Initializing Arrays in Java

1. Overview

An array is a data structure that allows us to store and manipulate a collection of elements of the same data type. Arrays have a fixed size, determined during initialization, that cannot be altered during runtime.

In this tutorial, we’ll see how to declare an array. Also, we’ll examine the different ways we can initialize an array and the subtle differences between them.

2. Understanding Arrays

In Java, arrays are objects that can store multiple elements of the same data type. We can access all elements in an array through their indices, which are numerical positions starting from zero. Also, the length of an array represents the total number of elements it can hold:

In the image above, we have a one-dimensional array of eight items.

Notably, if we try to access an element outside the valid index range of the array, it throws ArrayIndexOutOfBoundsException.

3. Declaring and Initializing One-Dimensional Arrays

We can easily declare an array by specifying its data type followed by square brackets and the array name:

int[] numbers;

In the code above, we declare an uninitialized array of int type.

Alternatively, we can place the square brackets after the array name, but this approach is less common:

int numbers[];

Furthermore, we must initialize an array to use it. Initialization involves allocating memory using the new keyword and specifying the array length:

var numbers = new int[7];

In the code above, we initialize a numbers array to hold seven elements of int type.

After initialization, we can assign values to individual elements using their indices:

numbers[0] = 10;
numbers[1] = 20;

Here, we add 10 and 20 to indices 0 and 1, respectively.

Furthermore, we can retrieve an element value by using its index:

assertEquals(20, numbers[1]);

In the code above, we assert that the element in index one is 20.

It’s possible to declare and initialize an array in a single step:

int[] numbers = new int[5];

Notably, the length of an array is always fixed and cannot be extended after initialization.

Alternatively, we can specify the length of an array using a variable:

int length = 7;
int[] numbers = new int[length];

Here, we declare a variable used as the length of the array. Importantly, length can only be of type int. Any other type apart from int throws an incompatible type error.

4. Declare Array of Unknown Size

When we declare an array, knowing the size is unnecessary. We can either assign an array to null or an empty array:

int[] numbers = null;

int[] numbers = new int[0];

But we need to know the size when we initialize it because the Java virtual machine must reserve the contiguous memory block for it. As we discussed earlier, we can initialize an array with the new keyword:

int[] numbers =  new int[5];

If we want to resize the array, we can do it by creating an array of larger size and copying the previous array elements to the new array:

int newSize = 10; // New desired size
int[] newArray = new int[newSize];

// Copy elements from the old array to the new array
System.arraycopy(numbers, 0, newArray, 0, numbers.length);

numbers = newArray // reference to new array

If we’re allowed to use ArrayList, then for dynamic resizing we should always use ArrayList:

// we can add elements dynamically without specifying the size
List<Integer> numbers = new ArrayList<>();
numbers.add(1);
numbers.add(2);
numbers.add(3);

ArrayList internally uses an array and System.arrayCopy() to support dynamic resizing.

5. Default Values for Array Elements

Upon initialization, the elements of an array are automatically assigned default values based on the data type of the array. These values represent the array elements’ initial state before we explicitly assign any values.

Arrays of int, short, long, float, and double data types set all elements to zero by default:

int[] array = new int[5];
assertArrayEquals(new int[] { 0, 0, 0, 0, 0 }, array);

Furthermore, for boolean arrays, the default value for all elements is false:

boolean[] array = new boolean[5];
assertArrayEquals(new boolean[] { false, false, false, false, false }, array);

Finally, for arrays of object types, such as String, the default value for all elements is set to null:

String[] array = new String[5];
assertArrayEquals(new String[] { null, null, null, null, null }, array);

Notably, when we use or access an element of an array without explicitly assigning a value, we are automatically using or accessing the default value.

6. Initializing Arrays With Values

Also, we can assign values to an array during initialization. This is often called array literals:

String[] brand = new String[] { "Toyota", "Mercedes", "BMW", "Volkswagen", "Skoda" };

In the code above, we initialize a string array with five brand names. The total number of elements within the curly braces determines the length/size of the array.

