Arrays

An array holds a fixed number of values of the same type, stored in order and accessed by index: a week of temperatures, a list of scores, the pixels of an image. Arrays are Java's most basic collection and the building block underneath many others.

Creating arrays

JAVA

void main() {
    int[] scores = {72, 88, 95, 61};
    String[] days = new String[3];
    days[0] = "Mon";
    days[1] = "Tue";

    System.out.println(scores[2]);
    System.out.println(scores.length);
    System.out.println(days[1] + " " + days[2]);
}

Output

95
4
Tue null

Either list the values in braces, or use new Type[size], which fills the array with default values (0, false, or null for objects). length is a field, not a method — no parentheses.

Changing elements

JAVA

void main() {
    int[] stock = {5, 0, 12};
    stock[1] = 20;
    stock[2] += 3;
    System.out.println(stock[0] + ", " + stock[1] + ", " + stock[2]);
}

Output

5, 20, 15

An array's size is fixed when it's created; to grow, create a bigger array and copy — or use an ArrayList (lesson 40).

Looping over arrays

JAVA

void main() {
    double[] temps = {18.5, 21.0, 19.5, 24.0, 22.5};
    double sum = 0;
    double max = temps[0];
    for (double t : temps) {
        sum += t;
        if (t > max) max = t;
    }
    System.out.println("Average: " + sum / temps.length);
    System.out.println("Max: " + max);
}

Output

Average: 21.1
Max: 24.0

Printing an array

JAVA

import java.util.Arrays;

void main() {
    int[] numbers = {3, 1, 2};
    System.out.println(numbers.length > 0);
    System.out.println(Arrays.toString(numbers));
}

Output

true
[3, 1, 2]

Printing an array directly shows something like [I@1b6d3586 (its type and a hash code). Arrays.toString shows the contents.

The Arrays utility class

JAVA

import java.util.Arrays;

void main() {
    int[] values = {42, 7, 19, 3, 25};
    Arrays.sort(values);
    System.out.println(Arrays.toString(values));
    System.out.println(Arrays.binarySearch(values, 19));

    int[] copy = Arrays.copyOf(values, 7);
    System.out.println(Arrays.toString(copy));

    int[] filled = new int[4];
    Arrays.fill(filled, 9);
    System.out.println(Arrays.toString(filled));
    System.out.println(Arrays.equals(new int[]{1, 2}, new int[]{1, 2}));
}

Output

[3, 7, 19, 25, 42]
2
[3, 7, 19, 25, 42, 0, 0]
[9, 9, 9, 9]
true

Arrays sorts, searches (in a sorted array), copies, fills and compares. Note Arrays.equals: == on arrays only checks whether they're the same object.

Arrays are references

JAVA

import java.util.Arrays;

void main() {
    int[] original = {1, 2, 3};
    int[] alias = original;
    int[] copy = original.clone();
    alias[0] = 99;
    System.out.println(Arrays.toString(original));
    System.out.println(Arrays.toString(copy));
}

Output

[99, 2, 3]
[1, 2, 3]

An array variable holds a reference. Assigning it to another variable doesn't copy the array — both names point to the same one. Use clone() or Arrays.copyOf for a real copy.

Index out of bounds

JAVA

void main() {
    String[] sizes = {"S", "M", "L"};
    System.out.println(sizes[3]);
}

Output

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
    at Main.main(Main.java:3)

Java checks every index. Going outside the array is an error rather than silently reading random memory, as in some older languages.

Try it yourself

Count how many values in an array are above the average.

Output

Average 224.2, 2 days above average

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