Arrays in Java are fixed-size, zero-indexed collections of a single type. Knowing how to declare, initialize, and access them correctly is the foundation of the entire Unit 4 exam weight.
Three Ways to Initialize an Array
// 1. Declare with size -- fills with default values
int[] scores = new int[5]; // [0, 0, 0, 0, 0]
double[] vals = new double[3]; // [0.0, 0.0, 0.0]
String[] names = new String[4];// [null, null, null, null]
// 2. Initializer list -- size inferred
int[] primes = {2, 3, 5, 7, 11};
// 3. Declare then assign
int[] data;
data = new int[10];
Default Values
Type
Default value
int
0
double
0.0
boolean
false
Object / String
null
Accessing Elements
int[] arr = {10, 20, 30, 40, 50};
System.out.println(arr[0]); // 10 -- first element
System.out.println(arr[arr.length-1]); // 50 -- last element
System.out.println(arr.length); // 5 -- number of elements
⚠ length vs length(): Arrays use arr.length (no parentheses). Strings use str.length() (with parentheses). This is one of the most common errors on the AP exam.
📝 Practice Question 1
What are the contents of arr after the following declaration? boolean[] arr = new boolean[4];
📝 Practice Question 2
Which of the following correctly accesses the LAST element of an array data of unknown length?
✅ Exam Tip: The AP exam frequently tests whether you know that array indices run from 0 to length - 1. The most common runtime error is accessing arr[arr.length] instead of arr[arr.length - 1].
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