Unit 4 Cycle 1 Day 1: Array Declaration and Initialization
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Array Declaration and Initialization
Section 4.1 — Array Creation and Access
Key Concept
Arrays in Java are fixed-size, ordered collections of elements of the same type. Declaration uses square brackets: int[] arr = new int[5] creates an array of 5 integers, all initialized to 0. Array elements are accessed by index starting at 0, so valid indices for an array of length n range from 0 to n - 1. Once created, an array's length cannot change. The length property (not a method — no parentheses) returns the number of elements.
Consider the following code segment.
What is printed?
Answer: (A) 0
When an int array is created with new int[5], all elements are initialized to 0 by default. Only indices 0 and 1 are explicitly set. Index 2 retains its default value of 0.
Why Not the Others?
(B) 20 is at index 1, not index 2.
(C) null is the default for object arrays, not primitive arrays. int defaults to 0.
(D) Index 2 is within bounds (0-4). No ArrayIndexOutOfBoundsException.
Common Mistake
Primitive arrays default to 0 (int), 0.0 (double), or false (boolean). Object arrays default to null. Unassigned elements are NOT empty or undefined.
AP Exam Tip
Know the default values: int=0, double=0.0, boolean=false, String/Object=null. This is tested frequently on the AP exam.