📓6.1: Array Basics

Array Creation and Access
⭐️ Summary

📖 This page is a condensed version of CSAwesome Topic 6.1

Using a GitHub Template for class notes

Go to the public template repository for our class: BWL-CS Java Template
Click the button above the list of files then select Create a new repository
Specify the repository name: CS2-Unit-6-Notes
Click

Now you have your own personal copy of this starter code that you can always access under the Your repositories section of GitHub!
Now on your repository, click and select the Codespaces tab
Click Create Codespace on main and wait for the environment to load, then you’re ready to code!
📝 Take notes in this Codespace during class, coding along with the instructor.

Array Creation and Access

To keep track of 10 exam scores, we could declare 10 separate variables: int score1, score2, score3, … , score10; But what if we had 100 exam scores? That would be a lot of variables!

📋 Most programming languages have a simple data structure for a collection of related data that makes this easier. In some programming languages, this is called a list. In Java and many programming languages, this is called an array.

Array: A block of memory that stores a collection of data items (elements) of the same type under one variable name.

Arrays are useful whenever you have many elements of data of the same type that you want to keep track of, but you don’t need to name each one. Instead you use the array name and a number (called an index) for the position of an item in the array.

Index: An integer that represents a location in a collection such as an Array. In Java, the first element in an array is stored at index 0.

You can make arrays of ints, doubles, Strings, and even object classes that you have written like Students. Here’s a fun that introduces the concept of an array and gives an example.

ANALOGY

An array is like a row of small lockers, except that you can’t cram lots of stuff into it. You can only store one value at each locker:

An array index is like a locker number. It helps you find a particular place to store your stuff and retrieve stuff. You can get or store a value from or to an array using its index.

Arrays and lists in most programming languages start counting elements from the number 0, so the first element in an array is at index 0. This is similar to how Strings are indexed in Java – the first character is at index 0.

💬 DISCUSS: Can you think of another example of something that is like an array?

Declaring an Array

When we declare a variable, we specify its type and then the variable identifier (name). To make a variable into an array, we put square brackets after the data type:

   // Declaration for a single int variable
   int score;
   // Declaration for an ARRAY of ints
   int[] scores;

For example, int[] scores means we have an array called scores that contains int type values.

The declarations do not create the actual array. Arrays are objects in Java, so any variable that declares an array holds a 👉 reference to an object. If the array hasn’t been created yet and you try to print the value of the variable, it will print null (meaning it doesn’t reference any object yet).

There are two ways to create an array. You can use the keyword new to get new memory or use an initializer list to set up the values in the array.

Watch the following which shows the two ways of creating an array with a physical model of Java memory.

Using `new` to Create Empty Arrays

To create an empty array after declaring the variable, use the new keyword with the type and the size of the array (the number of elements it can hold). This will actually create the array in memory. You can do the declaration and the creation all in one step, see the String array names below. The size of an array is set at the time of creation and cannot be changed after that.

  //declare an array variable
  int[] highScores;
  // create the array
  highScores = new int[5];
  // declare and create array in 1 step!
  String[] names = new String[5];

Add two more array declarations:

One that creates an array of 5 doubles called prices
Another array of 5 Strings called names.

What happens in the computer’s memory when you declare arrays?

Array elements are initialized to default values like the following:

0 for elements of type int
0.0 for elements of type double
false for elements of type boolean
null for elements of type String or any other object

Two 5 element arrays with their values set to the default values for integer and object arrays.

Initializer Lists to Create Arrays with Values

Another way to create an array is to use an initializer list. You can initialize (set) the values in the array to a list of values in curly braces ({}) when you create it, like below. In this case you don’t specify the size of the array, it will be determined from the number of values that you specify.

  int[ ] highScores = {99,98,98,88,68};
  String[ ] names = {"Jamal", "Emily", "Destiny", "Mateo", "Sofia"};

When you create an array of a primitive type (like int) with initial values specified, space is allocated for the specified number of items of that type and the values in the array are set to the specified values. When you create an array of an object type (like String) with initial values, space is set aside for that number of object references. The objects are created and the object references set so that the objects can be found.

Watch the following which shows an array of String objects with a physical model of Java memory.

