Introduction to Collections in Java

A collection is a general term that means something like "a bunch of objects stored in a structured manner". Examples might include:

a simple list of strings;
a mapping or association of strings to strings (e.g. country names to the names of their capitals);
a list that imposes some constraint, e.g. keeping strings in alphabetical order, or not allowing duplicates in the list.

Java provides a number of standard implementations of structures such as lists and maps. Together, they're sometimes referred to as the Java Collections Framework or just (Java) Collections. Java also provides utility methods to perform functions such as sorting a list of data.

There are a couple of ways to approach collections in Java:

if you just need a collection of "standard" objects (boring old Strings, Numbers etc provided by the Java class libraries), then you can take a fairly practical approach and start using the standard collections classes without worrying too much about how they work– we'll take a practical look at lists, sets and maps.
If you're more or less familiar with what collections are, but are having trouble deciding which class to use, see the section on How to choose which Java collection class to use.
if you need to store instances of your own class in some types of collection, then you will generally need to understand a little bit (but not all the details) about how they work, in particular how HashMaps work via the technique of hashing– however, you may wish to consult this site's hash function guidelines if you don't want to get too bogged down in the theory;
in some cases, you may need to have a fuller understanding about how collection classes work in order to make decisions relating to performance.

Advanced use of hashing techniques

More advanced readers may wish to take a look at the section on advanced use of hash codes. In this section, we see how to exploit some statistical properties of hash codes to reduce the amount of memory needed to perform certain tasks, notably duplicate elimination.

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