Passing unknown ArrayList type to method

Question

Say I have these two ArrayLists:

ArrayList<Book> book = new ArrayList<>();
ArrayList<Journal> journal = new ArrayList<>();

Book and Journal are two different classes.

Both Book and Journal objects have a getYear() method. I want to make a method that passes in an unknown ArrayList type and compares a passed in year to an object in the list. The following code is in main:

public static void fooBar(int year, ArrayList<?> list)
{
    if(list.get(0).getYear() == year) // does not work!
    {
    }
}

If an unknown type is passed into the method, I cannot use that object's methods (getYear()). How can I do this without making two methods that do the same thing (one for Book and one for Journal)?

Answer 1

You can make an interface (if it doesn't already exist) (possibly named HasYear) that declares the getYear() method, and have Book and Journal implement it.

Then you can have your fooBar method take an ArrayList of some type parameter that is a HasYear.

public static void fooBar(int year, ArrayList<? extends HasYear> list)

The ? extends is due to the fact it's a consumer and not a producer.

Answer 2

Like JonK said, if they share a superclass or interface that defines getYear() then this will be made easier. Actually, the have to have either of those two relationships to make your proposed relationship.

interface HasYear {
    public int getYear();
}

Book.java

public class Book implements HasYear{
    //...
    public int getYear(){/*YOUR IMPLEMENTATION*/}
}

Journal.java

public class Journal implements HasYear{
    //...
    public int getYear(){/*YOUR IMPLEMENTATION*/}
}

Now, you can create and use your ArrayList like this:

public static void fooBar(int year, ArrayList<? extends HasYear> list){
    if(list.get(0).getYear() == year){
        //yay
    }
}