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2015-03-26_arraylists-part-2.md

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Objectives

  • To be able to create an Iterator from an ArrayList.
  • To be able to loop over an ArrayList in 3 different ways.

Looping over lists

Before we get started, let's create an ArrayList we can play with. Create a class Adder and import the ArrayList. We're going to use an ArrayList of integers. We can't add primitive types but Java has a "wrapper class", a reference type that wraps a primitive type, that we can use. It's call Integer. Create an ArrayList holding objects of type Integer.

ArrayList<Integer> numbers = new ArrayList();

Exercise: Write a program that asks the user for some integers and adds them to your ArrayList. Use a while loop and break when the user gives you a -2.

Give me some numbers to add:
1
1
2
-2
Thanks!

For loop

On Tuesday, we saw that we could loop over a list like this:

for (int i = 0; i < numbers.size(); i++) {
    System.out.println( numbers.get(i) );
}

For each loop

We're going to look at two different ways of iterating over ArrayLists. The first is a for each loop. The syntax is this:

for (Integer num : numbers) {
    System.out.println(num);
}

Exercise: Using a for each loop, calculate the sum of numbers that the user input.

Give me some numbers to add:
1
1
2
-2
Thanks! Your sum is 4.

Iterator

Finally, we can iterate using an object of type Iterator. First, we need to import the class:

import java.util.Iterator;

Next, we want to create an object of type Iterator by calling the iterator() method on numbers; you can call this method on any collection:

Iterator<Integer> numbersIter = numbers.iterator();

Notice we have to give the Iterator a reference type, just like the ArrayList. Now we can use numberIter's methods hasNext() and next() to loop or iterate over the collection:

while (numbersIter.hasNext()) { // numbersIter.next() will return the next integer }

An iterator is stateful, meaning as you call the method next(), the contents of the iterator change. You cannot access them again! Try it...

while (numbersIter.hasNext()) {
    numbersIter.next();
}
numbersIter.next(); // Exception in thread "main" java.util.NoSuchElementException

When would we want to do this? Perhaps if our input is stateful and we do not want the user to be able to "go back". For example, the Scanner class is an Iterator. You can't call nextInt() twice in a row for the same user input.

Exercise: Rewrite your sum functionality to use an Iterator rather than a for each loop.

clear()

ArrayLists have a nice method clear() that will remove any added objects. Try it:

numbers.add(1);
numbers.clear();
System.out.println(numbers.size()); // 0

Exercise: Update your program so that the adder's stored values are cleared if the user inputs -1. It will keep waiting for more positive integers until the user types -2.

Give me some numbers to add:
2
3
4
-1
33
33
-2
66

In-class assignment: Counting and graphing the number of subway entrances in NYC

Parsing the data

NYC Open Data is a public repository of data about New York City. We're going to be analyzing and graphing the dataset for subway entrances. Our end goal is to produce a colored and labeled bar chart comparing the number of entrances per line. Don't worry, you'll get to use two classes that will help you with this!

I've already downloaded the data, but you can also do so here. The text file should look like this:

http://www.mta.info/nyct/service/,Smith St & Bergen St At Ne Corner (To Manhattan And Queens Only),F-G
http://www.mta.info/nyct/service/,Court St & Montague St At Sw Corner,2-3-4-5-N-R
http://www.mta.info/nyct/service/,Court St & Montague St At Sw Corner,2-3-4-5-N-R
http://www.mta.info/nyct/service/,Clinton St & Montague St At Nw Corner,2-3-4-5-N-R
http://www.mta.info/nyct/service/,Flatbush Ave & Empire Blvd At Sw Corner,B-Q-S
...

Use the provided File class to read in the text file. You create a new file like this:

File f = new File("/Subway/src/subway-data.txt");

Next, you can call getLines on the File instance and it will return you an ArrayList of ArrayList of Strings. Each inner ArrayList is a line and each String is the string between the commas. Call the method, store it in a variable, and play with it. How can you get the number of subway entrances per line?

Graphing the data

Use the DrawingPanel class--credit to Marty Stepp for writing this--to create a Graphics2D object. Don't worry about how all this works; just use this snippet:

DrawingPanel panel = new DrawingPanel(500, 250);
Graphics2D g = panel.getGraphics();

Now g is a Graphics2D object. Use its API to draw the necessary objects. You will want to use setColor(), setStroke(), drawString(), and drawLine().

Refer to the stencil file for more specific instructions.

Bonus questions

Use setStroke and the BasicStroke class create a thicker bar graph.

You can use Color.BLACK, Color.WHITE, etc. to create the bar graph. Now use the Color constructor, which takes a hexidecimal number, to print more interesting colors.