Scala – When to use case class or regular class

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

Case classes can be seen as plain and immutable data-holding objects that should exclusively depend on their constructor arguments.

This functional concept allows us to

• use a compact initialization syntax (Node(1, Leaf(2), None)))
• decompose them using pattern matching
• have equality comparisons implicitly defined

In combination with inheritance, case classes are used to mimic algebraic datatypes.

If an object performs stateful computations on the inside or exhibits other kinds of complex behaviour, it should be an ordinary class.