public class Util {
  public static void printBag(Bag<?> bag ) {
    System.out.println(bag.toString()); 
  }
}
Bag<String> myBag1 = new Bag("Hello");
Bag<Integer> myBag2 = new Bag(23);
Util.printBag(myBag1);  // Hello
Util.printBag(myBag2);  // 23

public static <T> void printBag(Bag<T> bag ) {
  System.out.println(bag.toString()); 
}

public static <T> Bag<T> getBag(Bag<T> bag ) {
  return bag;
}

public static void printBag(Bag<? extends Number> bag ) {
  System.out.println(bag.toString()); 
}

//Wildcard Lower Bounds
//A lower bound wildcard restricts the wildcard to a class or interface and any of its parent types.
//There are some general guidelines provided by Java as to when to use what type of wildcard:

//An upper bound wildcard should be used when the variable is being used to serve some type of data to our code.
//A lower bound wildcard should be used when the variable is receiving data and holding it to be used later.
//When a variable that serves data is used and only uses Object methods, an unbounded wildcard is preferred.
//When a variable needs to serve data and store data for use later on, a wildcard should not be used (use a type parameter instead).
//An upper bound restricts the type parameter to a class or any of its sub-classes and is done this way: SomeClass<? extends SomeType>. A lower bound restricts the type parameter to a class or any of its parent classes and is done this way: SomeClass<? super SomeType>.
public class Util {
  public static void getBag(Bag<? super Integer> bag ) {
    return bag;
  }
}