Furthermore, we can omit the array type for primitive data types:

int[] array = { 1, 2, 3, 4, 5 };

Additionally, when using the var keyword for type inference, we cannot initialize an array literal without the new keyword:

var array = {1, 2, 3, 4, 5};

The code above results in a compilation error.

To use the var keyword with array literals, we must introduce the new keyword, allowing the compiler to infer the type from the right-hand side of the assignment:

var arr = new int[]{1,2,3,4,5};

Here, the compiler can infer it’s an array of integers.

7. Adding Values to Arrays Using Arrays.fill()

The java.util.Arrays class has several methods named fill() that accept different types of arguments and fill the whole array with the same value:

long array[] = new long[5];
Arrays.fill(array, 30);

This method can be useful when we need to update multiple elements of an array with the same value.

The method also has several alternatives that set a given range of an array to a particular value:

int[] array = new int[5];
Arrays.fill(array, 0, 3, -50);

Here, the fill() method accepts the initialized array, the index to start the filling, the index to end the filling (exclusive), and the value itself respectively as arguments. The first three elements of the array are set to -50 and the remaining elements retain their default value which is zero.

Notably, if the range between the start and the end index is greater than the size of the array, it throws ArrayIndexOutOfBoundsException.

8. Copying Arrays Using Arrays.copyOf()

The method Arrays.copyOf() creates a new array by copying elements from an existing array. The method has many overloads, which accept different types of arguments.

Let’s see a quick example:

int[] array = { 1, 2, 3, 4, 5 };
int[] copy = Arrays.copyOf(array, 5);

There are a few things to note in this example:

• The method accepts two arguments –  the source array and the desired length of the new array to be created.
• If the length is greater than the length of the array to be copied, then the extra elements will be initialized using their type’s default value.
• If the source array hasn’t been initialized, then a NullPointerException is thrown.

9. Adding Values to Arrays Using Arrays.setAll()

The method Arrays.setAll() sets all elements of an array using a generator function. This method can be useful when we need to add values that follow a specific pattern or logic to an array.

Let’s see an example using the Arrays.setAll() method:

int[] numbers = new int[5];
Arrays.setAll(numbers, i -> i * 2);
assertArrayEquals(new int[] {0, 2, 4, 6, 8}, numbers);

In the code above, we create an array of int data types and use the setAll() method to populate it with even numbers.

Here’s another example that demonstrates a more complex generator function:

int[] array = new int[20];
Arrays.setAll(array, p -> p > 9 ? 0 : p);

// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

Here, the generator function sets the first ten elements of the array to their respective indices and sets the remaining elements to zero.

If the generator function is null, then a NullPointerException is thrown.

10. Cloning an Array Using ArrayUtils.clone()

Let’s utilize the ArrayUtils.clone() API from Apache Commons Lang 3, which initializes an array by creating a direct copy of another array:

char[] array = new char[] {'a', 'b', 'c'};
char[] copy = ArrayUtils.clone(array);

Note that this method is overloaded for all primitive types.

11. Declaring and Initializing Two-Dimensional Arrays

Moreover, let’s see how to declare a two-dimensional array:

int[][] matrix;

In the code above, we declare a two-dimensional array of int data type by specifying two sets of square brackets. Let’s initialize the array and specify the number of rows and columns:

matrix = new int[2][5];

We can think of this as an array with two rows and five columns.

Let’s assign value to the individual elements using nested indices, where the first index represents the row and the second index represents the column:

matrix[0][0] = 10;
matrix[0][1] = 20;
matrix[0][2] = 30;
matrix[0][3] = 40;
matrix[0][4] = 50;
matrix[1][0] = 60;
matrix[1][1] = 70;
matrix[1][2] = 80;
matrix[1][3] = 90;
matrix[1][4] = 100;

We can retrieve the value in a two-dimensional array by specifying its row and column indices:

assertEquals(20, matrix[0][1]);

In the code above, we retrieve the value of an array at row 0 and column 1.

