Lecture Notes 23: Generics and Data Structures.

Objectives

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

Understand how to use Generics.
Utilize the ArrayList Java API.
Demonstrate understanding of the differences between ArrayList and Java arrays.

Before Starting

If you do not have your Codio course ready, use any text editor or simple IDE. Some possibilities are:

Write using Sublime; Compile and run in the Terminal
Use IDEs like: IntelliJ, DrJava, or JGrasp

ArrayList

Data-structures are means for organizing and accessing data. They are one of the main topics in CSCI1112.

Examples we've seen so far:

Two dimensional arrays for organizing spreadsheets, mazes, and game state.
FBUsers and Nodes used to keep complex data and connect them to each other.
Strings for holding collections of characters.

Please take a few minutes to browse the ArrayList Javadoc page.

Activity 1 [Group Work]: Questions -

What do you think that this means:
```
public class ArrayList<E>
```
What is the "<E>"?
ArrayLists are similar to arrays. How do we find the value at an index into the ArrayList?
How do we add/change a value at a specific index?
What is the equivalent of array's ".length"?
If we want to get the data in the ArrayList in an array?
What if we want to do something similar to String's ".substring" method?

Generics

Data-structures contain data. We want the type of the data to be different, depending on what data we're organizing

Are we organizing contact information in a list of friends?
Are we organizing doubles in our spreadsheet?
Are we organizing Strings in a word processor?

How do we have different arrays containing different types of data?

      FBUser[] friends = new FBUser[n];
      double[][] spreadsheet = new double[4][10];
      String[] sentences = new String[numSentences];

More complex data-structures must be implemented in Java classes (often by us!).

However, we don't know how to write a class that can store data of a type that will be later specified by a client, or user of our class.

Generics enable us to write a class that stores data of a type specified by the user of our class.

What do generics look like?

```
public class ArrayList<E>
```
E is the type of data we want to hold in the ArrayList

Creating an ArrayList that holds Strings:

      ArrayList<String> sentences = new ArrayList<String>();

Note: we are specifying that we want to create an array list of strings

Activity 2 [Group Work]: Write a program that creates an ArrayList of Strings, put three strings into it, and then print out the contents of the ArrayList as well as its size.

Activity 3 [Group Work]: Add code to your program to do the following:

Find the index of one of the strings.
Remove an item from the ArrayList and replace it with another String.
Add Strings until you have a total of five, and then create an array of Strings that contains the middle three Strings in the ArrayList.

For each of these, use no loops and only use the ArrayList methods.

Other Data-Structures

Check out the LinkedList Javadoc.

Activity 4 [Group Work]: Questions -

How do you add values to a linked list?
How do you find the value at an index?
Having a little Deja vu? ^{("It happens when they change something.")}

Activity 5 [Group Work]: What is the difference between arrays, ArrayLists, and LinkedLists? Why does Java have three implementations of ways of storing data?

An aside aboout class-types for primitives

It turns out that all the familiar oprimitives (int, double, boolean, char, etc) have Class types that define some methods and properties for peerforming high level tasks with them.

For "int"s, you have a class type of Integer
For "double"s, you have a class type of Double
For "boolean"s, you have a class type of Boolean
For "char"s, you have a class type of Character

We've used these when converting to and from string:


public class CopyExample3 {

  public static void main (String[] argv)
  {
    String s = "123";
    int num = Integer.parseInt(s);
    System.out.println (num + 4);
  }

}

This code willl print a 127 rather than 1234.

When using generics:
If you want to use generics with "primitives", you must use the Class type instead of the primitive verision.

Activity 6 [Group Work]: Add the following code to GenericsExample.java. Then, fix the code so that it compiles and runs.

ArrayList<int> al = new ArrayList<int>();
al.add(10);
System.out.println(al.get(0));

Note that there are corresponding types for

double → Double
float → Float
boolean → Boolean

Using these classes, Java automatically converts code such as

ArrayList<Integer> al = new ArrayList<Integer>();
al.add(10);

ArrayList<Integer> al = new ArrayList<Integer>();
al.add(Integer(10));

Check out the HashMap Javadoc.

Activity 7 [Group Work]: Questions -

What does the <K,V> mean in "Class HashMap<K,V>"?
The main operations for a HashMap are "get" and "put". What do you think a HashMap does? What is its main use?
How is it different from LinkedList and ArrayList?

Activity 8 [Group Work]: Write a program for the IRS that lets us store people, and later look them up by their social security number (SSN). Use the person class below. Add three people objects to a HashMap, using their social security number as the key. Look them up, and print out their names.

public class Person {
  private String name;
  private int    ssn;
  public Person(String n, int ssn) {
    name = n;
    this.ssn = ssn;
  }
  public String getName() {
    return name;
  }
  public long getSsn() {
    return ssn;
  }
}

JavaDoc Treasure Hunt

Activity 9 [Group Work]: Write a program that

Creates an HashMap of People.
Populates it with a number of people (key = ssn, value = Person object).
Creates an ArrayList with the same contents as all of the values in the HashMap.
...without using loops. It should work just the same if the HashMap has 3 People, or 3 million.

CS 1111