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 Collection and 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.

Conclusions

I think Traits would be a useful addition to Java, what do others think?

interface List2 extends List { void sort(); }
 
abstract class AbstractList extends AbstractCollection implements List2 {
  ...
  public void sort() { Collections.sort( this ); }
}

source 7;
 
interface List {
  ...
  @Version(7) void sort();
}
 
abstract class AbstractList extends AbstractCollection implements List {
  ...
  @Version(7) public void sort() { Collections.sort( this ); }
}

interface Addable {
  int get();
  Addable instance( int value );
  Addable add( int increment ) { return instance( get() + increment ); }
}
 
class Adder implements Addable {
  private final int value;
  Adder( int value ) { this.value = value; }
  public int get() { return value; }
  public Adder instance( int value ) { return new Adder( value ); }
}

Object trait1 = new Traits().with( new CU() ).traited();
Object trait2 = new Traits().with( new CU() ).traited();
 
trait1.getClass() != trait2.getClass();
trait1 instanceof ?