Arrays

An array is a set of values of one type stored under one name. Instead of ten variables price1, price2 and so on, you create one array prices. An array has a fixed length: it is set at creation and never changes afterwards.

Creating an array

An array type is written as the element type with square brackets: int[], String[]. If the values are known right away, list them in curly braces:

void main() {
    int[] scores = {90, 75, 82};
    IO.println(scores[0]);
    IO.println(scores.length);
}

Output:

90
3

An element is accessed by its index — a number in square brackets. Numbering starts at zero, so three elements have indexes 0, 1 and 2. The length field holds the array's length.

Changing elements

An array element works like a regular variable: you can assign it a new value by index:

void main() {
    String[] names = {"Anna", "Oleg", "Maria"};
    names[1] = "Igor";
    IO.println(names[0] + ", " + names[1] + ", " + names[2]);
}

Output:

Anna, Igor, Maria

An empty array of a given length

When the values will come later, the array is created with the word new and the length in brackets. The elements immediately get default values: 0 for numbers, false for boolean, null for strings and other objects:

void main() {
    int[] counts = new int[3];
    String[] labels = new String[2];
    counts[0] = 5;
    IO.println(counts[0] + " " + counts[1] + " " + counts[2]);
    IO.println(labels[0]);
}

Output:

5 0 0
null

null means "no object". You cannot call a method on such an element — the program would crash with an error.

Looping over an array

Arrays are almost always processed in a loop. A regular for gives you the index, for-each gives you the values directly:

void main() {
    double[] prices = {4.5, 3.0, 2.5};
    double total = 0;
 
    for (int i = 0; i < prices.length; i++) {
        IO.println((i + 1) + ". $" + prices[i]);
    }
    for (double price : prices) {
        total += price;
    }
 
    IO.println("Total: $" + total);
}

Output:

1. $4.5
2. $3.0
3. $2.5
Total: $10.0

Printing an array

To print a whole array, use Arrays.toString() — it collects all elements into a string in square brackets:

void main() {
    int[] numbers = {3, 1, 2};
    IO.println(Arrays.toString(numbers));
}

Output:

[3, 1, 2]

The Arrays class can also sort, copy and compare arrays — see the lesson on the Arrays class.

Common mistake

If you print an array directly, you get cryptic text instead of the elements:

void main() {
    int[] numbers = {3, 1, 2};
    IO.println(numbers);
}

Output (the digits after @ change from run to run):

[I@27c6e487

This is an internal description of the object: [I means "an int array", followed by its hash code. To print the contents, always use Arrays.toString().