Java initialize Array  


Array Java

Java initialize ArrayPin

Arrays in java are the most widely used data structure that stores multiple values of the same data type in sequential order. The array has a fixed length and the index starts from 0 to n-1 where n is the length of an array. We can use arrays class in Java to store any type of value like String, integer, character, byte, and even user-defined objects. We will learn how to initialize array in Java?

Below is the diagrammatic representation of a single-dimensional array of integers having 11 elements.

Arrays in javaPin

Table of Contents

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Java Arrays Features  

  • The array has a fixed size and cannot be changed
  • Since the array is index-based, it is easy to access random elements
  • Allocates continuous memory for array elements.
  • Can store both primitive and non-primitive data values

How to declare an Array in Java?  

Java Arrays declaration

An array can be declared in the below ways. Array declaration contains 2 parts, first is the datatype of the elements which we need to store in an array(like int, String, etc) and followed by the array name. []  brackets denote that it is an array. When we declare an array, it just tells the compiler that the variable is an array and does not actually create an array.

datatype[] arrayName; (or)

datatype []arrayName; (or)

datatype arrayName[]; –> Normally we do not prefer to use this method though it is valid.

Example of array declaration

int[] arrNumbers;
String[] arrNames;
float[] arrValues;
boolean[] arrBoolean;
char[] arrLetters;
byte[] arrBytes;
double[] arrWeight;

How to create an Array in Java?  

String Array in Java

We create an array using the new operator. In this, we specify the size of an array in [] which denotes the amount of memory required to store the array variable.

arrname = new  datatype[size];

We can also declare and create an array in a single statement as below. The first statement creates an integer array named numbers of size 5. The second creates a String array named names of size 2

int[] arrNumbers = new int[5];
String[] arrNames = new String[2];

How to initialize array in Java?  

How to instantiate an array?

Array initialization or instantiation means assigning values to an array based on array size. We can also create and initialize (instantiate) an array together (Refer to Method 1 below). In this case, the number of elements denotes the length or size of an array. In Method 2, we are assigning values separately t0 each element. Since the array index starts with 0 and array size here is 3, the 3rd element occupies 2nd position which is n-1 where n is the size of the array.

//Method 1
int[] arrNumbers = {1,2,3};

//Method 2
int[] arrNumbers = new int[3];
arrNumbers[0] = 1;
arrNumbers[1] = 2;
arrNumbers[2] = 3;

Accessing Array elements In Java  

We access array elements by using its index value. Generally, we use For loop or For each loop to access the array elements since all the elements are of the same type and have a fixed size.

See also
Java Continue

Example: Create, Initialize and Access Array elements

Here, we are creating and initializing an array of strings in a single statement and accessing each element using for loop

public class ArrayDemo1 {

  public static void main(String[] args) {
    String[] arrMonths = {"May","June","July"};
    System.out.println("Length of array is: " + arrMonths.length);
    for(int i=0;i<arrMonths.length;i++)
    {
      System.out.println(arrMonths[i]); 
    }

  }

}
Output:
Length of array is: 3
May
June
July

Example: Another method of initializing array and accessing array elements

In the below example, we first declare and create an array of integers and then assign values to individual array elements. Here, we are using for each loop to access the array elements.

public class ArrayDemo2 {

  public static void main(String[] args) {
    int[] numbers = new int[5];
    numbers[0] = 100;
    numbers[1] = 101;
    numbers[2] = 103;
    numbers[3] = 104;
    numbers[4] = 105;
    
    for(int i: numbers)
    {
      System.out.println(i);
    }

  }

}
Output:
100
101
103
104
105

types of arrays in Java  

There are 2 types of arrays in Java:

  • Single dimensional array – This contains only 1 row and 1 column. All the above examples belong to a single dimensional array
  • Multidimensional array – This contains multiple rows and multiple columns. In other words, it is an array of arrays where all rows have same number of columns. Eg: 2*2 matrix
  • Jagged array – Each row contains a different number of columns

Multidimensional Arrays In java

Multidimensional arrays can have multiple rows and columns. The index in the first [] represents rows and the second [] represents columns.

Eg: int[][] a = new int[2][3]

This means the array contains 2 rows and 3 columns. Below is the diagrammatic representation of a multidimensional array

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Example of creating multidimensional arrays of Strings

The below example shows how to create, declare, and access multidimensional array elements. Here, we directly access array elements using the row and column index.

public class ArrayMulti {

  public static void main(String[] args) {
    String[][] arrNames = {{"John","Jacob"},{"Thomas","Martin"}};
    System.out.println(arrNames[0][0] + " " + arrNames[0][1]);
    System.out.println(arrNames[1][0] + " " + arrNames[1][1]);

  }

}
Output:
John Jacob 
Thomas Martin

Example of a 2D array of integers

Here, we are creating a 2-dimensional array of integers having 2 rows and 3 columns. We assign the values to these array elements inside for loop. The 1st for loop denotes rows and 2nd for loop denotes columns.

public class ArrayMulti {

  public static void main(String[] args) {
    //Declare and create multidimensional array
    int[][] arrnum = new int[2][3];
    for(int i=0;i<2;i++) {
      for(int j=0;j<3;j++) {
        //Assign values to array elements
        arrnum[i][j] = i+1;
        System.out.print(arrnum[i][j] + " ");
      }
      System.out.println();
    }

  }

}
Output:
1 1 1 
2 2 2 

Jagged Arrays in Java

A jagged array is also a 2-dimensional array having a different number of columns. In other words, each row has a different number of columns. Initializing a jagged array is different from that of a normal 2D array.

