Array in Java

Sometimes in our programs, we need to deal with the collection of data that is connected with some logic together. It would be great to put them all in one box and carry them all together. The array is the data type for that exact purpose, It can hold a collection of data.

The array is a collection of data of the same type.

• The array is a fixed-size collection of data. We need to specify the size of the array while creating it, and the size cannot be changed.
• We have random access to each element in the array by using its index. Indexes start with 0.
• Array works with references as Objects do.

Let’s see the syntax:

String[] colors = new String[5];
colors[0] = "red";
colors[1] = "blue";
colors[2] = "white";
colors[3] = "yellow";
colors[4] = "green";

In the above example, we created an array of type String with size 5.

[] — square brackets after type means it’s an array.

new String[5] — here we are specifying the size for our new array.

colors[0] = "red";

by using the reference of our array and the index of the element we can assign the value. Also, to read the value we use similar syntax

System.out.println(colors[0]); // red

Let’s see one more example

int[] numbers = new int[3];
numbers[0] = 11;
numbers[1] = 25;
numbers[2] = 39;

In this example, we have created an array of type int with size 3. The data type is an int but the syntax looks similar.

We can create the array and assign values in the same statement as well

String[] names = {"John", "Bek", "Alex", "Donald"};
System.out.println(names[0]); // John
System.out.println(names[3]); // Donald

We can create an array of almost any data type including all primitives and objects as well. Even we can create an array of array, basically, when the element of the array is another array. We call them multidimensional arrays.

int[][] numbers = new int[5][5];
numbers[0][0] = 1;
numbers[0][1] = 2;
numbers[0][2] = 3;
// ..

Another detail we need to know about arrays. The array values initially have default values

int[] arrNum = new int[3];
System.out.println(arrNum[0]); // 0
System.out.println(arrNum[1]); // 0
System.out.println(arrNum[2]); // 0
String[] colors = new String[3];
System.out.println(colors[0]); // null
System.out.println(colors[1]); // null
System.out.println(colors[2]); // null

The default value for objects is null, for the numeric data type is 0, and for boolean is false.

Ok, now we are familiar with the syntax, let’s talk more about how the array works internally and about the fixed size.

We created an array of type int with size 3 in the below example.
int[] arrNum = new int[3];
If we will imagine memory as the canvas it will look like this:

Java will allocate space for 3 elements and the name of the array(reference) will point there. You see, all elements of the array are stored one by one after each other. So that’s why we have random access with arrays. Each element has an index that starts from 0.

Let’s see what will happen if try to access 4th element in our array which does not exist because we created an array of size 3.

int[] arrNum = new int[3];
arrNum[3] = 9; // ArrayIndexOutOfBoundsException

We will get runtimeArrayIndexOutOfBoundsException exception. The compiler cannot catch this issue because it does know the size of the array and only during the runtime we will get this exception because the element with index 3 does not exist for this array.

Ok, we said array works with references, similarly to objects. Let’s take a look at an example

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] myArr = {1, 55, 3, 8, 21, 25, 73, 90};
         
        // [1, 55, 3, 8, 21, 25, 73, 90]      
        System.out.println(Arrays.toString(myArr)); 

        makeAllSeven(myArr); 

        // [7, 7, 7, 7, 7, 7, 7, 7]
        System.out.println(Arrays.toString(myArr)); 
    }

    public static void makeAllSeven(int[] arrNum) {
        for (int i = 0; i < arrNum.length; i++) {
            arrNum[i] = 7;
        }
    }
}

• we have our void method makeAllSeven that accepts int[] arrNum as argument and will assign value to each element as 7.
• we can see that after using this method the original value of the array got changed. The reason it got changed is that we are passing the reference that points to the actual array in the memory. The argument of the method will simply take a copy of the reference. It means it will also point to the original array. That’s the reason in the main method when using our myArr references, we see the all sevens after the method run.

We can check the size of our array by using length instance variable. It’s not a method so we don’t put () after it.

String[] colors = new String[6];
System.out.println(colors.length); // 6

let’s see another example

public void printArray(String[] arr) {
    // we can use .length to loop over our array
    for (int i = 0; i < arr.length; i++) {
        System.out.println(arr[i]);
    }
}

we can use for each loop to loop over our array as well

public void printArray(String[] arr) {
    for (String str : arr) {
        System.out.println(arr);
    }
}

if we would like to print array values as strings, to sort or just compare two arrays with each other, we can use java.util.Arrays class. It’s a helper class and it has many usefully static methods to work with arrays.

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] myArr = {1, 55, 3, 8, 21, 25, 73, 90};

        // print using Arrays
        // [1, 55, 3, 8, 21, 25, 73, 90]
        System.out.println(Arrays.toString(myArr));

        // sort  using Arrays
        Arrays.sort(myArr);

        // [1, 3, 8, 21, 25, 55, 73, 90]
        System.out.println(Arrays.toString(myArr));
        
        int[] myArr2 = {0, 4, 5, 3};
        // compare two arrays using Arrays
        System.out.println(Arrays.equals(myArr, myArr2));
    }
}

Let’s discuss the advantages and disadvantages of the array. The first advantage is random access. If we know the correct index we can read or update elements' values in one step using the index.

arrNum[0] = 7; // assign to first element value 7

In another hand, the biggest disadvantage is fixed size. For example, I need an array in my program and I’m not quite sure how many elements it should have. I will create an array with the approximate size and let’s say in the middle of my program I need to put more elements than the array size. I will need to resize my array, resizing is expensive because I need to create a new array with a bigger size and then copy all elements from an existing array. Creating a new array with a different size is the only way to resize an array.

That’s all for today. Happy coding!

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This article is part of the series of articles to learn Java programming language from Tech Lead Academy:
Introduction to programming 
OS, File, and File System
Working with terminal 
Welcome to Java Programming Language
Variables and Primitives in Java
Convert String to numeric data type
Input from the terminal in Java
Methods with Java
Java Math Operators and special operators
Conditional branching in Java
Switch statement in Java
Ternary operator in Java
Enum in Java
String class and its methods in Java
Loops in Java
Access modifiers in Java
Static keyword in Java
The final keyword in Java
Class and Object in Java
Object-Oriented Programming in Java
OOP: Encapsulation in Java
OOP: Inheritance in Java
OOP: Abstraction in Java
OOP: Polymorphism in Java
The method Overriding vs Overloading in Java
Array in Java
Data Structures with Java
Collection framework in Java
ArrayList in Java
Set in Java
Map in Java
Date and Time in Java
Exception in Java
How to work with files in Java
Design Patterns
Generics in Java
Multithreading in java
Annotations in Java
Reflection in Java
Reflection & Annotations - The Powerful Combination
Run terminal commands from Java
Lambda in Java
Unit Testing in Java
Big O Notation for coding interviews
Top Java coding interview questions for SDET