10 Examples of an Array in Java

Along with the String, the array is the most used data structure in Java. In fact, String is also backed by a character array in Java and other programming languages. It's very important for a Java programmer to have good knowledge of array and how to do common things with array e.g. initialization, searching, sorting, printing array in a meaningful way, comparing array, converting an array to String or ArrayList, and doing some advanced slicing and dicing operation with an array in Java. Like my previous tutorials 10 examples of HashMap in Java, I'll show you some practical examples of an array in Java. If you think, any important operation is not included, you can suggest their examples and I'll add them to this list.

This is a must-read article for Java beginners but some intermediate Java developers can also benefit from this as it will provide a good overview of an array in Java.

But, before we start with the examples, let's revisit some of the important properties of the array in Java:

1) Unlike C and C++ array is an object in Java.

2) The length attribute of the array gives the length of an array, this is different from the length() method of String.

3) The length of an array is fixed and cannot be changed once created. The only way to increase or decrease length is to create a new array and copy the contents of an old array to a new array.

4) You can access elements of an array using the index, which is a non-negative integer value e.g. a[1] will give you a second element.

5) Array index starts at zero and ranges till length -1.

6) Unlike C and C++ Java performs bound checks while accessing array elements. Trying to access an invalid index in the array will throw java.lang.ArrayIndexOutOfBoundsException.

7) An array is typed in Java, you cannot store Integer in an array of String in Java. It will throw ArrayStoreException at runtime.

8) You can create an array of both primitive and reference types in Java.

9) Array elements are stored in the contiguous memory location, hence creating a big array of JVM may throw java.lang.OutOfMemoryError: Java heap space if that big chunk of memory is not available.

10) The array is the backbone of many useful collection classes in Java e.g. ArrayList and HashMap both are backed by an array.

These are some of the important points about Array in Java. If you want to learn more about the array, I suggest you pick a good data structure and algorithm book or course like Data Structures and Algorithms: Deep Dive Using Java on Udemy is a good one to find out more about essential data structure in depth.

10 Examples of Array Data Structure in Java 

Now, let's see some examples of using an array in Java. This is the most comprehensive list of how to use an array in Java and covers everything from initializing an array to retrieving elements from an array. 

1) How to initialize an array in Java?

There are multiple ways to initialize an array in Java. You can only declare them or initialize them inline as shown below:

int[] primes = new int[10]; // elements will be initialize with default value
int[] even = new int[]{2, 4, 6, 8}; // inline initialization
int[] odd = {1, 3, 5};

Similarly, you can declare and initialize a two-dimensional array in Java. There are several ways to do so as I have shown in my articles about how to initialize a two-dimensional array in Java. Read that if you want to learn more about different ways to initialize a 2D array in Java.

2) How to get an element from the array?

You can retrieve an element of the array using index e.g. primes[0] will return the first element from the prime array, primes[1] will return the second element, and primes[primes.length - 1] will return the last element from the array.

Just remember that the index must be a non-negative integer value and starts with zero and ends with length - 1, trying to access an invalid index will throw ArrayIndexOutOfBoundsExcepiton in Java.

Btw, if you are a complete beginner on data structure and algorithms then you can also refer to a good course to learn essential data structures like Algorithms and Data Structures - Part 1 and 2 courses on Pluralsight is a nice course to learn algorithms and Fundamental data structure.

3) How to loop over an array in Java?

There are multiple ways to loop over an array in Java (see here), but here are two of the most common ways to iterate or loop over an array in Java:

i) classic for loop
ii) enhanced for loop

Here is the sample program:

int[] primes = {2, 3, 5, 7, 11};

// looping over array using for loop
for(int i =0; i< primes.length; i++){
System.out.printf("element at index %d is %d %n", i, primes[i]);
}

// iterating over array using enhanced for loop
for(int prime : primes){
System.out.println("current number is " + prime);
}

Output
element at index 0 is 2 
element at index 1 is 3 
element at index 2 is 5 
element at index 3 is 7 
element at index 4 is 11 
current number is 2
current number is 3
current number is 5
current number is 7
current number is 11

There are both advantages and disadvantages to each of the approaches. You must be careful with the end condition while using for loop e.g. a condition <= length is a common cause of java.lang.ArrayIndexOutOfBoundsException, and you don't have to worry about bounds when you use enhanced for loop for iterating over an array.

Similarly, when you use enhanced for loop, you don't have access to the current index so you cannot implement an algorithm that requires an index like reversing the array in place.

4) How to Sort an Array in Java?

There are two ways to sort an array in Java e.g. first use the library method Array.sort() which is also the preferred approach or write your own method to sort an array using quicksort or any sorting algorithm of your choice.

