Array anatomy

An array holds a fixed number of values of one type, numbered from 0:

int[] scores = new int[4];          // {0, 0, 0, 0} — zero-filled
int[] primes = {2, 3, 5, 7};        // literal form
primes[0] = 2;                       // index (read/write)
int n = primes.length;               // LENGTH IS A FIELD — no parentheses!

length without () is the array idiosyncrasy (Strings use length(), ArrayLists use size() — the exam mixes all three on purpose).

Bounds: valid indices are 0 .. length − 1. Index 4 on a length-4 array compiles fine and throws ArrayIndexOutOfBoundsException at runtime — the most common crash in student code.

Default values: new int[4] is all zeros; new boolean[4] all false; new String[4] all null (an array of references, not objects).

The two traversal loops:

for (int i = 0; i < arr.length; i++) {   // when you need the index
    System.out.println(arr[i]);
}

for (int x : arr) {                      // enhanced for: values only
    System.out.println(x);
}

The enhanced for cannot modify the array (its variable is a copy) and gives no position — use it for read-only visits.