Using collections in Java: maps

In programming terms, a map is a set of associations between pairs of objects. It crops up extremely frequently in programming in all sorts of cases where we want to deal with cases of "for X, what is the Y"? For example:

"for a given language code, what is the message to display meaning 'Please log in'?"
"for a given word in the document I'm processing, how many times did it occur do far?"
"for a given row ID number, what is the cached copy of the row from the database"?
"for a given user on my web server, what is their current session status"?

Notice the third item here: a common use for a map is as a simple cache.

Creating and using a map

The syntax for creating a map is largely similar to lists and sets. But this time we need to specify the type of object we want to map from and the type to map to. Let's say we want to store an association of country codes with country names, in other words, String to String:

Map<String,String> countryNames = new HashMap<String,String>(200);

In this case, we use the common map implementation HashMap. As with the set, we supply an estimate of the number of mappings we expect to add, to help the map operate more efficiently¹. To add an association to the map, we actually use a method called put():

countryNames.put("GB", "Great Britain");
countryNames.put("FR", "France");
countryNames.put("IT", "Italy");
countryNames.put("FW", "Far Far Away");

Then, to retrieve the country name for a particular code, we call:

String name = countryNames.get("IT");

Other things to note about Maps:

The items that we associate from are called keys; the items they are associated to (the country names here) are called values.
If for a particular key there is no associated value, then get() returns null.
A given key can be associated with only one value. If you put("X", "Y") then subsequently put("X", "Z"), then get("X") will return Z, the last association made with that key.
The put() method actually returns the previous association with the given key (or null if there was none).

More on maps

For more advanced uses, it is useful to understand how maps work, in particular about the technique called hashing from which the HashMap and HashSet get their name.

1. Every time a collection becomes "full", it generally has to re-organise itself to accommodate the extra capacity needed. For example, an ArrayList needs to create a new, larger array. In the case of HashSet or HashMap, it turns out that this re-organisation is quite an "expensive" operation. So if we can anticipate roughly how many items will be added in the first place, we cut down on this expensive re-organisation.

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