Difference between object and class in Scala

Question

I m just going over some Scala tutorials on the Internet and have noticed in some examples an object is declared at the start of the example   What is the difference between class and object in Scala

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Defining an object in Scala is like defining a class in Java that has only static methods  However  in Scala an object can extend another superclass  implement interfaces  and be passed around as though it were an instance of a class   So it s like the static methods on a class but better

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A class is just like any other class in other languages  You define class just like any other language with some syntax difference   class Person val name  String  val me   new Person  My name     However  object is a class with single object only  This makes it interesting as it can be used to create static members of a class using companion object  This companion object has access to private members of the class definition and it has the same name as the class you re defining   class Person var name  String       import Person      def hi    String   sayHello name     object Person     private def sayHello name  String   String    Hello     name    val me   new Person  My name   me hi     Also  noteworthy point is that object class is lazily created which is another important point  So  these are not instantiated unless they are needed in our code   If you re defining connection creation for JDBC  you can create them inside object to avoid duplication just like we do in Java with singleton objects

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An object has exactly one instance  you can not call new MyObject   You can have multiple instances of a class   Object serves the same  and some additional  purposes as the static methods and fields in Java

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In scala  there is no static concept  So scala creates a singleton object to provide entry point for your program execution  If you don t create singleton object  your code will compile successfully but will not produce any output  Methods declared inside Singleton Object are accessible globally  A singleton object can extend classes and traits   Scala Singleton Object Example  object Singleton        def main args Array String              SingletonObject hello              No need to create object                  object SingletonObject        def hello              println  Hello  This is Singleton Object                   Output   Hello  This is Singleton Object   In scala  when you have a class with same name as singleton object  it is called companion class and the singleton object is called companion object   The companion class and its companion object both must be defined in the same source file   Scala Companion Object Example  class ComapanionClass        def hello              println  Hello  this is Companion Class                  object CompanoinObject        def main args Array String              new ComapanionClass   hello             println  And this is Companion Object                    Output   Hello  this is Companion Class  And this is Companion Object    In scala  a class can contain   1  Data member  2  Member method   3  Constructor Block   4  Nested class   5  Super class information etc   You must initialize all instance variables in the class  There is no default scope  If you don t specify access scope  it is public  There must be an object in which main method is defined  It provides starting point for your program  Here  we have created an example of class   Scala Sample Example of Class  class Student        var id Int   0                             All fields must be initialized       var name String   null        object MainObject        def main args Array String              var s   new Student                    Creating an object           println s id     s name                  I am sorry  I am too late but I hope it will help you

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The formal difference -    you can not provide constructor parameters for Objects Object is not a type - you may not create an instance with new operator  But it can have fields  methods  extend a superclass and mix in traits     The difference in usage    Scala doesn t have static methods or fields  Instead you should use object  You can use it with or without related class  In 1st case it s called a companion object  You have to    use the same name for both class and object put them in the same source file     To create a program you should use main method in object  not in class   object Hello     def main args  Array String         println  Hello  World           You also may use it as you use singleton object in java

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The object is similar to the static class in Java to some extend  the static characteristic means the static class need not to create an object when putting to the JVM  it can be used by it s class name directly and the same instance same data state  is shared wherever it is used

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A class is a definition  a description  It defines a type in terms of methods and composition of other types   An object is a singleton -- an instance of a class which is guaranteed to be unique  For every object in the code  an anonymous class is created  which inherits from whatever classes you declared object to implement  This class cannot be seen from Scala source code -- though you can get at it through reflection   There is a relationship between object and class  An object is said to be the companion-object of a class if they share the same name  When this happens  each has access to methods of private visibility in the other  These methods are not automatically imported  though  You either have to import them explicitly  or prefix them with the class object name   For example   class X        class X can see private members of object X      Prefix to call   def m x  Int    X f x        Import and use   import X     def n x  Int    f x     private def o   2    object X     private def f x  Int    x   x       object X can see private members of class X   def g x  X          import x       x o   o    fully specified and imported

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If you are coming from java background the concept of class in scala is kind of similar to Java  but class in scala cant contain static members   Objects in scala are singleton type you call methods inside it using object name  in scala object is a keyword and in java object is a instance of class

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Object is a class but it already has is  an instance  so you can not call new ObjectName  On the other hand  Class is just type and it can be an instance by calling new ClassName

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The object keyword creates a new singleton type  which is like a class that only has a single named instance  If you   re familiar with Java  declaring an object in Scala is a lot like creating a new instance of an anonymous class   Scala has no equivalent to Java   s static keyword  and an object is often used in Scala where you might use a class with static members in Java

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Scala class same as Java Class but scala not gives you any entry method in class  like main method in java  The main method associated with object keyword  You can think of the object keyword as creating a singleton object of a class that is defined implicitly   more information check this article  class and object keyword in scala programming

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As has been explained by many  object defines a singleton instance  The one thing in the answers here that I believe is left out is that object serves several purposes    It can be the companion object to a class trait  containing what might be considered static methods or convenience methods  It can act much like a module  containing related subsidiary types and definitions  etc  It can implement an interface by extending a class or one or more traits  It can represent a case of a sealed trait that contains no data  In this respect  it s often considered more correct than a case class with no parameters  The special case of a sealed trait with only case object implementors is more or less the Scala version of an enum  It can act as evidence for implicit-driven logic  It introduces a singleton type    It s a very powerful and general construct  What can be very confusing to Scala beginners is that the same construct can have vastly different uses  And an object can serve many of these different uses all at once  which can be even more confusing

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tl dr   class C defines a class  just as in Java or C    object O creates a singleton object O as instance of some anonymous class  it can be used to hold static members that are not associated with instances of some class  object O extends T makes the object O an instance of trait T  you can then pass O anywhere  a T is expected  if there is a class C  then object C is the companion object of class C  note that the companion object is not automatically an instance of C    Also see Scala documentation for object and class   object as host of static members  Most often  you need an object to hold methods and values variables that shall be available without having to first instantiate an instance of some class  This use is closely related to static members in Java   object A     def twice i  Int   Int   2 i     You can then call above method using A twice 2    If twice were a member of some class A  then you would need to make an instance first   class A       def twice i  Int   Int   2   i    val a   new A   a twice 2    You can see how redundant this is  as twice does not require any instance-specific data   object as a special named instance  You can also use the object itself as some special instance of a class or trait  When you do this  your object needs to extend some trait in order to become an instance of a subclass of it   Consider the following code   object A extends B with C             This declaration first declares an anonymous  inaccessible  class that extends both B and C  and instantiates a single instance of this class named A   This means A can be passed to functions expecting objects of type B or C  or B with C   Additional Features of object  There also exist some special features of objects in Scala   I recommend to read the official documentation    def apply      enables the usual method name-less syntax of A      def unapply      allows to create custom pattern matching extractors if accompanying a class of the same name  the object assumes a special role when resolving implicit parameters

[scala] Difference between object and class in Scala

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