Using Wildcard-Type Parameters in Java - dummies

Using Wildcard-Type Parameters in Java

By Doug Lowe

Java is a great way to program, but sometimes you will need to get creative. To do this, you can use wildcard-type parameters. Suppose you have a method that’s declared like this:

public void addItems(ArrayList<Object> list)

{

// body of method not shown

}

Thought question: Does the following statement compile?

addItems(new ArrayList<String>());

Answer: Nope.

That’s surprising because String is a subtype of Object. So you’d think that a parameter that says it accepts an ArrayList of objects accepts an ArrayList of strings.

Unfortunately, inheritance doesn’t work quite that way when it comes to formal type parameters. Instead, you have to use another feature of generics, called wildcards.

In short, if you want to create a method that accepts any type of ArrayList, you have to code the method like this:

public void addItems(ArrayList<?> list)

In this case, the question mark indicates that you can code any kind of type here.

That’s almost as good as inheritance, but what if you want to actually limit the parameter to collections of a specific superclass? For example, suppose you’re working on a payroll system that has an Employee superclass with two subclasses named HourlyEmployee and SalariedEmployee, and you want this method to accept an ArrayList of Employee objects, HourlyEmployee objects, or SalariedEmployee objects?

In that case, you can add an extends clause to the wildcard, like this:

public void addItems(ArrayList<? extends Employee> list)

Then you can call the addItems method with an ArrayList of type Employee, HourlyEmployee, or SalariedEmployee.

Now, before you call it a day, take this example one step further: Suppose this addItems method appears in a generic class that uses a formal type parameter <E> to specify the type of elements the class accepts, and you want the addItems method to accept an ArrayList of type E or any of its subclasses. To do that, you’d declare the addItems method like this:

public void addItems(ArrayList<? extends E> list)

Here the wildcard type parameter <? extends E> simply means that the ArrayList can be of type Evor any type that extends E.