Job Ready Java

Lesson 23Exploring Maps

This lesson continues the discussion of using collections by exploring the Map data structure, which is used for saving key/value pairs. In the lesson, we will primarily focus on the HashMap implementation and how to work with it in your code.

LEARNING OBJECTIVES

By the end of this lesson, you will be able to:

• Describe the Map interface
• Understand the HashMap implementation of Map
• Work the HashMap in your code

MAP INTERFACE

A Map is an object that maps keys to values. In general, a map models the concept of a mathematical function. In a map, each key can map to one and only one value, so it cannot contain duplicate keys.

While Maps are part of the Java Collections Framework, the Map interface itself is not a Collection (i.e., it does not extend the Collection interface, and it is not iterable); however, we can get a Collection of either the keys or the values of a Map or a Collection of Entry objects (both Key and Value).

The Map interface has many methods. In this lesson, we will cover the most often used methods, which are as follows:

• get
• put
• keySet
• size
• values

NOTE

We cover a few examples using these methods in this lesson; however, it is recommended that you should read the Javadoc associated with each to become familiar with how they work. You can find the Map interface in the Javadocs at docs.oracle.com/en/java/javase/15/docs/api/java.base/java/util/Map.html for Version 15, or at docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Map.html ...