Containers with generics

All.java

package acme.jelly;

public class Apple extends Fruit {}

package acme.jelly;

public class Box<T extends Fruit> {
  private T item;

  public Box(T item) {
    this.item = item;
  }

  public T get() {
    return item;
  }

  public void replace(T item) {
    this.item = item;
  }
}

package acme.jelly;

public class Fruit {}

package acme.jelly;

public class RealBox<T> {
  private T item;

  public RealBox(T item) {
    this.item = item;
  }

  public T get() {
    return item;
  }

  public void replace(T item) {
    this.item = item;
  }
}

package acme.jelly;

public class RunMe {
    public static void main(String[] args) {
        RealBox<? extends Fruit> box = new RealBox<>(new Apple());
        System.out.println(box.get());
//        box.replace(new Apple());       // will fail compilation
//        System.out.println(box.get());  // will fail compilation
    }
}    

fruit.scala

class Plant
class Fruit extends Plant
class Apple extends Fruit

// 'var' is not allowed for covariant type
// class RealBox[+T](var item: T) {
class RealBox[+T](item: T) {
  def get: T = item
  def getOrElse[U >: T](otherItem: U, really: Boolean): U =
    if (really) item else otherItem

  // Not allowed for covariant type
  // def replace(item: T): Unit = this.item = item
}

class Box[T <: Fruit](var item: T) {
  def get: T = item
  def replace(item: T): Unit = this.item = item
}

//val appleBox = new RealBox(new Apple)
//val plantBox = new Box[Plant](new Plant)
val appleBox = new Box(new Apple)
appleBox.get  // do not use println in worksheet

val fruitRealBox = new RealBox[Fruit](new Fruit)
fruitRealBox.getOrElse("aa", false)