Lesson 22 of 31

6 mins read

Array Indexing & Traversal

In Plain English

Java arrays are 0-indexed: the first element is at index `0` and the last element is at `array.length - 1`. Attempting to access an index outside this range throws an `ArrayIndexOutOfBoundsException`.

The Mental Model

"Think of hotel room floors in European numbering: Ground floor is index 0. The 4th room in a 4-room hallway is at slot 3. Trying to visit room 4 causes a "room does not exist" error."

Deep Dive: How It Works

The `.length` property (no parentheses) gives the total number of elements in the array.

Looping pattern: `for (int i = 0; i < arr.length; i++) { ... }`

Modifying in place: `arr[i] = arr[i] * 2;` directly updates the value in memory.

Syntax Blueprint

for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

Condition i < arr.length safely stops before the out-of-bounds index.

Core Rules to Remember

Zero-Based Indexing: Valid indices run from 0 to length - 1.

The length Field: Array property without parentheses (arr.length).

Out of Bounds Safety: Java throws ArrayIndexOutOfBoundsException on invalid index.

Common Beginner Traps & How to Fix Them

Mistake:Using i <= arr.length in loop condition

Why it happens: When i reaches arr.length, accessing arr[i] throws an ArrayIndexOutOfBoundsException.

How to fix: Always use strictly less than: i < arr.length.

Pro Tip: To get the last element in any non-empty array: arr[arr.length - 1].

Live Interactive Example

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Traversing an Array with Index Loop

Java 13

Output Console

Click "Run Code" to view the rendered output.

How it works: The loop visits indices 0, 1, 2, 3 cleanly without throwing out-of-bounds errors.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Calculate sum of array elements

Given int[] values = {5, 10, 15};
Loop through the array, calculate total sum, and print "Total: " + sum.

Sandbox Output

Click "Run & Check" to test your solution.

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