Working with Maps

Terminology

Definition

A map is an unordered collection of values each with distinct keys. The key-value pair stored within the map is called an entry.

The key is a distinct object that represents the entry as a whole.

The value is any object.

A map is sometimes also called a dictionary.

Using Maps

A Map might be used to associate student IDs with complete student records, or words to their definitions (like a real dictionary). What other examples of maps can you think of?

What kind of collection do you think would be used to store the keys in a map?

What kind of collection do you think would be used to store the values in a map?

Using TreeMap

In Java, the class TreeMap, is used to model the concept of a map given above. In the remainder of this activity, you will use one or more TreeMap objects to discover several interesting statistics about words in a file: how many times a word appears in a given file, or on which lines a word appears in a file.

There is one class defined for this activity: WordLines. You should start by reading over the code in this class and making sure you understand it. In particular, explain the following details about the code:

1. What is the purpose of the Integer object and how is it used in this class?
2. Why is the output printed in sorted order?
3. How does the method printResults work for any kind of map (i.e., why is the ? character required instead of simply using the superclass Object)?

Problems

After you understand the current implementation, you should implement the following additional methods and test them by inspecting their output.

1. Complete the method, getLineNumbers, that creates a new Map with words as the key and the set of line numbers on which that word appears as the value. This method should get the data for each word from the Scanner passed it.
2. Complete the method, getFrequencies, that creates a new Map with letters as the key and the set of words which start with that letter as the value. This method should get the data for each word from the Scanner passed it.