Perhaps I'm slow, but I don't see the difference between a trait and a Java interface other than the fact that traits appear to be more of a run-time construct.
Java interfaces are actually a very nice compromise between multiple and single inheritance.
You can not get rid of the method with Java interface. Plus, interface is only
declaration, you must implement methods in each class which use interface. AFAIK (last touched Java 5 years ago)
Look at example: While the purpose of this paper is not to argue the merits of traits per se (see [SDNB03, BSD03] for such arguments), it is helpful to understand the motivation for traits. In languages with single inheritance, such as Smalltalk, it is often the case that inheritance does not provide sufficient flexibility for structuring a class hierarchy. Consider the case of two classes in different subtrees of the inheritance hierarchy and assume that they both implement some common protocol. If this protocol is not implemented by a common superclass, then each class must provide its own implementation, which results in code duplication. On the other hand, if we lift the implementation of the protocol up to the common superclass, we pollute the interface of the superclass, which affects all of its subclasses. Furthermore, if the protocol is defined by building on methods defined in intermediate classes, we will have to add these methods to the common superclass as well. This problem resul ts from the dual nature of classes. Classes serve as both object generators and as superclasses. In the former case, the implementation of a class must be complete, whereas in the latter case the class implementation may have abstract methods that are implemented by a subclass. Traits provide a mechanism to separate these two aspects of classes and allow code to be reused across the class hierarchy. Multiple inheritance [Str94] and mixins [BC90, FKF98] represent two other attempts to solve this problem, but they both introduce semantic complexities and ambiguities (e.g., multiple copies of instance variables in the case of multiple inheritance, and problems with the order of method definition in the case of mixins) [SDNB03].
It is from link i provided in previous post.
-Dmitry Dorofeev.
