Arrays

Introduction to Arrays

We have seen how to store single pieces of data in variables. What happens when we need to store a group of data? What if we have a list of students in a classroom? Or a ranking of the top 10 horses finishing a horse race?

If we were storing 5 lottery ticket numbers, for example, we could create a different variable for each value:

int firstNumber = 4;
int secondNumber = 8;
int thirdNumber = 12;
int fourthNumber = 16;
int fifthNumber = 20;

That is a lot of ungainly repeated code. What if we had 100 lottery numbers? It is more clean and convenient to use a Java array to stroe the data as a list.

An array hold a fixed number of values of one type. Arrays hold doubles, ints, booleans, or any other primitives. Arrays can also contains Strings as well as object references!

Each index of an array corresponds with a different value. Here is a diagram of an array filled with integer values:

elements	4	8	12	16	20
indices	0	1	2	3	4

Similar to C and Python, the indexes start at 0! The element at index 0 is 4, while the element at index 1 is 8. This array has a length of 5, since it holds five elements, but the highest index of the array is 4.

Creating an Array Explicitly

Imagine that we're using a program to keep track of the prices of different items we want to buy. We would want a list of the prices and a list of the items they correspond to. To create an array, we first declare the type of data it holds.

double[] prices;

Then, we can explicitly initialize the array to contain the data we want to store:

prices = {13.15, 15.87, 14.22, 16.55};

Just like with simple variables, we can declare and initialize in the same line:

double[] prices = {13.15, 15.87, 14.22, 16.55};

We can use arrays to hold Strings and other objects as well as primitives:

String[] clothingItems = {"Tank Top", "Beanie", "Funny Socks", "Pants"};

Importing Arrays

If we want to have a descriptive printout of an array, we need a toString() method that is provided by the Arrays package in Java.

import java.util.Arrays;

We put this line at the top of the file, before we even define the class!

When we import a package in Java, we are making all of the methods of that package available in our code.

The Arrays package has many useful methods, including Arrays.toString(). When we pass an array into Arrays.toString(), we can see the contents of the array printed out.

import java.util.Arrays;

public class Lottery(){

    public static void main(String[] args){
        int[] lotteryNumbers = {4, 8, 12, 16, 20};
        String betterPrintout = Arrays.toString(lotteryNumbers);
        System.out.println(betterPrintout);
    }
}

This code will print:

[4, 8, 12, 16, 20]

Get Element By Index

Now that we have an array declared and intitialized, we want to be able to get values out of it.

We use square brackets [] to access data at a certain index:

double[] prices = {13.1, 15.87, 14.22, 16.55}

System.out.println(prices[1]);

This command will print out 15.87.

This happens because 15.87 is the item at the 1 index of the array.

If we try to access an element outside of its appropriate index range, we will receive an ArrayIndexOutOfBoundsException error.

For example, if we were to run the command System.out.println(prices[5]), we would get the following output:

java.lang.ArrayIndexOutOfBoundsException: 5

Creating an Empty Array

We can also create empty arrays and then fill the items one by one. Empty arrays have to be intitialized with a fixed size:

String[] menuItems = new String[5];

Once you declare this size, it cannot be changed! This array will always be of size 5.

After declaring and intializing, we can set each index of the array to be a different item:

menuItems[0] = "Veggie hot Dog";
menuItems[1] = "Potato salad";
menuItems[2] = "Cornbread";
menuItems[3] = "Roasted broccoli";
menuItems[4] = "Coffee ice cream";

This group of commands has the same effect as assigning the entire array at once:

String[] menuItems = {"Veggie hot dog", "Potato salad", "Cornbread", "Roasted broccoli", "Coffee ice cream"};

We can also change an item after it has been assigned! Let's say this restaurant is changing its broccoli dish to a cauliflower one:

menuItems[3] = "Baked cauliflower";

Now the array looks like:

["Veggie hot dog", "Potato salad", "Cornbread", "Baked cauliflower", "Coffee ice cream"]

Array Length

What if we have an array storing all the usernames for our program, and we want to quickly see how many users we have? To get the length of an array, we can access the length field of the array object:

String[] menuItems = new String[5];
System.out.println(menuItems.length);

This command would print 5, since the menuItems array has 5 slots, even though they are all empty.

If we print out the length of the prices array:

double[] prices = {13.1, 15.87, 14.22, 16.55};

System.out.println(prices.length);

We would see 4, since there are four items in the prices array!

String[] args

When we write main() methods for our programs, we use the parameter String[] args. Now that we know about array syntax, we can start to parse what that means.

