Java Arrays

Context: FIT2099_MOC · store many values of one type under one name · fixed length · iterated with a for-each loop Problem it solves: create a fixed-size collection of same-typed items, then access/update by index.

Quick Revision

• 🎯 Trigger: many values of the same type ➔ an array, not 100 separate variables.
• ⚡ Key Constraint: arrays are 0-based and fixed length; index the last item with arr.length - 1 (note length is a field, no ()).

🔧 Minimal Working Example

String[] students = {"Adam", "Billy", "Charlie"};   // literal, length 3
System.out.println(students[0]);                     // access: "Adam" (0-based)
students[2] = "Chuck";                                // update by index
System.out.println(students.length);                 // 3  (field, not method)

Expected output: Adam; then students[2] becomes Chuck; length 3.

• Create (literal) ➔ String[] a = {"x","y"}; — values known up front.
• Create (empty, fixed size) ➔ String[] a = new String[5]; then assign a[0] = "x".
• Access / update ➔ a[i] reads or writes position i (0-based).
• Length ➔ a.length — an attribute (no parentheses), unlike String.length().

🔀 Variations

• Iterate with index ➔ for (int i = 0; i < a.length; i++) … when you need i.
• Iterate values ➔ for (String s : a) … (for-each) to just visit each element.

✍️ Practice

Practice 1: Make a 5-element int array, fill it with the squares 1²…5², and print each on its own line.

Reference solution

int[] sq = new int[5];
for (int i = 0; i < sq.length; i++) sq[i] = (i + 1) * (i + 1);
for (int v : sq) System.out.println(v);

• Key move: index-for to fill (needs i), for-each to print (values only).

⚠️ Common Mistakes

• 💡 length has no parentheses ➔ array a.length is a field; only String uses .length() (a method).
• 💡 Fixed size + 0-based ➔ new String[5] holds indices 0–4; a[5] throws ArrayIndexOutOfBoundsException.