now let us create an object

scala> val order = new Order(1,"2014-10-10",100,"COMPLETED")
I am inside order constructor
order: Order = Order(1,2014-10-10,100,COMPLETED)

scala> order.orderId
res4: Int = 1

scala> order.orderDate
res5: String = 2014-10-10

if you give order.<tab> you will get all supported methods
scala> order.
asInstanceOf   copy           orderCustomerId     orderDate     orderId     orderStatus     productArity     productIterator   toString   
canEqual       isInstanceOf   orderCustomerId_=   orderDate_=   orderId_=   orderStatus_=   productElement   productPrefix                

if you give order.productElement<tab> it will give the args of that method.

scala> order.product
productArity   productElement   productIterator   productPrefix

scala> order.productArity
   def productArity: Int

scala> order.productArity
   def productArity: Int

scala> order.productArity
   def productArity: Int

scala> order.productArity
   def productArity: Int

scala> order.productArity
res6: Int = 4

scala> order.productElement
   def productElement(Int): Any

scala> order.productElement
   def productElement(Int): Any

scala> order.productElement(1)
res7: Any = 2014-10-10

scala> order.productElement(0)
res8: Any = 1

scala> order.productElement(2)


scala> order.product
productArity   productElement   productIterator   productPrefix

scala> order.productIterator
res10: Iterator[Any] = non-empty iterator

scala> order.productIterator.for
forall   foreach

scala> order.productIterator.foreach(println(
     | )
     | ';
<console>:3: error: unclosed character literal
       ';
       ^

scala> order.productIterator.foreach(println)
1
2014-10-10
100
COMPLETED

scala> order.product
productArity   productElement   productIterator   productPrefix

scala> order.productPrefix
   def productPrefix: String

scala> order.productPrefix
   def productPrefix: String

scala> order.productPrefix
res12: String = Order