A String[] is an array made up of Strings. Examples of String arrays:

String[] humans = {"Nick", "Alyssa", "Matt", "Nathan"};
String[] robots = {"R2D2", "Marvin", "Wall-E", "Bender"};

The args parameter is another example of a String array. In this case, the array args contains the arguments that we pass in from the terminal when we run the class file. (So far args has been empty.)

So how can you pass arguments to main()> Let's say we have this class HelloYou:

public class HelloYou {
    public static void main(String[] args){
        System.out.println("Hello " + args[0];)
    }
}

When we runn the file HelloYou in the terminal with an argument of "Laura":

java HelloYou Laura

We get the output:

Hello Laura

The String[] args would be interpreted as an array with one element, "Laura". When we use args[0] in the main method, we can access that element like we did in HelloYou.

We can actually create if-else statements that run based on the input:

if (args[0].equals("A")){
    // do something
} else if (args[0].equals("B")){
    // do something
} else {
    // do something
}

Arrays Review

We have now seen how to store a list of values in an array. We can use this knowledge to make organized programs with more complex variables.

Throughout these notes, we have leared about:

• Creating arrays explicitly, using { and }.

• Accessing an index of an array, using [ and ].

• Creating empty arrays of a certain size, and filling the indices one by one.

• Getting the length of an array using .length.

• Using the argument args that is passed into the main() method of a class.

Let's create a small program that holds student names and test scores with the following characteristics:

• Make an array of strings called students with the following names: Sade, Alexus, Sam, Koma.

• Create an empty array of doubles called mathScores of size 4.

• Sade got a 94.5 on the test. Store this value in the same indice that she is listed in the students array.

• Sam got a 76.8. Store this value in the appropriate spot in the mathScores array.

• Finally, add a print statement that says: "The number of students in the class is numStudents." using the .length operator.

import java.util.Arrays;

public class Classroom {

    public static void main(String[] args){
        String[] students = {"Sade", "Alexus", "Sam", "Koma"};
        double[] mathScores = new double[4];
        mathScores[0] = 94.5;
        mathScores[2] = 76.8;

        System.out.println("The number of students in the class is " + students.length + ".");
    }
}

Introduction to ArrayLists

When we work with arrays in Java, we've been limited by the fact that once an array is created, it has a fixed size. We can't add or remove elements.

But what if we needed to add to the book lists, newsfeeds, and other structures we were using arrays to represent?

To create multiple and dynamic lists, we can use Java's ArrayLists. ArrayLists allow us to:

• Store object references as elements

• Store elements of the same type (just like arrays)

• Access elements by index (just like arrays)

• Add elements

• Remove elements

Remember how we had to import java.util.Arrays in order to use additional array methods? To use an ArrayList at all, we need to import them from Java's util package as well:

import java.util.ArrayList;

Creating ArrayLists

To create an ArrayList, we need to declare the type of object it will hold, just as we do with arrays:

ArrayList<String> babyNames;

We use angle brackets (<>) to declare the type of the ArrayList. These symbols are used for generics. Generics are a Java construct that allows us to define classes and objects as parameters of an ArrayList. For this reason, we can't use primitive types in an ArrayList:

// This code won't compile:
ArrayList<int> ages;

// This code will compile:
ArrayList<Integer> ages;

The <Integer> generic has to be used in an ArrayList instead. You can also use <Double> and <Char> for types you would normally declare as doubles or chars.

We can intialize to an empty ArrayList using the new keyword:

// Declaring:
ArrayList<Integer> ages;
// Intializing:
ages = new ArrayList<Integer>();

// Declaring and intializing in one line:
ArrayList<String> babyNames = new ArrayList<String>();

Adding Items to an ArrayList

Now we have an empty ArrayList, but how do we get it to store values?

ArrayList comes with an add() method which inserts an element into the structure. There are two ways we can use add().

If we want to add an element to the end of the ArrayList, we'll call add() using only one arguement that represents the value we are inserting. In this example, we'll add objects from the Car class to an ArrayList called carShow:

ArrayList<Car> carShow = new ArrayList<Car>();

carShow.add(ferrari);
// carShow now holds [ferrari]
carShow.add(thunderbird);
// carShow now holds [ferrari, thunderbird]
carShow.add(volkswagen);
// carShow now holds [ferrari, thunderbird, volkswagen]

If we want to add an element at a specific index of our ArrayList, we'll need two arguments in our method call: the first arguement will define the index of the new element while the second argument defines the value of the new element:

// Insert object corvette at index 1
carShow.add(1, corvette);
// carShow now holds [ferrari, corvette, thunderbird, volkswagen]

// Insert object porsche at index 2
carShow.add(2, porsche);
// carShow now holds [ferrari, corvette, porsche, thunderbird, volkswagen]

By inserting a value at a specified index, any elements that appear after this new element will have their index shift over by 1.

