Chapter 9. Traits

In Java, a class can implement an arbitrary number of interfaces. This model is very useful for declaring that a class implements multiple abstractions. Unfortunately, it has one major drawback. For many interfaces, much of the functionality can be implemented with boilerplate code that will be valid for all classes that use the interface.

Often, an interface has members that are unrelated (“orthogonal”) to the rest of the members in the implementating class. The term mixin is used for such focused and reusable state and behavior. Ideally, we would maintain this behavior independent of any concrete types that use it.

Before version 8, Java provided no built-in mechanism for defining and using such reusable code. Instead, Java programmers must use ad hoc conventions to reuse implementation code for a given interface. In the worst case, the developer just copies and pastes the same code into every class that needs it. A better, but imperfect solution has been to implement a separate class for the behavior, keep an instance of it in the original class, and then delegate method calls to the support class. This approach works, but it adds lots of needless overhead and error-prone boilerplate.

Interfaces in Java 8

Java 8 changes this picture. Interfaces can now define methods, called defender methods or just default methods. A class can still provide its own implementation, but if it doesn’t the defender method is used. So, Java 8 interfaces behave more like Scala’s ...

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