Recall that generic code can have an upper bound when defined using a type parameter or a wildcard. We can also provide a lower bound when working with wildcards. A lower bound wildcard restricts the wildcard to a class or interface and any of its parent types. For example:

public class Util {
  public static void getBag(Bag<? super Integer> bag ) {
    return bag;
  }
}

In the example above we’ve used the super keyword to restrict the argument to getBag() to be a Bag of Integer, Number, or Object. If a call to getBag() with Bag<Double> was made it would result in an exception because Double is not an Integer or one of its parents.

Some important things to note about lower bounds are:

• They cannot be used with generic type parameters, only wildcards.
• A wildcard cannot have both a lower bound and upper bound, in this case, it’s best to use a type parameter.

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).

Let’s practice adding a lower bound to a generic method.