Alternatively, we can initialize a two-dimensional array using a less frequently used syntax:

int[] array[] = new int[5][5];

Also, we can initialize a two-dimensional array with literals using nested curly brackets:

 int [][] twoDArray = { { 1, 2, 3 }, { 4, 5, 6 } };

In the code above, we initialized a two-dimensional array of int type containing two one-dimensional arrays, which represent the two rows of our array.

12. Adding Values to an Array Using a for Loop

We can assign values to each element of an array individually using a loop. This approach can be useful when we need to populate an array based on specific conditions or calculations.

Let’s look at an example that uses a loop-based approach:

int[] array = new int [3];
for (int i = 0; i < array.length; i++) {
    array[i] = i + 2;
}

In the example above, we use a loop to assign values to each element. In this case, we assign the value i + 2 to the element at index i.

For multi-dimensional arrays, we can use nested loops to iterate over each dimension and assign values to the elements:

int[][] matrix = new int[3][4];
for (int i = 0; i < matrix.length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        matrix[i][j] = i * 4 + j;
    }
}

Here, we use nested for loops to iterate over the rows and columns, respectively. We assign each element the value i * 4 + j, which is calculated based on the row and column indices.

13. Initializing an Array Using the Stream API

The Stream API provides convenient methods for creating arrays from streams of elements, including methods such as Arrays.stream(), IntStream.of(), DoubleStream.of(), and many others. These methods allow us to initialize arrays with specified values.

Let’s see an example that uses the Instream.of() method to initialize and populate an array:

 int[] values = IntStream.of(1, 2, 3, 4, 5).toArray();

Here, we create an IntStream with five values and use the toArray() method to convert it to an array of integers.

Moreover, we can initialize a higher-dimensional array using the Stream API. Let’s use this approach to initialize a two-dimensional array:

int[][] matrix = IntStream.range(0, 3)
  .mapToObj(i -> IntStream.range(0, 4).map(j -> i * 4 + j).toArray())
  .toArray(int[][]::new);

First, we generate a stream of integers using IntStream.range(0, 3), corresponding to the indices of the rows in the matrix. Next, we use the mapToObj() method to convert each integer in the original stream into an array of integers. Then, we generate another stream of integers corresponding to the indices of the columns in each row of our matrix.

Finally, we convert the stream of integer arrays into a two-dimensional array using the toArray(int[][]::new) method, which is a method reference that creates a new two-dimensional array.

14. Higher-Dimensional Arrays

Furthermore, we can have arrays of more than two dimensions. Depending on the specific use case, we can have a three-dimensional array, a four-dimensional array, and so on. The number of square brackets determines the dimensions.

Let’s see how to declare and initialize a three-dimensional array:

int[][][] threeDArray = new int[3][4][5];

In the code above, we have three square brackets to indicate this is a three-dimensional array. The first square bracket [3] represents the depth of the array. It indicates the number of two-dimensional arrays that the three-dimensional array contains. In this case, we have three two-dimensional arrays in the three-dimensional array.

Let’s add an element to the first 2D array:

threeDArray[0][0][1] = 4;

Here, the first index [0] indicates the first 2D array within the 3D array, the second index [0] represents the first row of the first 2D array, and the third index [1] represents the second column in the first column of the first 2D array.

Additionally, let’s add an element to the second 2D array:

threeDArray[1][0][0] = 6;

First, we indicate the second 2D array via its index [1]. Then, we add an element to the first column of the first row of that 2D array.

Simply put, a three-dimensional array is an array of two-dimensional arrays.

Finally, a four-dimensional array is essentially an array of three-dimensional arrays.

15. Conclusion

In this article, we explored different ways of initializing arrays in Java. Also, we learned how to declare and allocate memory to arrays of any type, including one-dimensional and multi-dimensional arrays.

Additionally, we saw different ways to populate arrays with values, including assigning values individually using indices, and initializing arrays with values at the time of declaration.