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I see that traits in Scala are similar to interfaces in Java (but interfaces in Java extend other interfaces, they don't extend a class). I saw an example on SO about traits usage where a trait extends a class.
What is the purpose of this? Why can traits extend classes?
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Yes they can, a trait that extends a class puts a restriction on what classes can extend that trait - namely, all classes that mix-in that trait must extend that class .
Although Scala has abstract classes, it's much more common to use traits than abstract classes to implement base behavior. A class can extend only one abstract class, but it can implement multiple traits, so using traits is more flexible.
Traits are used to define object types by specifying the signature of the supported methods. Scala also allows traits to be partially implemented but traits may not have constructor parameters. A trait definition looks just like a class definition except that it uses the keyword trait.
Traits are used to share interfaces and fields between classes. They are similar to Java 8's interfaces. Classes and objects can extend traits, but traits cannot be instantiated and therefore have no parameters.
Yes they can, a trait that extends a class puts a restriction on what classes can extend that trait - namely, all classes that mix-in that trait must extend that class.
scala> class Foo defined class Foo scala> trait FooTrait extends Foo defined trait FooTrait scala> val good = new Foo with FooTrait good: Foo with FooTrait = $anon$1@773d3f62 scala> class Bar defined class Bar scala> val bad = new Bar with FooTrait <console>:10: error: illegal inheritance; superclass Bar is not a subclass of the superclass Foo of the mixin trait FooTrait val bad = new Bar with FooTrait ^ If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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