See also
Replace Elements with Greatest Element on Right Side Leetcode Solution

Initialization of Jagged Array

During array creation, we specify the number of rows. In this example, it is 2. In the next 2 statements, for each row array, we specify the number of columns. Here, 1st row has 3 columns and 2nd row has 4 columns.

int[][] arrnum = new int[2][];
arrnum[0] = new int[3];
arrnum[1] = new int[4];

Example of a jagged array by assigning values in for loop

public class JaggedArray {

  public static void main(String[] args) {
    int[][] arrnum = new int[2][];
    arrnum[0] = new int[3];
    arrnum[1] = new int[4];

    int val=1;
    //Assign values
    for(int i=0;i<arrnum.length;i++) {
      for(int j=0;j<arrnum[i].length;j++) {
        arrnum[i][j] = val;
      }
    }
    
    //Print the values
    for(int i=0;i<arrnum.length;i++)
    {
      for(int j=0;j<arrnum[i].length;j++)
      {
        System.out.print(arrnum[i][j] + " ");
      }
      System.out.println();
    }
  }

}
Output:
1 1 1 
1 1 1 1 

Jagged array example by initializing the values during array creation

public class JaggedArray {

  public static void main(String[] args) {
    int[][] arr = {{4,5,6},{1,2},{7,9,8}};
    for(int i=0;i<3;i++)
    {
      for(int j=0;j<arr[i].length;j++) {
        System.out.print(arr[i][j] + " ");
      }
      System.out.println();
    }
  }
}
Output:
4 5 6 
1 2 
7 9 8 

Java Array Methods  

Below are the direct methods supported by Arrays in Java

MethodDescription
void clone()Clones the existing array values where references are not copied
Boolean equals(Object 0)Checks whether some other object is equal to the current object
Class getClass()Returns the classname
String toString()Returns a string representation of the object
int length()Returns length of the array

Java Array Exceptions  

Arrays in Java throws the below exception:

  • ArrayIndexOutOfBoundsException: This occurs when the index value we specify is greater than the length of an array or when it is negative. This occurs mainly while assigning value or accessing array elements.

Copy an array  

We can copy elements from one array to another using the method arraycopy of the class System.

See also
Count Pairs With Given Sum

Copy array syntax

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

src-source array object to copy from

srcPos – starting position in the source array

dest – destination array object to copy to

destPos – starting position in the destination array

length – number of array elements to copy

Example of copying an array

In the below example, we are copying 4 elements from the source array to destination array. Hence the output prints “java””

public class ArrayCopy {

  public static void main(String[] args) {
    char[] a = {'d','l','h','y','j','a','v','a','g','r','t'};
    char[] b = new char[4];
    
    System.arraycopy(a, 4, b, 0, 4);
    System.out.println(String.valueOf(b));

  }

}
Output:
java

Pass array to a method  

In Java, we can pass an array object to a method for further manipulation or other operations. The below example shows how we can pass an array object of integers to a method and perform the addition of all array elements.

public class ArrayDemo3 {

  public static void main(String[] args) {
    int[] num = {5,7,3,2,8};
    add(num);

  }
  
  public static void add(int[] num) {
    int sum = 0;
    for(int i=0;i<num.length;i++) {
      sum = sum + num[i];
    }
    System.out.println("Sum of array elements is : " + sum);
  }

}
Output:

Sum of array elements

 is : 25

Return array from a method  

We can also return an array object from method to the main method after performing the required operation.

public class ArrayDemo4 {

  public static void main(String[] args) {
    String[] arr = getArrayValues();
    for(String cities:arr) {
      System.out.println(cities);
    }
      

  }
  
  public static String[] getArrayValues() {
    String[] arrCities = {"Chennai","Bangalore","Delhi"};
    return arrCities;
  }

}
Output:
Chennai
Bangalore
Delhi

Array manipulations  

Arrays in Java belong to java.util package. There are several operations supported by java.util.Array class as mentioned below:

  • We can use copyOfRange method of Array class to copy a range of elements from one array to another
  • Search an array for specific value based on an index (Binary search)
  • Compare to arrays to check equality using the equals method
  • Use the fill method to fill an array to place a specific value at an index
  • Sorting an array using the sort method
See also
OOPs concepts in Java

Creating an array using a user-defined object  

In java, we can also create a user-defined object just like how we create an array of strings, integer, etc. This is an example of how we can create a Student array object and initialize the array object.

public class ArrayDemo5 {

  public static void main(String[] args) {
    Student[] s = new Student[2];
    s[0] = new Student(111,"Ajit");
    s[1] = new Student(112,"Biju");
  }

}
 class Student{
  public int rollno;
  public String name;
  public Student(int rno,String n) {
    rollno = rno;
    name = n;
  }
}

Conclusion  

This tutorial provides a detailed description of Arrays Class in Java, types of arrays in Java, declare, create and initialize arrays with various illustrations.

Reference

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