Apart from the interview, you should always use Arrays.sort() method for sorting an array in Java. This method allows you to sort an array in both ascending and descending order of elements as shown below:

 int[] primes = {2, 17, 5, 23, 11};

 System.out.println("array before sorting : " + Arrays.toString(primes));
 System.out.println("sorting array in ascending order");

 Arrays.sort(primes);
 System.out.println("array after sorting : " + Arrays.toString(primes));
        
 String[] fruits = {"apple", "banana", "orange", "grapes"};
 System.out.println("String array before sorting : " 
                           + Arrays.toString(fruits));
 System.out.println("Sorting array in descending order");
        
 // only work with object arrays, not with 
 // primitive arrays
 Arrays.sort(fruits, Collections.reverseOrder());
 System.out.println("array after sorting : " + Arrays.toString(fruits));

Output
array before sorting : [2, 17, 5, 23, 11]
sorting array in ascending order
array after sorting : [2, 5, 11, 17, 23]
String array before sorting : [apple, banana, orange, grapes]
Sorting array in descending order
array after sorting : [orange, grapes, banana, apple]

The Arrays.sort() method allows only increasing order sorting for primitives but both normal and reverse order sorting for object arrays. In order to sort the primitive array in descending order, just sort the array in increasing order and reverse the array in place as shown here.

5. How to print an array in Java?

Even though the array is an object in Java, unfortunately, it doesn't override the toString() method, which means directly printing array as System.out.println(array) will not print its content instead it will print something which is not very helpful as shown below:

String[] fruits = {"apple", "banana", "orange", "grapes"};
System.out.println(fruits);
[Ljava.lang.String;@33909752

It actually prints the type of array and then the hashcode in hexadecimal. Since most of the time we want to print elements of an array, this is not going to work, instead, we need to use Arrays.toString() method which prints the elements as shown below:

System.out.println(Arrays.toString(fruits));
[orange, grapes, banana, apple]

Btw, things are a little bit tricky with a two-dimensional array in Java. If you directly pass the 2D array to the Arrays.toString() method then it will not print the content as shown below:

int[][] cubes = { 
  {1, 1},
  {2, 4},
  {3, 9},
  {4, 16}
};
System.out.println(Arrays.toString(cubes));

Output
[[I@33909752

The right way to print a two-dimensional array is to use the deepToString() method as shown below:

System.out.println(Arrays.deepToString(cubes));

Output
[[1, 1], [2, 4], [3, 9], [4, 16]]

So, use Arrays.toString() method to print one dimensional array and Arrays.deepToString() method to print two or multi-dimensional array in Java. If you are interested to learn more about different types of arrays, I suggest checking out this Free Data Structure and Algorithm Courses which covers not only an array but other essential data structures as well.

6) How to check if two arrays are equal in Java?

What do you think? What should have been the natural way to check if two arrays are equal or not? well, it could have been by using the == operator, but unfortunately, Java doesn't support operator overloading.

It doesn't even have the equals() method, so you cannot even do an array.equals(other). So, how do we check if two arrays are equal or not? Well, you can use Arrays.equals() and Arrays.deepEquals() to check if two arrays are equal or not.

Two arrays are said to be equal if they are of the same type, have the same length, and have the same element at the corresponding index.

Here is an example of comparing two arrays in Java:

int[] primes = {3, 5, 7};
int[] odds = {3, 5, 7};

boolean isEqual = Arrays.equals(primes, odds);

if (isEqual) {
  System.out.printf("array %s and %s are equal %n",
                    Arrays.toString(primes),
                    Arrays.toString(odds));
} else {
   System.out.printf("array %s and %s are not equal %n",
                    Arrays.toString(primes),
                    Arrays.toString(odds));
}

Output
array [3, 5, 7] and [3, 5, 7] are equal 

Similarly, you can use Arrays.deepToEquals() method to compare two-dimensional arrays in Java as shown in this example here.

7) How to convert an Array to String in Java?

You can convert an array to a String like a comma-separated String by writing some utility method as shown below. This method accepts a String array and a delimiter and returns a big String where array elements are separated by a given delimiter.

You can use this method to convert a string array to comma separated String, pipe-separated string, or a colon-separated String in Java. Btw, This is also our 7th example of an array in Java.

   /**

     * Java method to convert an array to String delimited by given delimiter
     *
     * @param array
     * @param delimiter
     * @return String where array elements separated by delimiter
     */
    public static String arrayToString(String[] array, String delimiter) {
        String result = "";

        if (array.length > 0) {
            StringBuilder sb = new StringBuilder();

            for (String s : array) {
                sb.append(s).append(delimiter);
            }

            result = sb.deleteCharAt(sb.length() - 1).toString();
        }
        return result;
    }

Here is the result of some testing, where we have used this method to convert an array to CSV and colon-delimited String in Java

String[] currencies = {"USD", "INR", "AUD", "GBP"};
System.out.println("array is : " + Arrays.toString(currencies));

// array to comma separated String
String output = arrayToString(currencies, ",");
System.out.println("CSV string: " + output);
        
// array to colon separated String
output = arrayToString(currencies, ":");
System.out.println("colon string: " + output);

Output
array is : [USD, INR, AUD, GBP]
CSV string: USD,INR,AUD,GBP
colon string: USD:INR:AUD:GBP

You can see that our String contains all the elements from the array and they are also separated by comma and colon.  Btw, If you are preparing for a coding interview, then I suggest you join Data Structure and Algorithms - Interview!! course on Udemy, it's a great course to learn about essential data structure and prepare for interviews.

