BeginnerPractice in the editor

Java Arrays Class

Writing a loop every time you want to print or sort an array gets tiring. The java.util.Arrays class has ready-made methods for the common jobs. Like Scanner, it is in java.util, so the program needs an import line.

Printing an array

Printing an array directly gives something unreadable like [I@1b6d3586 (its type and a code). Arrays.toString gives the values:

ExampleTry it in the editor

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] numbers = {5, 3, 9, 1};
        System.out.println(Arrays.toString(numbers));
        String[] names = {"Zara", "Amit", "Leo"};
        System.out.println(Arrays.toString(names));
    }
}

Output

[5, 3, 9, 1]
[Zara, Amit, Leo]

Sorting and searching

Arrays.sort sorts the array itself, from small to large (or alphabetically for strings). On a sorted array, Arrays.binarySearch finds the index of a value very quickly:

ExampleTry it in the editor

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] numbers = {42, 7, 19, 3, 25};
        Arrays.sort(numbers);
        System.out.println(Arrays.toString(numbers));
        System.out.println(Arrays.binarySearch(numbers, 19));
        String[] cities = {"Pune", "Agra", "Kochi"};
        Arrays.sort(cities);
        System.out.println(Arrays.toString(cities));
    }
}

Output

[3, 7, 19, 25, 42]
2
[Agra, Kochi, Pune]

binarySearch only works on a sorted array. If the value is not there, it returns a negative number.

Filling and copying

ExampleTry it in the editor

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] scores = new int[5];
        Arrays.fill(scores, 10);
        System.out.println(Arrays.toString(scores));
        int[] original = {1, 2, 3, 4, 5};
        int[] firstThree = Arrays.copyOf(original, 3);
        int[] bigger = Arrays.copyOf(original, 7);
        int[] middle = Arrays.copyOfRange(original, 1, 4);
        System.out.println(Arrays.toString(firstThree));
        System.out.println(Arrays.toString(bigger));
        System.out.println(Arrays.toString(middle));
    }
}

Output

[10, 10, 10, 10, 10]
[1, 2, 3]
[1, 2, 3, 4, 5, 0, 0]
[2, 3, 4]

Copying is how you "grow" an array: since its length is fixed, you make a new, longer copy. The ArrayList topic shows a list that grows by itself.

Comparing arrays

== on two arrays only checks whether they are the very same array object. To compare the contents, use Arrays.equals:

ExampleTry it in the editor

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] a = {1, 2, 3};
        int[] b = {1, 2, 3};
        System.out.println(a == b);
        System.out.println(Arrays.equals(a, b));
    }
}

Output

false
true

Sum, minimum and maximum

Arrays.stream turns an array into a stream of values with handy methods. Streams get their own topic, Streams, later; these three are useful right away:

ExampleTry it in the editor

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] marks = {67, 89, 74, 95, 58};
        System.out.println("Sum: " + Arrays.stream(marks).sum());
        System.out.println("Max: " + Arrays.stream(marks).max().getAsInt());
        System.out.println("Average: " + Arrays.stream(marks).average().getAsDouble());
    }
}

Output

Sum: 383
Max: 95
Average: 76.6

When to use these methods

Reach for the Arrays class before writing a loop of your own: its methods are tested, fast and easy to read. A loop is still the right tool when you need something special, such as counting only some values or stopping early, as in the Arrays topic.

Practice exercises

Sort the array {"pear", "apple", "fig", "banana"} and print it.
Make an array of 10 numbers all equal to 7 with Arrays.fill and print it.
Print the smallest value of {12, -4, 33, 0, 8} using Arrays.stream(...).min().