上下文界定的类型参数形式为T：M的形式，其中M是一个泛型，这种形式要求存在一个M[T]类型的隐式值：
/**
* 上下文界定
*/
@Test
def testOrdering_Class_Context() = {
class User(val userName: String, val age: Int) class UserOrderingObject extends Ordering[User] {
override def compare(x: User, y: User): Int = x.age - y.age
} class Pair[T: Ordering](val first: T, val second: T) {
def smaller(implicit ord: Ordering[T]) = {
if (ord.compare(first, second) > 0)
second
else
first
}
} val u1 = new User("u1", 12)
val u2 = new User("u2", 13)
//提供隐式值，隐式值需要提供在创建Pair之前
implicit val uo = new UserOrderingObject
val p = new Pair(u1, u2) println(p.smaller.age) } /**
* 上下文界定
*/
@Test
def testOrdering_Object_Context_1() = {
class User(val userName: String, val age: Int) //TODO 隐式object伴生对象
implicit object UserOrderingObject extends Ordering[User] {
override def compare(x: User, y: User): Int = x.age - y.age
} class Pair[T: Ordering](val first: T, val second: T) {
def smaller() = {
//TODO 获取隐式对象
val ord = implicitly[Ordering[T]]
if (ord.compare(first, second) > 0)
second
else
first
}
} val u1 = new User("u1", 12)
val u2 = new User("u2", 13)
// 有隐式伴生对象不再需要提供隐式值
val p = new Pair(u1, u2) println(p.smaller.age) } /**
* 上下文界定
*/
@Test
def testOrdering_Object_Context_2() = {
class User(val userName: String, val age: Int) object UserOrderingObject extends Ordering[User] {
override def compare(x: User, y: User): Int = x.age - y.age
} class Pair[T: Ordering](val first: T, val second: T) {
def smaller() = { val ord = implicitly[Ordering[T]]
if (ord.compare(first, second) > 0)
second
else
first
}
} val u1 = new User("u1", 12)
val u2 = new User("u2", 13)
implicit val uo = UserOrderingObject
val p = new Pair(u1, u2) println(p.smaller.age) }