Right now, we know that a variable can store a single value of a specific type, but sometimes it is more efficient to have a single variable hold multiple values. To make a variable hold multiple values of a specific type, we use a concept known as arrays and, in Java, ArrayLists.

Array

In Java, an array is a data structure that holds a fixed amount of values of a specific type. To create an array, we use the following syntax:

int[] array           = new int[3];
// or, if we know the values we will use
int[] arrayWithValues = { 3, 8, 72 };

This might remind you of how we create a new instance of a class by using the new keyword. However, you will notice that instead of using parenthesis, we use square brackets. Inside the square brackets, we enter the amount of values the array can hold. We can visualize this array as follows:

       | col 0 | col 1 | col 2 |
-------|-------|-------|-------|
 row 0 |  [0]  |  [1]  |  [2]  |
-------|-------|-------|-------|

You might notice that this looks like a spreadsheet, or a table of values. That's because that exactly what an array is!

To access a value of the array, use brackets on the name of the variable. Inside the brackets, type the index, or the location, of the element inside the array. For example, to access the second value of the array:

int secondVal = array[1];

2D Array

Before, we created what is known as a 1D array, meaning that it only has one dimension of values. To create a 2D array, we use 2 square brackets instead of one.

int[][] array2d = new int[2][3];

This array can be visualized as:

       | col 0  | col 1  | col 2  |
-------|--------|--------|--------|
 row 0 | [0][0] | [0][1] | [0][2] |
-------|--------|--------|--------|
 row 1 | [1][0] | [1][1] | [1][2] |
-------|--------|--------|--------|

In Java, arrays are usually accessed in what is called row-major order, meaning that in a 2D array, the first pair of brackets refers to the row of the array, and the second bracket pair refers to the column of the array. Using this principle, to access the second column of the first row:

int secondColFirstRow = array2d[0][1];

Looping through arrays

Writing loops through arrays is a fundamental part of Java. Arrays have a length property that allows us to determine how long the array is. We can use this length property in the ending condition of a for loop like this:

int[] array = new int[4];
​
for(int i = 0; i < array.length; i++) {
    int element = array[i]; // accesses the i'th element of array
    // ...
}

This for loop will run through every element of the array until it is complete.

We can use this same looping principle with 2D arrays:

int[] array2d = new int[4][3];
​
for(int row = 0; row < array.length; row++) {
    // each element of a 2d array is a 1d array
    int[] array1d = array2d[row];
    for(int col = 0; col < array1d.length; col++) {
        int element     = array1d[col];
        // or...
        int sameElement = array2d[row][col];
    }
}

ArrayLists

Now we know how to make a variable hold a fixed amount of multiple values. But what if we need the variable to hold a dynamic amount of values? Introducing ArrayList.

ArrayLists are dynamic arrays that allow us to do a lot more than regular arrays. The main advantage of ArrayLists over arrays is that we can create, read, update, and delete (CRUD) elements.

To use an ArrayList, the first thing we need to do is at the top of our Java program, import java.util.ArrayList.

import java.util.ArrayList;

This tells Java that we will be using ArrayLists in this program.

Here is how you create a new ArrayList:

ArrayList<Double> list = new ArrayList<Double>();

Notice that you can't immediately specify which values you need in the ArrayList. Also, what's with the angle brackets < and > ? Those are called generic type specifiers, a higher level Java principle. Basically, they tell the ArrayList what type of values it can accept. In this case, the ArrayList<Double> list can only accept values of double type.

Let's go over some of the methods of an ArrayList:

list.add(57.0);    // adds a value to the list
list.get(0);       // gets the first value of the list
list.remove(0);    // removes the first value of the list
list.remove(57.0); // removes the given value from the list
list.set(0, 89.0); // sets the first value of the list to the given value

To see a full list of the methods on the ArrayList class, read the complete ArrayList API.

Using integers with ArrayList

You might notice that the get, set, and remove methods all take integers as a parameter. Say you are using Integer as the generic type for an ArrayList:

ArrayList<Integer> list = new ArrayList<Integer>();
list.add(90);

Now, the list has the element 90 and index 0. What if we wanted to remove the value 90 from the list?

list.remove(90); // java.lang.IndexOutOfBoundsException

We receive an error from Java that notifies us that there is no 3rd element in the ArrayList. Why wouldn't it just remove the value 2? Java thinks, when you enter an int as the parameter for this method, that you are requesting to remove the given index from the ArrayList. To fix this issue, we have to cast the int into an Integer.

list.remove((Integer)90);

This is just one of the quirks of the Java language.

Looping through ArrayLists

Similarly to the length property on arrays, ArrayLists offer a size method that returns the size of the list. This is how you would loop through an ArrayList using the size method:

ArrayList<Integer> list = new ArrayList<Integer>();
​
for(int i = 0; i < list.size(); i++) {
    int element = list.get(1); // accesses the i'th element of array
    // ...
}

When using an ArrayList, you have to use the get method to get the element of a specific index, unlike in arrays when you can use a bracket pair.