泛型

上面是相似于java的泛型，有三个类Shape（形态）、Rectangle（长方形）、Square（正方形）。Shape是Rectangle的父类，Rectangle是Square的父类。所以咱们定义变量的时候，能够申明他为Shape类型，在new的时候给具体的类型。
Draw1这里有个泛型T，咱们能够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

下面不能编译是因为Draw1[Shape]和new Draw1[Rectangle]并没有父子关系，Scala通过协变能够让他们有父子关系：
在泛型T后面加一个+

class Draw2[+T]() {

}

而后在main函数中就能够调用了：

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后面加一个-

class Draw3[-T]() {

}

而后在main函数中就能够调用了：

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。
比方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办法是Shape2的办法，所以printInfo中能够间接调用。t.printInfo()办法，
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的办法。

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() // 形态
    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("形态")
  }
}

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())
  }
}