Scala - 泛型

泛型 下面是类似于java的泛型,有三个类Shape(形状)、Rectangle(长方形)、Square(正方形)。Shape是Rectangle的父类,Rectangle是Square的父类。所以我们定义变量的时候,可以声明他为Shape类型,在new的时候给具体的类型。
Draw1这里有个泛型T,我们可以看到定义变量的时候,这里的类型是不能有父子类关系的。

object GenericDemo { def main(args: Array[String]): Unit = { val shape1: Shape = new Shape() var shape2: Shape = new Rectangle() var shape3: Shape = new Square() val draw1: Draw1[Shape] = new Draw1[Shape]() // val draw2: Draw1[Shape] = new Draw1[Rectangle]() // error // val draw3: Draw1[Shape] = new Draw1[Square]() // error } }class Shape {}class Rectangle extends Shape {}class Square extends Rectangle {}class Draw1[T]() {}

协变和逆变 上面不能编译是因为Draw1[Shape]和new Draw1[Rectangle]并没有父子关系,Scala通过协变可以让他们有父子关系:
在泛型T前面加一个+
class Draw2[+T]() {}

然后在main函数中就可以调用了:
val draw2: Draw2[Shape] = new Draw2[Rectangle]() val draw3: Draw2[Shape] = new Draw2[Square]()

scala中还有逆变,就是颠倒Draw3[T]的父子关系:
在泛型T前面加一个-
class Draw3[-T]() {}

【Scala - 泛型】然后在main函数中就可以调用了:
val draw4: Draw3[Rectangle] = new Draw3[Shape]() val draw5: Draw3[Square] = new Draw3[Shape]()

上界下界 上界下界就是定义泛型的边界。
比如Draw2通过<:Shape2来定义泛型的上界,那Draw2的参数就是Shape2类以及子类。printInfo方法是Shape2的方法,所以printInfo中可以直接调用。t.printInfo()方法,
Draw3通过>:Rectangle2来定义泛型的下界,那Draw3的参数就是Rectangle2类以及父类,此时他只有Any的方法。
object GenericDemo2 { def main(args: Array[String]): Unit = { val draw1: Draw2[Shape2] = new Draw2(new Shape2()) val draw2: Draw2[Shape2] = new Draw2(new Rectangle2()) val draw3: Draw2[Shape2] = new Draw2(new Square2()) draw1.printInfo() // 形状 draw2.printInfo() // 长方形 draw3.printInfo() // 正方形 println("-------------") val draw4: Draw3[Shape2] = new Draw3(new Shape2()) val draw5: Draw3[Rectangle2] = new Draw3(new Rectangle2()) //val draw6: Draw3[Square2] = new Draw3(new Square2()) // error draw4.printInfo() // class com.scala.learn12.Shape2 draw5.printInfo() // class com.scala.learn12.Rectangle2 // draw3.printInfo() } }class Shape2 { def printInfo(): Unit = { println("形状") } }class Rectangle2 extends Shape2 { @Override override def printInfo(): Unit = { println("长方形") } }class Square2 extends Rectangle2 { @Override override def printInfo(): Unit = { println("正方形") } }class Draw2[T <: Shape2](t: T) { def printInfo(): Unit = { t.printInfo() } }class Draw3[T >: Rectangle2](t: T) { def printInfo(): Unit = { println(t.getClass()) } }

    推荐阅读