Use of def val and var in scala

Question

class Person val name String var age Int   def person   new Person  Kumar  12  person age   20 println person age    These lines of code outputs 12  even though person age 20 was successfully executed  I found that this happens because I used def in def person   new Person  Kumar  12   If I use var or val the output is 20  I understand the default is val in scala  This   def age   30 age   45      gives a compilation error because it is a val by default  Why do the first set of lines above not work properly  and yet also don t error

User · Answer

With   def person   new Person  Kumar   12     you are defining a function lazy variable which always returns a new Person instance with name  Kumar  and age 12  This is totally valid and the compiler has no reason to complain  Calling person age will return the age of this newly created Person instance  which is always 12   When writing  person age   45   you assign a new value to the age property in class Person  which is valid since age is declared as var  The compiler will complain if you try to reassign person with a new Person object like  person   new Person  Steve   13      Error

User · Answer

Let s take this   class Person val name String var age Int   def person  new Person  Kumar  12  person age 20 println person age    and rewrite it with equivalent code  class Person val name String var age Int   def person  new Person  Kumar  12   new Person  Kumar   12   age   20  println  new Person  Kumar   12   age    See  def is a method  It will execute each time it is called  and each time it will return  a  new Person  Kumar   12   And these is no error in the  assignment  because it isn t really an assignment  but just a call to the age   method  provided by var

User · Answer

As Kintaro already says  person is a method  because of def  and always returns a new Person instance  As you found out it would work if you change the method to a var or val   val person   new Person  Kumar  12    Another possibility would be   def person   new Person  Kumar  12  val p   person p age 20 println p age    However  person age 20 in your code is allowed  as you get back a Person instance from the person method  and on this instance you are allowed to change the value of a var  The problem is  that after that line you have no more reference to that instance  as every call to person will produce a new instance    This is nothing special  you would have exactly the same behavior in Java   class Person      public int age      private String name     public Person String name  int age          this name   name          this age   age          public String name    return name       public Person person        return new Person  Kumar   12       person   age   20  System out println person   age     -- gt  12

User · Answer

To provide another perspective   def  in Scala means something that will be evaluated each time when it s used  while val is something that is evaluated immediately and only once  Here  the expression def person   new Person  Kumar  12  entails that whenever we use  person  we will get a new Person  Kumar  12  call  Therefore it s natural that the two  person age  are non-related   This is the way I understand Scala probably in a more  functional  manner   I m not sure if  def defines a method val defines a fixed value  which cannot be modified  var defines a variable  which can be modified    is really what Scala intends to mean though  I don t really like to think that way at least

User · Answer

I d start by the distinction that exists in Scala between def  val and var     def - defines an immutable label for the right side content which is lazily evaluated - evaluate by name  val - defines an immutable label for the right side content which is eagerly immediately evaluated - evaluated by value  var - defines a mutable variable  initially set to the evaluated right side content    Example  def  scala gt  def something   2   3   4  something  Int scala gt  something     now it s evaluated  lazily upon usage res30  Int   14   Example  val   scala gt  val somethingelse   2   3   5    it s evaluated  eagerly upon definition somethingelse  Int   17   Example  var  scala gt  var aVariable   2   3 aVariable  Int   6  scala gt  aVariable   5 aVariable  Int   5   According to above  labels from def and val cannot be reassigned  and in case of any attempt an error like the below one will be raised   scala gt  something   5   6  lt console gt  8  error  value something   is not a member of object  iw        something   5   6            When the class is defined like   scala gt  class Person val name  String  var age  Int  defined class Person   and then instantiated with   scala gt  def personA   new Person  Tim   25  personA  Person   an immutable label is created for that specific instance of Person  i e   personA    Whenever the mutable field  age  needs to be modified  such attempt fails   scala gt  personA age   44 personA age  Int   25   as expected   age  is part of a non-mutable label  The correct way to work on this consists in using a mutable variable  like in the following example   scala gt  var personB   new Person  Matt   36  personB  Person   Person 59cd11fe  scala gt  personB age   44 personB age  Int   44       value re-assigned  as expected   as clear  from the mutable variable reference  i e   personB   it is possible to modify the class mutable field  age    I would still stress the fact that everything comes from the above stated difference  that has to be clear in mind of any Scala programmer

User · Answer

There are three ways of defining things in Scala    def defines a method val defines a fixed value  which cannot be modified  var defines a variable  which can be modified    Looking at your code   def person   new Person  Kumar  12    This defines a new method called person  You can call this method only without    because it is defined as parameterless method  For empty-paren method  you can call it with or without       If you simply write   person   then you are calling this method  and if you don t assign the return value  it will just be discarded   In this line of code   person age   20   what happens is that you first call the person method  and on the return value  an instance of class Person  you are changing the age member variable    And the last line   println person age    Here you are again calling the person method  which returns a new instance of class Person  with age set to 12   It s the same as this   println person   age

[scala] Use of def, val, and var in scala

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