Hash tables

A dataset of key-value pairs is usually implemented as a hash table. It is a data structure in which the key acts like an index into the set, much like page numbers in a book or line numbers in a table. This direct access is much faster than sequential access, which is like searching through a book page-by-page for a certain word or phrase.

In Java, we usually use the java.util.HashMap<Key,Value> class to implement a key-value pair dataset. The type parameters Key and Value are specified classes. (There is also an older HashTable class, but it is considered obsolete.)

Here is a data file of seven South American countries:

Figure 2-1 Countries data file

Here is a Java program that loads this data into a HashMap object:

Listing 2-1 HashMap example for Countries data

The Countries.dat file is in the data folder. Line 15 instantiates a java.io.File object named dataFile to represent the file. Line 16 instantiates a java.util.HashMap object named dataset. It is structured to have...