97 Things Every Java Programmer Should Know

Chapter 48. Know Your Collections

Nikhil Nanivadekar

Collections are a staple in any programming language. They constitute one of the basic building blocks of commonly developed code. The Java language introduced the Collections framework a long time ago in JDK 1.2. Many programmers reach for ArrayList as their de facto collection to use. However, there’s more to collections than ArrayList, so let’s explore.

Collections can be classified as ordered or unordered. Ordered collections have a predictable iteration order; unordered collections do not have a predictable iteration order. Another way to classify collections is sorted or unsorted. The elements in a sorted collection are sequenced from start to end based on a comparator; unsorted collections have no particular sequence based on elements. Although sorted and ordered have similar meanings in English, they cannot always be used interchangeably for collections. The important distinction is that ordered collections have a predictable iteration order but no sort order. Sorted collections have a predictable sort order, hence they have a predictable iteration order. Remember: all sorted collections are ordered collections, but not all ordered collections are sorted collections. There are various ordered, unordered, sorted, and unsorted collections in the JDK. Let’s take a look at a few of them.

List is an interface for ordered ...