Choosing a Collection Type

When faced with such a wide variety of collections, it is often difficult to decide which collection type you should use. The general rule is that you should use the collection type that is the most restrictive and still accomplishes your needs. The reason for this is that collections tend to slow down as their abilities increase. First, you should decide whether you need values that can be looked up by a key. If so, then you are restricted to a Map type. If not, then you can use a more general Collection type. If you decide to use a Collection type, the next thing you need to ask is whether you need duplicates in your collection. If so, then you will need to use a List; otherwise, a Set should suffice. After you have made these decisions, you should decide whether you need the objects to be sorted. This decision is much hazier than the other decisions. Almost all collections will need to be sorted at one time or another by a user. However, there are other ways of doing this sorting, such as copying the collection and sorting it in a list. You should opt for SortedSets or SortedMaps only if there is a reason why the collection should always be sorted in a specific manner. Sorted collections and maps must maintain the sorting order, so they tend to be significantly slower than other collections and take up more memory. You should incur this performance hit only if you need the functionality. As for choosing the implementations, you need to consider the merits of each type of implementation before determining which is the best. Screenshot-5 should help you make that decision.

Screenshot-5. Choosing the right collection

In this tutorial, we deal only with the JDK standard collection types. However, the Apache Jakarta Commons project has a library of other collection types that broadens the selection scope significantly. I highly encourage developers to seek out this project at http://jakarta.apache.org/commons/collections.html and take a look at other options.