Topic 4.3 Notes – Array Creation and Access

1. What a 1D Array Is

A 1D array is a single variable that holds multiple values of the same data type.

• All elements must be:
  • primitive values (int, double, boolean), or
  • object references (String, Student, etc.)
• Each stored value is called an element.
• Each element has a numbered position called an index.
• Arrays use zero-based indexing. The first element is at index 0.

Picture an array like this:

1D array with zero-based indexing

One variable name → many indexed slots.

Arrays are objects

Even though they feel different from other objects, arrays:

• Are created using new
• Have a built-in length attribute
• Store data in memory like any object

Fixed size

The length is set when the array is created and cannot change.

If you create an array with 6 elements, it will always have 6 elements. You can change the values inside, but not the size.

Key vocabulary:

• Element → value inside the array
• Index → position number
• Length → total number of elements

2. Declaring and Creating Arrays

There are two separate ideas here: declaring the variable and creating the array.

Declaration

int[] numbers;
String[] words;

This only creates a reference variable. No array exists yet.

If you try to use it now, you’ll get a NullPointerException.

Creation with new

numbers = new int[4];

Now the array object exists.

• Length is permanently 4
• Valid indices are 0, 1, 2, 3

Default Values

When you use new, Java automatically fills the array:

• int → 0
• double → 0.0
• boolean → false
• Object references → null

Example:

String[] names = new String[3];

All three elements currently store null. The array exists, but no String objects are inside yet.

That distinction shows up a lot on quizzes.

Initializer Lists

If you already know the values:

int[] ages = {15, 16, 15, 17};
String[] colors = {"red", "blue", "green"};

• Java figures out the length automatically.
• You don’t write new int[...] in this form.

You’ll see both styles in multiple choice.

3. Accessing and Modifying Elements

Use square brackets with an index:

ages[1] = 18;       // modify
int x = ages[2];    // access

Same syntax for reading and writing.

Index Rules

Valid index values are:

0 through array.length - 1

In math form:

0 ≤ index < array.length

If array.length is 5, valid indices are 0-4.

The length Attribute

Every array has:

array.length

Notice:

• No parentheses
• It is an attribute, not a method

Common loop pattern:

for (int i = 0; i < array.length; i++) {
    // process array[i]
}

The < is critical.

4. Index Errors and How to Avoid Them

ArrayIndexOutOfBoundsException

This happens if the index is:

• Negative
• Equal to length
• Greater than length - 1

Example mistake:

for (int i = 0; i <= array.length; i++)

That <= will crash when i == array.length.

Most AP questions that involve arrays are testing whether you understand that the last valid index is length - 1.

NullPointerException with Arrays

There are two different null problems.

1. The array reference is null

int[] nums;
nums[0] = 5;   // crash

The array was declared but never created.

2. The elements are null

String[] names = new String[2];
int len = names[0].length();  // crash

The array exists, but names[0] is null.

On multiple choice, they love mixing these up.

5. How This Appears on the AP Exam

Arrays show up constantly.

You’ll be asked to:

• Trace loops that use array.length
• Identify which line causes an exception
• Write methods that process arrays
• Use arrays as instance variables in classes

If you see array code, immediately:

1. Identify the length.
2. Determine valid index range.
3. Watch for <=.
4. Check for null references.

Arrays are the foundation for 2D arrays later. Everything about indexing and length carries forward.