AP CSA — Topic 4.1

The big idea

An array is a fixed-size collection of items of the same type. Each item lives in a numbered slot called an index.

On AP CSA, you must be comfortable reading array code and predicting values at specific indexes.

Way	Code	What it means
Specify size	`int[] scores = new int[5];`	Creates 5 `int` slots (default values at first).
Initializer list	`int[] scores = {10, 20, 30};`	Creates an array with those values (size becomes 3).

int[] a = new int[4];     // size 4, indexes 0..3
int[] b = {2, 4, 6, 8};   // size 4, already filled

If you don’t fill an array yet, Java gives each slot a default value:

int[] nums = {5, 7, 9};  // length = 3
System.out.println(nums[3]);  // ❌ invalid (last index is 2)

Use brackets [ ] to access or change a specific element.

int[] nums = {10, 20, 30};

int first = nums[0];   // access
nums[1] = 99;          // update

// nums is now {10, 99, 30}

Index	0	1	2
Value	10	99	30

The index tells you the position; the value is what’s stored there.

Arrays use length (field), not length() (method).

int[] nums = {4, 8, 12, 16};
int n = nums.length;        // 4
int last = nums[nums.length - 1];  // 16

// Off-by-one bug:
int[] a = new int[5];
a[a.length] = 1; // ❌ last valid index is a.length - 1