Composing independent traits
Given two independent traits:
trait T1 {
def x = 42
}
trait T2 {
def x = 0
}
If I try to define a class mixing in these two traits like:
class C extends T1 with T2
I get a compiler error:
error: overriding method x in trait T1 of type => I开发者_开发技巧nt;
method x in trait T2 of type => Int needs `override' modifier
class C extends T1 with T2
^
one error found
Now suppose T1 and T2 have been developed independently, hence no override as they do not override anything. How can C be defined then? Like this:
class C extends T1 with T2 {
override def x = super.x
}
?
This is known as the diamond problem. In Scala there are two ways to solve this:
trait T1 {
def x = 0
}
trait T2 {
def x = 42
}
class C extends T1 with T2 {
override def x = super.x
}
class D extends T2 with T1 {
override def x = super.x
}
If you call new C().x
now, you will get 42
because Scala uses the implementation of the trait you mixed in last. new D().x
will yield 0
by the way. This means, that in order to solve the diamond problem, you have to define explicitly which implementation you would like to use.
The other way is as follows:
trait T {
def x: Int
}
trait T1 extends T {
override def x = 0
}
trait T2 extends T {
override def x = 42
}
class C extends T1 with T2
A call to new C().x
will still yield 42
because T2 is the last trait mixed in. The difference is that you don't have to define x
in C
.
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