What are Traits?Traits are like interfaces, but can have a method body and a class that implements a Trait inherits the method body. In many ways Traits are similar to abstract classes, but they don’t have constructors, don’t have initializers, don’t have fields, can be multiply inherited (like an interface), and super does not refer to a method in an inherited trait, instead of super you have to qualify the intended name [TraitName].[methodName]( … ).By way of example consider current best practice: you write an interface, e.g. List, write a useful abstract class to make writing lists easier, AbstractList, and then implement some lists, e.g. ArrayList. This is all well and good, but suppose you now want to add some extra methods to List, e.g. sort. You can’t because every class that implements List would need to add a sort method and you don’t have control over all the source that uses List. So instead you use a helper class, Collections, and add sort as a static method. The user then calls sort( list ) (assuming a static import is used). A better technique is a Trait, with a trait you modify List:
public interface List extends Collection {
…
void sort() { … } // same syntax as an interface but allows method bodies
}
ArrayList now automatically pick up sort and the user does not need a special import, instead just like any other instance method the user writes list.sort().
Details
With Traits, if there is a conflict due to multiple inheritance then you have to resolve this conflict (this is a key difference from Mixins – see below). E.G.:
interface X { int m() { return 1; } }
interface Y { int m() { return 2; } }
interface Z extends X { int m() { return 3; } }
class XY implements X, Y { // conflict – 2 m’s
public int m() { return X.m(); } // resolve conflict
}
class XZ implements X, Z {} // no conflict – Z’s m overrides X’s
Note on Mixins
The difference between a Trait and a Mixin is that order is important. In the class XY example given above, with a Trait you have to explicitly say which m you want. With a Mixin the order in which the interfaces are mixed in determines what happens. class XY implements X, Y {} is read as first mixing in X then mixing in Y; hence the m from Y overrides the m from X, since it is mixed in afterwards.
Note on Extension Methods
An alternative way of adding methods to a class is Extension Methods, these are proposed for Java 7, but have a number of problems.
Extending existing classes
To be able to add methods to an interface without recompiling, all clients of the interface, the class loader has to be extended to automatically add in the extra methods if they are not present and to flag an error if an unresolved conflict exists. Currently the class loader flags a missing method, the required extra action would be to see if the missing method is in an implemented interface and if it can be added without conflict.
As an alternative to abstract classes
Traits would be a viable alternative to abstract classes in many cases, for example the methods in AbstractCollection could be moved to Collectionand no one would need to use AbstractCollection in the future. But a Trait cannot have any fields, therefore some abstract classes, e.g. AbstractList (it contains a field) would still be needed, although some methods may be moved to an interface.
Reference
Supporting Java Traits in Eclipse
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