Overview

In this reading we will be introduced to the idea of a Map, a data structure that allows us to create an association between one object (a key) and another (a value).

You can think of a map as a special type of array. In a regular array, there is association between a number (the index) and the value stored at that index. A map is similar except that the index doesn’t have to be a number, it can be any type of value you want (e.g. a String or even a Turtle). In fact maps are sometimes called associative arrays.

For example, we could use a Map to create a dictionary, i.e. an association between a word (a String) and a definition (also a String). In this example, the word would be the key while the definition would be the value.

Required Reading

Section 13.3 (pp. 459-465) from the course textbook. You may skim over Section 13.3.2 (Downcasting) and ignore everything after Program 124 on page 465.

Learning Objectives

BASIC Learning Objectives

Each student will be responsible for learning and demonstrating proficiency in the following objectives PRIOR to the class meeting. The reading quiz will test these objectives.

1. Describe what is stored inside of a Java Map.
2. Write a Java statement that creates a HashMap object and assigns it to a Map variable, both with and without using generics.
3. Use the put method to add an entry into a given HashMap object.
4. Use the get method to retrieve a value from a given HashMap object.
5. Contrast the List and the Set interfaces in Java.
6. Use a for-each loop to print out all the keys/values in a given HashMap.

ADVANCED Learning Objectives

The following objectives should be mastered by each student DURING and FOLLOWING the class session through active work and practice.

1. Identify cases when you would use a HashMap to solve a problem.
2. Write, read, and trace Java code that uses a HashMap.

Pre-class Exercises

These exercises are geared towards mastering the BASIC learning objectives listed above. You are expected to submit them before class and it is highly recommended that you complete them before attempting the reading quiz.

Reminder: You should test all of your solutions in DrJava.

1. What line of code would you write to import the Map and HashMap classes?

2. Write a Java statement that creates a HashMap that maps a String object to a Picture object (i.e. a mapping between a title of a picture and the picture itself). Assign this new object to a Map variable named gallery. You should utilize generics to explicitly set the types that are being used by the map (namely String and Picture).

3. Part A: Write Java code to create three new Picture objects and add them to the gallery map, using whatever titles you want.

   Part B: Write Java code that uses the HashMap class’ get method to locate one of the items in your gallery. Assign the result of the get method to a Picture variable named foo.

4. Write a for-each loop that first prints out the title of each item in the gallery map, and then displays the associated picture object using the show method.