4.3 Array Creation and Access

What you need to know

• Declare and create: int[] nums = new int[5]; — five ints, all initialized to 0. double[] → 0.0, boolean[] → false, object arrays (String[]) → null.
• Initializer list: int[] nums = {4, 8, 15}; creates and fills in one step.
• Access: nums[i]. Indices run from 0 to nums.length - 1. length is a field (no parentheses), unlike String's length().
• Accessing index length or any negative index throws ArrayIndexOutOfBoundsException at run time.
• Array size is fixed once created. To "grow" you create a new, bigger array and copy.
• Arrays are objects: an array variable holds a reference. int[] b = a; makes b an alias of a — same array.
• An array of objects holds references; each element must be assigned an object before its methods are called, or you get a NullPointerException.

Worked example

int[] a = new int[4];        // {0, 0, 0, 0}
a[0] = 7;
a[3] = 2;
a[a.length - 1] = 9;         // last element → 9
System.out.println(a[2]);    // 0
System.out.println(a.length);// 4
// a[4] = 1;                 // ArrayIndexOutOfBoundsException

String[] words = new String[2];
// words[0].length();        // NullPointerException — element is null
words[0] = "hi";
System.out.println(words[0].length());   // 2

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• Two creation forms: new int[n] — n elements, all default; {a, b, c} — initializer, exactly those elements. You can't use an initializer after the declaration line (arr = {1, 2}; won't compile); use arr = new int[]{1, 2};.
• Defaults: 0, 0.0, false, null. An array of Strings or of your own class starts as all nulls; you must assign each element an object before calling methods on it. new Student[5] creates zero Students.
• length is a field: arr.length, no parentheses. (String's is a method: s.length(). ArrayList's is list.size().) Mixing these is a compile-error question.
• Valid indices: 0 through length − 1. Index length is one past the end — the exam's favorite exception. Negative indices also throw.
• Arrays are objects. The variable holds a reference. int[] b = a; aliases; modifying b[0] modifies a[0]. To copy, make a new array and loop.
• Fixed size forever. To add an element you create a bigger array, copy everything, and switch the reference. This is what ArrayList does internally — which is why ArrayList exists.
• Array of arrays = 2D array (4.11). Each row is a 1D array object with its own length.
• Passing an array to a method passes the reference: the method can change elements, not replace the array (3.6).

Mistakes that cost points

• arr[arr.length]. Out of bounds. Last element is length − 1.
• arr.length(). Compile error. No parentheses for arrays.
• Calling a method on an element of a new object array. It's null. NullPointerException.
• Expecting int[] b = a; to copy. It aliases.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

int[] arr = new int[6];

int[] x = {3, 6, 9};
int[] y = x;
y[1] = 0;
System.out.println(x[1]);