One-Dimensional Arrays and Traversals

Fixed length, zero-based indexing

An array holds a fixed number of values of the same type. The length is decided when the array is created and can never change: int[] a = new int[6]; creates six slots, and a.length is 6 forever. Note that length is a field, not a method — there are no parentheses, unlike String.length().

Valid indices run from 0 to a.length − 1. Touching any other index throws an ArrayIndexOutOfBoundsException at run time — the compiler will not catch it.

New arrays are filled with default values: 0 for int, 0.0 for double, false for boolean, and null for object types such as String.

Two ways to traverse

for (int i = 0; i < a.length; i++) {   // indexed loop
    a[i] = a[i] * 2;                     // CAN modify
}

for (int x : a) {                        // enhanced for (for-each)
    System.out.println(x);               // read only
}

The enhanced for loop copies each element into the loop variable. Assigning to that variable changes the copy, never the array. Use the indexed loop whenever you must write into the array, compare neighbouring elements, or know where you are.