Tutorial 17: Generic Classes

Consider this tutorial as an introduction to generic classes, which we will look into deeply in the tutorials under Java Collections.

When we refer to something as generic, we mean that it can assume many shapes or work with many different forms. For the purpose of this introductory tutorial, I will use ArrayList. There are several other generic classes, but we will deal with those in the Collections tutorials.

I will first start by telling you how generic classes were used in the past, i.e. in Java 5. Then, I will show you how we use ArrayLists today. This difference will help you understand Java deeper.

Generic Classes in Java 5

In Java 5, this is how we used ArrayList and any other class in the collections framework for that matter. 

We declare an ArrayList like this:

ArrayList flowers = new ArrayList();

Then we add items into the ArrayList like this:

flowers.add(“rose”);

flowers.add(“dandelion”);

flowers.add(“lilies”);

If we want to remove an item from the ArrayList, we can either refer to it by its index. Remember that just like arrays, the indices of objects in an ArrayList start from zero. So, if I want to remove the item “dandelion”, I will write the following:

flowers.remove(1);

If I want to output one of the contents of the ArrayList flowers, I can use the get() method as follows:

Figure 1: Generic classes in Java 5.

Notice that in order to output the actual strings of the flowers in the ArrayList, I use the toString() method. This is because Java will return an Object. So, we have to tell Java that we want the output to be, not of type Object, but of type String.

Generic Classes in Java 6 and Above

After Java 5, generic classes worked a bit differently. 

For example, when creating the generic class, you now could specify the datatype of objects it holds using angle brackets. For example, the modern way to declare the above ArrayList of flowers would be like this:

ArrayList<String> flowers = new ArrayList<String>();

Now, we add items to the ArrayList in much the same way as we did in Java 5, using the add() function like this:

flowers.add(“roses”);

flowers.add(“dandelion”);

flowers.add(“lilies”);

We also remove items from the ArrayList in the same way we did in Java 5, by referring to the object using its index in the remove() method like this:

flowers.remove(2);

The above statement would remove “lilies” from the ArrayList because the indices of the items in the ArrayList start at zero.

The importance of using the angle brackets as shown above becomes clear when we want to output the items or use them for any other operation. Consider outputting the items in the ArrayList onto the console. Here is how we would do it using a foreach loop:

for (String flower : flowers) {

            System.out.println(flower);

}

Notice that in Java 5, we had to call the toString() method because the ArrayList returned an Object. However, since we specified that we are creating an ArrayList to hold strings like this: ArrayList<String> animals = new ArrayList<String>(); , Java knows that the objects are of type String and we thus don’t need to call the toString() method. 

Figure 2: Generic classes in Java 6 and above.

In the next tutorial, I am going to teach you about the wildcard operator in generic classes.

I hope you have enjoyed reading this tutorial. It is one of the shorter ones. If you have any questions or comments, please make sure to post them in the comments section below and I will address them as soon as possible.

Until next time, take care.