Traits

Introduction#

Traits are structural construction objects in the Groovy language.

Traits enable implementation of interfaces. They are compatible with static type checking and compilation Traits are behaved as interfaces with default implementations and state.

Declaration of a trait is by using the trait keyword.

Traits methods scope support only public and private methods.

Basic Usage

A trait is a reusable set of methods and fields that can be added to one or more classes.

trait BarkingAbility {
    String bark(){ "I'm barking!!" }
}

They can be used like normal interfaces, using implements keyword:

class Dog implements BarkingAbility {}
def d = new Dog()
assert d.bark() = "I'm barking!!"

Also they can be used to implement multiple inheritance (avoiding diamond issue).

Dogs can scratch his head, so:

trait ScratchingAbility {
    String scratch() { "I'm scratching my head!!" }
}

class Dog implements BarkingAbility, ScratchingAbility {}
def d = new Dog()
assert d.bark() = "I'm barking!!"
assert d.scratch() = "I'm scratching my head!!"

   

Multiple inheritance problem

Class can implement multiple traits. In case if one trait defines method with the same signature like another trait, there is a multiple inheritance problem. In that case the method from last declared trait is used:

trait Foo {
  def hello() {'Foo'}
}
trait Bar {
  def hello() {'Bar'}
}

class FooBar implements Foo, Bar {}

assert new FooBar().hello() == 'Bar'