IntelliJ IDEA 2020.2 Help

Extract trait in Scala

With the Extract Trait refactoring you can extract selected members from a Scala class, object, or a trait into a new trait. IntelliJ IDEA supports def, val, and var type members.

Extract trait

  1. In the editor, select code members that you want to refactor.

  2. From the main menu, select Refactor | Extract | Trait.

    Alternatively, right-click anywhere in the editor to open the context menu and select Refactor | Trait.

  3. In the dialog that opens, enter a new trait name and select members you want to extract.

    If you need to select all of the listed members in the Members to extract section, press Ctrl+A first and thenSpace.

    the Extract Trait dialog
    Click Refactor.


Let's create a class Calculator with several methods and extract one of the methods (mul) into a trait.

class Calculator { def add(a: Int, b: Int): Int = a + b def mul(a: Int, b: Int): Int = a match { case 0 =>0 case _ => add(b, mul(a-1, b)) } }
trait Multiplier { this: Calculator => def mul(a: Int, b: Int): Int = a match { case 0 => 0 case _ => add(b, mul(a - 1, b)) } }
class Calculator extends Multiplier { def add(a: Int, b: Int): Int = a + b }

IntelliJ IDEA creates a new trait Multiplier that contains the mul method with its definition body. The class Calculator extends the trait Multiplier.

Extract trait dialog reference

Extract trait fromThis field contains the name of the class, object, or trait from which you are trying to extract a member.
Trait nameUse this field to enter a name for your new trait.
Package for traitUse this field to add a package for the newly created trait. We recommend that you use the same package as the original class, object, or trait from which you are extracting.
Members to extract Use this area to select members you want to extract.

When the extract abstracts checkbox is selected then the new trait will only contain a declaration without a definition body. For example, an abstract method that doesn't contain a body in such case will be overridden in the original class, and the definition will be left there.

When this checkbox is not selected, the method will be moved to the new trait with its definition as in the example.

Last modified: 08 May 2020