Also, note that an error will occur if we try to insert a value at an index that does not exist.

You are able to add multiple data types to the same array using add():

ArrayList assortment = new ArrayList<>();
assortment.add("Hello"); // String
assortment.add(12); // Integer
assortment.add(ferrari); // reference to Car
// assortment holds ["Hello", 12, ferrari]

In this case, the items stored in this ArrayList will be considered Objects. As a result, they won’t have access to some of their methods without doing some fancy casting. Although this type of ArrayList is allowed, using an ArrayList that specifies its type is preferred.

ArrayList Size

Let's say we have an ArrayList that stores items in a user's online shopping cart. As the user navigates through the site and adds items, their cart grows bigger and bigger.

If we wanted to display the number of items in the cart, we could find the size of it using the size() method:

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

shoppingCart.add("Goofy socks");
System.out.println(shoppingCart.size());
// 1 is printed
shoppingCart.add("Funny tie");
System.out.println(shoppingCart.size());
// 2 is printed
shoppingCart.add("HK-416 Assault Rifle");
System.out.println(shoppingCart.size());
// 3 is printed

In dynamic objects like ArrayLists, it's important to know how to access the amount of objects we have stored.

Accessing an Index

With arrays, we can use bracket notation to access a value at a particular index:

double[] ratings = {3.2, 5.5, 1.6};

System.out.println(ratings[1]);
// this will print 5.5

This code prints 5.5, the value at index 1 of the array.

For ArrayLists, bracket notation won't work. Instead we use the method get() to access an index:

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

shoppingCart.add("Goofy Socks");
shoppingCart.add("Funny tie");
shoppingCart.add("HK-416 Assault Rifle");

System.out.println(shoppingCart.get(2));

This code prints "HK-416 Assault Rifle", which is the value at index 2 of the ArrayList.

Changing a Value

When we were using arrays, we could rewrite entries by using bracket notation to reassign values:

String[] shoppingCart = {"Goofy Socks", "Funny tie", "HK-416 Assault Rifle"};

shoppingCart[0] = "Serious Socks";
// This overwrites the "Goofy Socks" string with "Serious Socks"

ArrayList has a slightly different way of doing this, using the set() method:

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

shoppingCart.add("Goofy Socks");
shoppingCart.add("Funny tie");
shoppingCart.add("HK-416 Assault Rifle");

shoppingCart.set(0, "Serious Socks");

// shoppingCart now holds ["Serious Socks", "Funny tie", "HK-416 Assault Rifle"]

Removing an Item

What if we wanted to get rid of an entry alltogether? For arrays, we would have to make a completely new array without the value.

Luckily, ArrayLists allow us to remove an item by specifying the index to remove:

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

shoppingCart.add("Goofy Socks");
shoppingCart.add("Funny tie");
shoppingCart.add("HK-416 Assault Rifle");

shoppingCart.remove(1);
// shoppingCart now holds ["Goofy Socks", "HK-416 Assault Rifle"]

We can also remove an item by specifying the value to remove:

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

shoppingCart.add("Goofy Socks");
shoppingCart.add("Funny tie");
shoppingCart.add("HK-416 Assault Rifle");

shoppingCart.remove("Funny tie");
// shoppingCart now holds ["Goofy Socks", "HK-416 Assault Rifle"]

Note: This command removes the FIRST instance of the value "Funny tie".

Getting an Item's Index

What if we had a really large list and wanted to know the position of a certain element in it? For instance, what if we had an ArrayList detectives with the names of fictional detectives in chronological order, and we wanted to know what position "Flecther" was.

// detectives holds ["Holmes", "Poirot", "Marple", "Spade", "Fletcher", "Conan", "Ramotswe"];
System.out.println(detectives.indexOf("Fletcher"));

This code would print 4, since "Fletcher" is at index 4 of the detectives ArrayList.

ArrayLists Review

Some crucial methods in ArrayLists:

• Adding a new ArrayList item using add().

• Accessing the size of an ArrayList using size().

• Finding an item by index using get().

• Changing the value of an ArrayList item using set().

• Removing an item with a specific value using remove().

• Retrieving the index of an item with a specific value using indexOf().