Array Length

Arrays know their length (how many elements they can store). It is a public read-only instance variable so you can use dot-notation to access the instance variable (arrayName.length).

Dot-notation is using variable name followed by a . and then the instance variable (property) name or a method name.

    System.out.println(highScores.length);

Try adding another value to the highScores initializer list and run again to see the length value change.

Note that length is an instance variable and not a method, unlike the String length() method, so you don’t add parentheses after length. The length instance variable is declared as a public final int. public means you can access it and final means the value can’t change.

Access and Modify Array Values

To access the items in an array, we use an indexed array variable which is the array name and the index inside of square bracket [ ]. Remember that an index is a number that indicates the position of an item in a list, starting at 0.

An indexed variable like arrayname[index] can be used anywhere a regular variable can be used, for example to assign a new value or to get a value from the array like below.

  // assign a new value 99 to the first element in the array
  highScores[0] = 99;
  // print the first element of the array
  System.out.println( highScores[0] );

The first value in an array is stored at index 0 and the index of the last value is the length - 1 (since length is the number of items and the starting index is 0). Use arrayname[index] to access or modify array items.

Watch the following which shows a physical model of Java memory setting array values.

Parallel Arrays

If you want to keep track of the top 5 highest scores in a game and the names of the people with those scores, you could use two parallel arrays. One array could keep track of the scores and the other the names.

With parallel arrays, you have to make sure you keep them in the same order so that the same index can be used to get correponding names and scores.

Try out the following code which has two parallel arrays, highScores and names.

Can you print out Mateo’s score?
Can you change Sofia’s score to 97 using an assignment statement in the code?
Can you modify the arrays so that they have 6 elements, add your name and score, and print them out?

public static void main(String[] args) {
    // declare, create, initialize arrays
    int[] highScores = {99, 98, 98, 88, 68};
    String[] names = {"Jamal", "Emily", "Destiny", "Mateo", "Sofia"};

    // Print corresponding names and scores
    System.out.println(names[0] + " has a score of " + highScores[0]);
    System.out.println(names[1] + " has a score of " + highScores[1]);
}

💬 DISCUSS: What happens if you try to access an element that is not there? Try to access a highScore or name at index 7 above to see what happens. The index must be between 0 and the length of the array - 1 or it will give an error message called ArrayIndexOutOfBoundsException.

⚠️ Using an index value outside of the range 0 - (length-1) will result in an ArrayIndexOutOfBoundsException being thrown.

One powerful feature in the array data abstraction is that we can use variables for the index! As long as the variable holds an integer, we can use it as an index:a

  // use a variable for the index
  int index = 3;
  System.out.println(  highScores[index] );

🗺 In-Class Challenge: Countries Arrays

Go to
Make sure you SIGN IN!
Complete the Programming Challenge: Countries Array activity in pairs. 🗺

🎲 Practice Game: Array Indices

Try the game below to practice! Click on Arrays and click on the element of the * array that would be printed out by the given code. If you’re stuck, check on Labels to see the indices. We encourage you to work in pairs and see how high a score you can get.

⭐️ Summary

Arrays represent collections of related data all of the same data type.
The size of an array is established at the time of creation and cannot be changed.
Arrays can store either primitive data or object reference data.
When an array is created using the keyword new, all of its elements are initialized with a specific value based on the type of elements:
- Elements of type int are initialized to 0
- Elements of type double are initialized to 0.0
- Elements of type boolean are initialized to false
- Elements of a reference type are initialized to the reference value null. No objects are automatically created.
Initializer lists can be used to create and initialize arrays.
Square brackets ([ ]) are used to access and modify an element in an array using an index. The indexed array variable, for example array[index], can be used anywhere a regular variable can be used, for example to get or assign values.
The valid index values for an array are 0 through one less than the number of elements in the array, inclusive. Using an index value outside of this range will result in an ArrayIndexOutOfBoundsException being thrown.

🛑 When class ends, don’t forget to SAVE YOUR WORK!

Navigate to the Source Control menu on the LEFT sidebar
Type a brief commit message in the box, for example: updated Main.java
Click the button on the LEFT menu
Click the button on the LEFT menu
Finally you can close your Codespace!

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.