Arrays & Bounds Thinking

An array is a fixed-length row of same-typed values, accessed by index:

int[] temps = {18, 21, 19, 25};       // literal: length fixed at creation
double[] prices = new double[10];     // 10 slots, all 0.0
temps[0] = 17;                        // index 0 is the FIRST element
int last = temps[temps.length - 1];   // length is a field, not a method

The rules that shape everything:

• Fixed size. An array cannot grow. temps.length never changes. When your data must grow, that is the sign to reach for ArrayList (Module 10).
• Zero-based indexing. Valid indexes run 0 .. length-1. Outside that range throws ArrayIndexOutOfBoundsException the moment it happens.
• Homogeneous and typed. An int[] holds ints — nothing else. Java checks at compile time.

Iterating is the module-4 loop with length as the boundary; the enhanced for covers the common read-everything case. Bounds thinking is the skill: every index you compute (i, i + 1, length - 1, midpoints) must be re-checked against 0 and length - 1 in your head. Swapping neighbors a[i] with a[i + 1] only works while i + 1 <= length - 1 — one comparison mistake and the JVM throws.

2D arrays are arrays of arrays — a grid:

int[][] grid = { {1, 2}, {3, 4} };
System.out.println(grid[1][0]);      // 3 — row 1, column 0
for (int[] row : grid) {
    for (int cell : row) { /* ... */ }
}

Next: Java's most-used class — String.