Arrays in Java - GeeksforGeeks

31 Mar, 2022
An array in Java is a group of like-typed variables referred to by a common name. Arrays in Java work differently than they do in C/C++. Following are some important points about Java arrays. 
In Java, all arrays are dynamically allocated. (discussed below)
Since arrays are objects in Java, we can find their length using the object property length. This is different from C/C++, where we find length using sizeof.
A Java array variable can also be declared like other variables with [] after the data type.
The variables in the array are ordered, and each has an index beginning from 0.
Java array can be also be used as a static field, a local variable, or a method parameter.
The size of an array must be specified by int or short value and not long.
The direct superclass of an array type is Object.
Every array type implements the interfaces Cloneable and java.io.Serializable.
An array can contain primitives (int, char, etc.) and object (or non-primitive) references of a class depending on the definition of the array. In the case of primitive data types, the actual values are stored in contiguous memory locations. In the case of class objects, the actual objects are stored in a heap segment.  
One-Dimensional Arrays: 
The general form of a one-dimensional array declaration is 
type var-name[];
OR
type[] var-name;
An array declaration has two components: the type and the name. type declares the element type of the array. The element type determines the data type of each element that comprises the array. Like an array of integers, we can also create an array of other primitive data types like char, float, double, etc., or user-defined data types (objects of a class). Thus, the element type for the array determines what type of data the array will hold. 
Example: 
// both are valid declarations
int intArray[]; 
or int[] intArray; 

byte byteArray[];
short shortsArray[];
boolean booleanArray[];
long longArray[];
float floatArray[];
double doubleArray[];
char charArray[];

// an array of references to objects of
// the class MyClass (a class created by
// user)
MyClass myClassArray[]; 

