Arrays & Indexes

An array stores several values of the same element type in order. int[] minutes means a variable naming an array of ints. An initializer such as {10, 25, 40} supplies three elements. Indexes start at zero, so those values occupy positions 0, 1 and 2. An index describes a position, not the element value.

Use minutes.length to get the number of elements. This array property has no parentheses; String.length() is a method and does have parentheses. The last valid index is length minus one, but an empty array has no last element at all. Check boundaries rather than guessing that length itself is a valid index.

Arrays have a fixed size

You can replace an element at an existing position. Doing so changes the array, not its length. You cannot create a new position by assigning at index length. new int[3] creates three elements initialized to zero; it does not create an empty growable container. The next list lesson introduces a growable alternative.

Print individual elements while learning. Printing an array variable directly does not produce a readable list of its contents; it prints an identity-style representation. We will use traversal to produce meaningful reports rather than relying on that representation.

public class Main {
    public static void main(String[] args) {
        int[] minutes = {10, 25, 40};

        System.out.println(minutes[0]);
        System.out.println(minutes.length);

        minutes[1] = 30;

        System.out.println(minutes[1]);
        System.out.println(minutes.length);
    }
}

10
3
30
3

Replacing the middle value changes only position 1. The array still has three slots. Draw three labeled boxes and keep indexes distinct from their stored minute amounts.

public class Main {
    public static void main(String[] args) {
        int[] slots = new int[3];
        int[] empty = {};

        System.out.println(slots[2]);
        System.out.println(slots.length);
        System.out.println(empty.length);
    }
}

0
3
0

slots has three real zero values; empty has no elements. Both are arrays, but only one permits reading index 2. A zero value is not the same thing as a missing element.

public class Main {
    public static void main(String[] args) {
        int[] values = {10, 20};
        System.out.println(values[values.length]);
    }
}

Your turn

public class Main {
    public static void main(String[] args) {
        // Create and update the array, then print three facts.
    }
}

public class Main {
    public static void main(String[] args) {
        int[] durations = {15, 25, 10};
        durations[0] = 20;

        System.out.println(durations.length);
        System.out.println(durations[0]);
        System.out.println(durations[durations.length - 1]);
    }
}

3
20
10

Change it and explain

Change the plan to a single element and check that length-1 still selects it. Then try an empty array: explain why the formula becomes -1 and must not be used. A formula that works on several nonempty examples still needs a stated precondition or a guard for empty input.

Connect this back to main’s frame: String[] args declares an array of text arguments. The browser supplies no command-line arguments, so that array is empty. Input-box text is standard input, a different channel taught on Day 6.