8. How to convert an Array to ArrayList in Java?

There are multiple ways to convert an Array to ArrayList in Java, as shown in this article, but I'll share the easiest way to do that. Yes, you guessed it right, I'll use the Arrays.asList() method to convert an Array to ArrayList in Java as shown below:

String[] hosting = {"Bluehost", "GoDaddy", "Google"};
List<String> listOfHosting = Arrays.asList(hosting);

Remember, the task is not completed yet as most of the developers will think. We have got a List, not the ArrayList, also the list we have is a read-only list where you cannot add or remove elements but you can replace elements with valid indices.

In order to convert it to our general-purpose ArrayList, we need to use the copy constructor provided by the Collection class as shown in the following example:

ArrayList<String> hostingCompanies = new ArrayList<>(Arrays.asList(hosting));

This is the right way to convert an Array to ArrayList in Java.

It is also used to declare ArrayList with values because you can replace the array with values anytime, as shown below:

ArrayList<String> hostingCompanies = new ArrayList<>(Arrays.asList("Sony",
                                                     "Apple", "Amazon"));

You can see this article to learn more ways to convert an Array to ArrayList in Java.

9. Searching in Array using Binary Search?

There are two ways to do a binary search in Java either write your own method as shown here or use the Arrays.binarySearch() method, which accepts an array and the element you want to search and return its index if the element is found or -1 otherwise.

Btw, If you are preparing for a coding interview, then I suggest you join Data Structures in Java: An Interview Refresher course on Educative, it's a great course to learn about essential data structure and prepare for interviews.

10. How to create a subarray from an array?

You can use the System.arrayCopy() method to create a subarray from an array in Java. This method is very flexible it allows you to specify the start index and how many elements are from the start index, this way you can copy any number of elements from an index from the array in Java.

Here is an example of creating a sub-array in Java:

String[] bestIndianCreditCards = {"ICICI Instant Platinum Credit Card",
                                "Standard Chartered Platinum
                                 Rewards Credit Card",
                                "Citibank Platinum Credit Card",
                                "SBI Gold Credit Card",
                                "Yatra SBI Credit Card",
                                "HDFC Platinum Plus Credit Card",
                                "HDFC Solitaire Credit Card"};
        
        
// let's create sub array in Java
String[] sbiCards = new String[2];
System.arraycopy(bestIndianCreditCards, 3, sbiCards, 0, 2);
        
System.out.println("original array: "
             + Arrays.toString(bestIndianCreditCards));
System.out.println("copy of array: " + Arrays.toString(sbiCards));

Output
original array: [ICICI Instant Platinum Credit Card, 
Standard Chartered Platinum Rewards Credit Card, Citibank Platinum Credit Card,
 SBI Gold Credit Card, Yatra SBI Credit Card, HDFC Platinum Plus Credit Card,
 HDFC Solitaire Credit Card]
copy of array: [SBI Gold Credit Card, Yatra SBI Credit Card]

You can see we have easily extracted only SBI credit cards from the big list of credit card arrays. Since we specified the source position as 3, System.arrayCopy() started copying elements from the 3rd index or fourth element and then copied it into the destination array starting from position zero. The last element is for how many elements to copy, we copied only two because there were only two SBI credit cards in the list.


That's all about 10 Examples of an array in Java. As I said, the array is the most useful data structure not just in Java but across all programming languages. Good knowledge of the array and how to do common things with the array should be at the fingertips of a Java programmer. If you think, any important array-related example is missing here then please suggest and I'll add it here.

Other Data Structure and Algorithms  You may like

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• How to reverse an array in Java? (solution)
• How to implement a binary search tree in Java? (solution)
• How to remove duplicate elements from the array in Java? (solution)
• How to implement a recursive preorder algorithm in Java? (solution)
• Recursive Post Order traversal Algorithm (solution)
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• 75+ Coding Interview Questions for Programmers (questions)
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• Postorder binary tree traversal without recursion (solution)
• 10 Free Data Structure and Algorithm Courses for Programmers (courses)

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P. S. - If you are looking for some Free Algorithms courses to improve your understanding of Data Structure and Algorithms, then you should also check these best Data Structure online courses on Udemy.