Scala | Stateful Object

Stateful Objects are that objects which have changeable members or mutable members, that may vary what transactions or operations were previously performed on the object. For a same operation performed many times, the result outputed may be different from previous result’s. It is very common to compare stateful objects with real world objects, where the state of the objects changes over-time.

Syntax:



class classname
{
     // declaring some states that are mutable
     var state1
     var state2
     def changestate
     {
          // some operation to change the states of the object
     }
}

When the members of an objects changes its value overtime for certain operation, which are performed on the object. Due to this the statefulness of the object changes that depends on the previously performed operations.

Below are some examples to understand stateful object.
Examples #1:

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// A Scala program to illustrate 
// stateful objects
  
// creating a class
class waterbottle
{
    // creating states
    var water: Int = 0
    def drinkwater =
        if(water > 0)
        {
            water = water-1
            println("water left = "+water)
        }
        else
        {
            println("waterbottle empty fill water")
        }
    }
      
    // Defining method
    def fillwater (c: Int)=
    {
        if(water + c > 5)
        {
            water = 5
        }
        else
        
            water = water + c
        }
    }
    override def toString= "water in bottle = " + water
}
  
// Creating object
object GFG 
{
    // Main method 
    def main(args: Array[String]) 
    
        // waterbottle object
        var w = new waterbottle
          
        // calling 
        w.fillwater(3)
        println(w)
          
        // Changing state
        w.drinkwater
        w.drinkwater
        w.drinkwater
        w.drinkwater
    
}

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Output :

water in bottle = 3
water left = 2
water left = 1
water left = 0
waterbottle empty fill water

As we can see in the above example state for w.drinkwater changes its result for the same operation to “waterbottle empty fill water” till we fill or update water. The mutable state depends on water which is a variable. Thus we can say that stateful objects are made from vars and not val but this is not true, a class can change its state without containing any vars.

For an object that changes its state to another state, the set of operation or the path taken to reach that particular state can be different, but at the end what state is achieved should be same. This is also known as operational equavalence where x & y are different objects but with the same states at the end of different set of operations.

Examples #2:

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// A Scala program to illustrate 
// stateful objects
  
// creating player class
class player
{
    // creating states
    var health: Int = 10
    def punch(p: player)
    {
        if(p.health > 0)
        {
            p.health = p.health-2
            println(p + " health is " + p.health)
            if(p.health < 1)
            {
                // checking the state
                dead(p)
            }
        }
        else
        {
            dead(p)
        }
    }
    def kick(p: player)
    {
        if(p.health > 0)
        {
            p.health = p.health-3
            println(p + " health is " + p.health)
            if(p.health < 1)
            {
                // checking the state
                dead(p)
            }
        }
        else
        {
            dead(p)
        }
    }
    def superp(p: player)
    {
        if(p.health > 0)
        {
            p.health = p.health - 5
            println(p + " health is " + p.health)
            if(p.health < 1)
            {
                // checking the state
                dead(p)
            }
        }
        else
        {
            dead(p)
        }
    }
    def dead(p:player)
    {
        println("Game Over")
        println(p +" is dead " + this + " is winner")
    }
}
  
// Creating object
object GFG
{
    // Main method 
    def main(args: Array[String]) 
    
        // Creating objects for player
        var p1 = new player
        var p2 = new player
        p1.kick(p2)
        p1.punch(p2)
        p1.superp(p2)
        p1.punch(p2)
  
    
}

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Output :

player@506e1b77 health is 7
player@506e1b77 health is 5
player@506e1b77 health is 0
Game Over
player@506e1b77 is dead player@4fca772d is winner
Game Over
player@506e1b77 is dead player@4fca772d is winner

Similarly in the above example we can tell that player is a stateful object without looking at the internal working of the class. Because the health cannot be reduced to negative so player has mutable states as the same operation returns different output at different or same inputs.



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