One-dimensional arrays

What is an array? It is a special data structure available in almost every programming language that allows you to represent a group of similar objects using a single common name.

Imagine you run a shelter for stray animals that has five cats. You may not remember the name of each cat, but each one has a tag with a number that uniquely identifies it. You can think of this as an array named "cats" of size five. Note that indexing starts from zero — this is the convention in Java. Without the possibility to create arrays, you would have to declare five separate variables with different names, which is not very convenient.

In Java, you can create arrays of any dimension — one-dimensional, two-dimensional, three-dimensional, and so on. In this lesson, we will explore all these options.

1. Declaring an Array in Java

One-dimensional arrays in Java are lists of variables of the same type. To create an array, you first declare a variable of the required type.

General syntax for declaring a one-dimensional array looks like this:

type[] variableName;

The type parameter defines the element type of the array, also known as the base type.

You can place the square brackets either before or after the variable name. However, it's more conventional to place them after the type, making it immediately clear that it's an array:

int monthDays[];
double[] monthSalaries;

2. Initializing an Array with the new Keyword

Declaring an array doesn't allocate memory. To allocate memory, use the new keyword, followed by the array type and its size in square brackets:

variableName = new type[size];

int[] values = new int[45];

Here's an example of declaring an int array of size 12. The line int[] monthDays = new int[12]; creates an array of 12 elements, each initialized to zero (the default value for int). You can access individual elements by index using square brackets:

public class ArrayExample1 {
    public static void main(String[] args) {
        int[] monthDays = new int[12];
        monthDays[0] = 31;
        monthDays[1] = 28;
        monthDays[2] = 31;
        monthDays[3] = 30;
        monthDays[4] = 31;
        monthDays[5] = 30;
        monthDays[6] = 31;
        monthDays[7] = 31;
        monthDays[8] = 30;
        monthDays[9] = 31;
        monthDays[10] = 30;
        monthDays[11] = 31;
        System.out.println("April has " + monthDays[3] + " days.");
    }
}

3. Array Initialization with an Initialization Block

If you already know the values for each element, use an array initialization block. Instead of new int[12], use curly braces with values separated by commas. The compiler infers the array size from the number of elements:

public class ArrayExample2 {
    public static void main(String[] args) {
        int[] monthDays = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        System.out.println("April has " + monthDays[3] + " days.");
    }
}

4. Anonymous Arrays in Java

Java also supports anonymous arrays, which can be used in two scenarios.

First, suppose you initialized an array like this:

int[] testScores = {1, 2, 3, 4};
...
testScores = {4, 7, 2}; // Compilation error

You can’t reassign values using the same initializer syntax. Instead, use an anonymous array to assign new values:

testScores = new int[]{4, 7, 2};

Second, anonymous arrays can be used as method arguments. In the following example, the print method accepts an array as a parameter. An anonymous array is passed directly to the method:

public class ArrayExample3 {
    public static void main(String[] args) {
        int[] testScores = {1, 2, 3, 4};
        for (int element : testScores) {
            System.out.print(element + " ");
        }
        System.out.println();
        testScores = new int[]{4, 7, 2};
        for (int element : testScores) {
            System.out.print(element + " ");
        }
        System.out.println();

        print(new int[]{4, 6, 2, 3});
    }

    public static void print(int[] array) {
        for (int element : array) {
            System.out.print(element + " ");
        }
    }
}