看了以前自己代码，发现下面例子。好智能，不用协变都能满足需求。
情景
scala> class A
defined class A
scala> class B extends A
defined class B
scala> class C[T]
defined class C
scala> object C{def apply[T](t: T) = new C[T]}
defined object C
scala> def f(x: C[A]) = 1
f: (x: C[A])Int
cala> f(C(new B))
res15: Int = 1
scala> val x = C(new B)
x: C[B] = C@30ec211a
scala> f(x)
<console>:18: error: type mismatch;
found : C[B]
required: C[A]
Note: B <: A, but class C is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
f(x)
写在一起，就能加入了下文信息自动把Array(new B)当成ArrayA
如果不写在一起，f(C(new B))那就需要写成
val b: A = new B
val cb: C[A] = C(b)
f(cb)
或者定义协变
scala> class C[+T]
scala> val x = C(new B)
x: C[B] = C@17550481
scala> f(x)
res0: Int = 1
编译
scala.this.Predef.println(f(scala.Array.apply[A](new B())((ClassTag.apply[A](classOf[TfIdf$$main$A]): scala.reflect.ClassTag[A]))))
在Array后面指定了A类型。