Object[]  ao,        // array of Object
Collection[] ca;  // array of Collection
                     // of unknown type
Although the first declaration establishes that intArray is an array variable, no actual array exists. It merely tells the compiler that this variable (intArray) will hold an array of the integer type. To link intArray with an actual, physical array of integers, you must allocate one using new and assign it to intArray. 
When an array is declared, only a reference of an array is created. To create or give memory to the array, you create an array like this: The general form of new as it applies to one-dimensional arrays appears as follows: 
var-name = new type [size];
Here, type specifies the type of data being allocated, size determines the number of elements in the array, and var-name is the name of the array variable that is linked to the array. To use new to allocate an array, you must specify the type and number of elements to allocate.
Example: 
int intArray[];    //declaring array
intArray = new int[20];  // allocating memory to array
OR 
int[] intArray = new int[20]; // combining both statements in one
Note : 
The elements in the array allocated by new will automatically be initialized to zero (for numeric types), false (for boolean), or null (for reference types). Refer Default array values in JavaObtaining an array is a two-step process. First, you must declare a variable of the desired array type. Second, you must allocate the memory to hold the array, using new, and assign it to the array variable. Thus, in Java, all arrays are dynamically allocated.
The elements in the array allocated by new will automatically be initialized to zero (for numeric types), false (for boolean), or null (for reference types). Refer Default array values in Java
Obtaining an array is a two-step process. First, you must declare a variable of the desired array type. Second, you must allocate the memory to hold the array, using new, and assign it to the array variable. Thus, in Java, all arrays are dynamically allocated.
In a situation where the size of the array and variables of the array are already known, array literals can be used. 
 int[] intArray = new int[]{ 1,2,3,4,5,6,7,8,9,10 }; 
 // Declaring array literal
The length of this array determines the length of the created array.
There is no need to write the new int[] part in the latest versions of Java.
Each element in the array is accessed via its index. The index begins with 0 and ends at (total array size)-1. All the elements of array can be accessed using Java for Loop.
 // accessing the elements of the specified array
for (int i = 0; i < arr.length; i++)
  System.out.println("Element at index " + i + 
                                " : "+ arr[i]);
Implementation:
Java
// Java program to illustrate creating an array// of integers,  puts some values in the array,// and prints each value to standard output. class GFG{    public static void main (String[] args)    {              // declares an Array of integers.      int[] arr;               // allocating memory for 5 integers.      arr = new int[5];               // initialize the first elements of the array      arr[0] = 10;               // initialize the second elements of the array      arr[1] = 20;               //so on...      arr[2] = 30;      arr[3] = 40;      arr[4] = 50;               // accessing the elements of the specified array      for (int i = 0; i < arr.length; i++)         System.out.println("Element at index " + i +                                      " : "+ arr[i]);             }}
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
Time Complexity: O(n)
Auxiliary Space : O(1)
You can also access java arrays using foreach loops.  
An array of objects is created like an array of primitive type data items in the following way. 
 Student[] arr = new Student[7]; //student is a user-defined class
The studentArray contains seven memory spaces each of the size of student class in which the address of seven Student objects can be stored. The Student objects have to be instantiated using the constructor of the Student class, and their references should be assigned to the array elements in the following way. 
Student[] arr = new Student[5];
Java
// Java program to illustrate creating//  an array of objects class Student{    public int roll_no;    public String name;    Student(int roll_no, String name)    {        this.roll_no = roll_no;        this.name = name;    }} // Elements of the array are objects of a class Student.public class GFG{    public static void main (String[] args)    {        // declares an Array of integers.        Student[] arr;         // allocating memory for 5 objects of type Student.        arr = new Student[5];         // initialize the first elements of the array        arr[0] = new Student(1,"aman");         // initialize the second elements of the array        arr[1] = new Student(2,"vaibhav");         // so on...        arr[2] = new Student(3,"shikar");        arr[3] = new Student(4,"dharmesh");        arr[4] = new Student(5,"mohit");         // accessing the elements of the specified array        for (int i = 0; i < arr.length; i++)            System.out.println("Element at " + i + " : " +                        arr[i].roll_no +" "+ arr[i].name);    }}
Element at 0 : 1 aman
Element at 1 : 2 vaibhav
Element at 2 : 3 shikar
Element at 3 : 4 dharmesh
Element at 4 : 5 mohit
Time Complexity: O(n)
Auxiliary Space : O(1)
JVM throws ArrayIndexOutOfBoundsException to indicate that the array has been accessed with an illegal index. The index is either negative or greater than or equal to the size of an array.
Java
public class GFG{    public static void main (String[] args)    {        int[] arr = new int[2];        arr[0] = 10;        arr[1] = 20;         for (int i = 0; i <= arr.length; i++)            System.out.println(arr[i]);    }}
Runtime error 
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2
    at GFG.main(File.java:12)
Output
10
20
Multidimensional arrays are arrays of arrays with each element of the array holding the reference of other arrays. These are also known as Jagged Arrays. A multidimensional array is created by appending one set of square brackets ([]) per dimension. Examples: 
int[][] intArray = new int[10][20]; //a 2D array or matrix
int[][][] intArray = new int[10][20][10]; //a 3D array
Java
public class multiDimensional{    public static void main(String args[])    {        // declaring and initializing 2D array        int arr[][] = { {2,7,9},{3,6,1},{7,4,2} };         // printing 2D array        for (int i=0; i< 3 ; i++)        {            for (int j=0; j < 3 ; j++)                System.out.print(arr[i][j] + " ");             System.out.println();        }    }}
2 7 9 
3 6 1 
7 4 2 
Like variables, we can also pass arrays to methods. For example, the below program passes the array to method sum to calculate the sum of the array’s values.
Java
// Java program to demonstrate// passing of array to method public class Test{       // Driver method    public static void main(String args[])    {        int arr[] = {3, 1, 2, 5, 4};                 // passing array to method m1        sum(arr);         }     public static void sum(int[] arr)    {        // getting sum of array values        int sum = 0;                 for (int i = 0; i < arr.length; i++)            sum+=arr[i];                 System.out.println("sum of array values : " + sum);    }}
sum of array values : 15
Time Complexity: O(n)
Auxiliary Space : O(1)
As usual, a method can also return an array. For example, the below program returns an array from method m1. 
Java
// Java program to demonstrate// return of array from method class Test{       // Driver method    public static void main(String args[])    {        int arr[] = m1();                 for (int i = 0; i < arr.length; i++)            System.out.print(arr[i]+" ");         }     public static int[] m1()    {        // returning  array        return new int[]{1,2,3};    }}
1 2 3 
Time Complexity: O(n)
Auxiliary Space : O(1)
Every array has an associated Class object, shared with all other arrays with the same component type. 
Java
// Java program to demonstrate// Class Objects for Arrays class Test{    public static void main(String args[])    {        int intArray[] = new int[3];        byte byteArray[] = new byte[3];        short shortsArray[] = new short[3];                 // array of Strings        String[] strArray = new String[3];                 System.out.println(intArray.getClass());        System.out.println(intArray.getClass().getSuperclass());        System.out.println(byteArray.getClass());        System.out.println(shortsArray.getClass());        System.out.println(strArray.getClass());    }}
class [I
class java.lang.Object
class [B
class [S
class [Ljava.lang.String;
Explanation: 
The string “[I” is the run-time type signature for the class object “array with component type int.”The only direct superclass of an array type is java.lang.Object.The string “[B” is the run-time type signature for the class object “array with component type byte.”The string “[S” is the run-time type signature for the class object “array with component type short.”The string “[L” is the run-time type signature for the class object “array with component type of a Class.” The Class name is then followed.
The string “[I” is the run-time type signature for the class object “array with component type int.”
The only direct superclass of an array type is java.lang.Object.
The string “[B” is the run-time type signature for the class object “array with component type byte.”
The string “[S” is the run-time type signature for the class object “array with component type short.”
The string “[L” is the run-time type signature for the class object “array with component type of a Class.” The Class name is then followed.
Now, as you know that arrays are objects of a class, and a direct superclass of arrays is a class Object. The members of an array type are all of the following: 
The public final field length, which contains the number of components of the array. Length may be positive or zero.
All the members inherited from class Object; the only method of Object that is not inherited is its clone method.
The public method clone(), which overrides the clone method in class Object and throws no checked exceptions.
When you clone a single-dimensional array, such as Object[], a “deep copy” is performed with the new array containing copies of the original array’s elements as opposed to references.
Java
// Java program to demonstrate// cloning of one-dimensional arrays class Test{       public static void main(String args[])    {        int intArray[] = {1,2,3};                 int cloneArray[] = intArray.clone();                 // will print false as deep copy is created        // for one-dimensional array        System.out.println(intArray == cloneArray);                 for (int i = 0; i < cloneArray.length; i++) {            System.out.print(cloneArray[i]+" ");        }    }}
false
1 2 3 
 
A clone of a multi-dimensional array (like Object[][]) is a “shallow copy,” however, which is to say that it creates only a single new array with each element array a reference to an original element array, but subarrays are shared. 
Java
// Java program to demonstrate// cloning of multi-dimensional arrays class Test{       public static void main(String args[])    {        int intArray[][] = {{1,2,3},{4,5}};                 int cloneArray[][] = intArray.clone();                 // will print false        System.out.println(intArray == cloneArray);                 // will print true as shallow copy is created        // i.e. sub-arrays are shared        System.out.println(intArray[0] == cloneArray[0]);        System.out.println(intArray[1] == cloneArray[1]);             }}
false
true
true
Related Articles: 
Jagged Array in Java
For-each loop in Java
Arrays class in Java
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