The Java collections framework is one of those areas of the Java language that is slightly forbidding. As is often the case, it also contains some of the most powerful features of the Java language! The ArrayList class is very powerful indeed because it reduces the amount of work required of the programmer. As you add data, objects of this class just expand automatically and in an efficient fashion. The automatic expansion helps to avoid issues such as exceeding the boundary of an array. However, it does introduce issues concerning efficiency because memory must be allocated behind the scenes. So, if you're adding many elements, you might notice a performance hit. The class also includes methods that can be called to improve the efficiency of additions.
The Java collection framework is very powerful and includes numerous capabilities not mentioned in this article, such as cloning, return values, removing elements, and more. All of these features are well described in the JDK documentation, and you can refer to them as you become more familiar with this rich set of classes.