How to use Array in Java? Example Tutorial

The array is one of the most important data structures in any programming language, at the same time different programming languages implement and treat the array differently. Anyone who is done some programming knows the value of an array and the importance of knowing about array type, using them in their program. Together with a linked list, the array forms a set of basic data structures. Though Java offers excellent Collection API and some of the collection classes like ArrayList and  HashMap, they are internally based on the array.

If you are coming from C or C++ background then you will find some differences about how the array behaves there and how it does in Java, a most notable difference between the array in C and the array in Java is bounds checking. 

The C compiler doesn't check if the program is accessing a valid array index, while in Java, JVM does that and throws ArrayIndexOutOfBoundException, if the program tries to access an invalid array index.

In this Java article, we will take a look at the array in Java, both primitive and object arrays. It's a collection of important things about Java array and their properties.

Java Array 101

1) Array in Java is an object and array instance is also created using the new operator. Array.length gives the length of the array. For example for following java array:

int[] intArray = new int[10];

System.out.println(intArray.length)

Output: 10

Array.length denotes its capacity because each index is initialized with a default value, as soon as the array is created.

2) The array index in Java starts with zero. Negative indexes are invalid in Java. Java will throw ArrayIndexOutOfBoundException if you try to access an array with an invalid index which could mean a negative index, index greater than or equal to the length of the array in Java.

3) Array is a fixed-length data structure. Once declared, you can not change the length of Array in Java.

4) Array is stored at a contiguous memory location in Java Heap. So if you are creating a large array it is possible that you might have enough space in the heap but still, Java throws OutOfmemoryError because of requested size might not be available on the contiguous memory location.

5) Different type of Array in Java have different types representing them for example in below example intArray.getClass() will be different than floatArray.getclass()

int[] intArray = new int[10];

float[] floatArray = new float[10];

6) You can not store a double value in an array of int, that would result in a compilation error.

int[] intArray = new int[10];

intArray[5]=1.2; //compilation error

if you try to do this operation in runtime, Java will throw ArrayStoreException

7) In Java Array can be created in different ways. here are a few examples of creating an array in Java

int[] intArray;   //creating array without initializing or specifying size

int intArray1[];  //another int[] reference variable can hold reference of an integer array

int[] intArray2 = new int[10]; //creating array by specifying size

int[] intArray3 = new int[]{1,2,3,4}; //creating and initializing array in same line.

you have the choice to initialize the java array while creating it alternatively you can initialize the array later by using it for loop. If you have noticed brackets that are used to denote an array can be placed on either side of an array variable.

The first approach is convenient for creating multiple arrays in Java e.g.

int[] array1, array2;

here both array1 and array2 are integer arrays, while in the second approach you need to place the bracket twice like

int array1[], array2[];

though not much difference it is just a matter of style, I read int[] as int array so the first approach fits perfect there.

8) If not initialized explicitly array elements are initialized with a default value of Type used to declare Java array. For example in the case of an uninitialized integer array, their element will have default value 0, for an uninitialized boolean array it would be false and for an Object array, it would be null.

9) You can access an element of Array by using the [] operator. Since the array index starts at zero [0] returns the first element and [length-1] returns the last element from the array in Java. For loop is a convenient way to iterate over the entire array in Java. 

You can use for loop to initialize the entire array by accessing each index or you can update/retrieve array elements. Java 5 also provides an enhanced for loop which will take care of array indexes by themselves and prevent ArrayIndexOutOfBoundException in Java. 

Here is an example of iterating array in Java using for loop.

Traditional approach

int[] numbers = new int[]{10, 20, 30, 40, 50};

for (int i = 0; i < numbers.length; i++) {

  System.out.println("element at index " + i + ": " + numbers[i]);

}

Output:

element at index 0: 10

element at index 1: 20

element at index 2: 30

element at index 3: 40

element at index 4: 50

Enhanced for loop approach

for(int i: numbers){

   System.out.println(i);

}

Output:

10

20

30

40

50

As you see in case enhanced for loop we don't need to check for indexes and it's an excellent way if you want to access all elements of the array one by one but at the same time since you don't have access to the index you can modify any element of Java array.

10. In Java you can easily convert an array into ArrayList. ArrayList is an index-based Java collection class that is a backed-up the array. advantage of ArrayList is that it can resize itself. resizing of ArrayList is just creating a larger array and copying content to that array as you can not change the size of the array in Java. check how to convert Array to ArrayList in Java for more details.

11. Java API Also provides several convenient methods to operate on Java array via java.util.Arrays class. By using the Arrays class you can sort an array in Java and you can also perform a binary search in Java.

12. java.lang.System class provides a utility method for copying elements of one array into another. System.arrayCopy is a very powerful and flexible method of copying contents from one array to another. You can copy the entire array or sub-array based on your need.

Syntax of System.arraycopy:

public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)

As you see arraycopy() method allow us to specify indexes and length which gives your flexibility to copy sub-array

and store it on desired location of destination or target array. here is an example of copying first three elements of source array into target array in Java:

public static void main(String args[]) {

        int[] source = new int[]{10, 20, 30, 40, 50};

        int[] target = new int[5];

      

        System.out.println("Before copying");

        for(int i: target){

            System.out.println(i);

        }

      

        System.arraycopy(source, 0, target, 0, 3);

      

        System.out.println("after copying");

        for(int i: target){

            System.out.println(i);

        }

    }

Output:

Before copying

0

0

0

0

0

after copying

10

20

30

0

0

You can see before copying all elements of the target array were zero(default value of int variable) and after copying the first 3 elements represent the first 3 items of the source array.

13. Java also supports multi-dimensional arrays, which can be very useful to represent 2D and 3D based data like rows and columns or matrices. multi-dimensional arrays in Java are also referred to as an array of arrays because in each index of the first array another array is stored of a specified length. here is an example of creating a multi-dimensional array in Java:

int[][] multiArray = new int[2][3];

This array has two rows and 3 columns or you can visualize it as an array of 3 arrays with length 2.  Here is how you can initialize a multi-dimensional array in Java:

int[][] multiArray = {{1,2,3},{10,20,30}};

System.out.println(multiArray[0].length);

System.out.println(multiArray[1].length);

Alternatively, you can also initialize multi-dimensional array individually by using for loop or manually. like:

multiArray[0] = new int[]{40,50,60};  will replace value of multiArray[0].

14.  Array is an extremely fast data structure and one should use it if you already know a number of elements going to be stored.

That's all about what is Array in Java and how to use it.  As you see Java arrays are a very powerful way of storing elements and provide the fastest access if you know the index. Though it also has limitations e.g. once created you can not change the size of the array, but if you ever need a dynamic array, consider using java.util.ArrayList class. It provides dynamic re-size functionality and auto re-size itself. It's also very easy to convert an Array into ArrayList and operate on that.

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