Java How To Create An Array
An array is a group of like-typed variables that are 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 have 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 an 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.
Array can contain primitives (int, char, etc.) as well as object (or non-primitive) references of a class depending on the definition of the array. In case of primitive data types, the actual values are stored in contiguous memory locations. In case of objects of a class, the actual objects are stored in heap segment.
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Creating, Initializing, and Accessing an Array
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 above establishes the fact 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.
Instantiating an Array in Java
When an array is declared, only a reference of array is created. To actually create or give memory to 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 specifies the number of elements in the array, and var-name is the name of array variable that is linked to the array. That is, 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 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 that will hold the array, using new, and assign it to the array variable. Thus, in Java all arrays are dynamically allocated.
Array Literal
In a situation, where the size of the array and variables of 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
Accessing Java Array Elements using for Loop
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:
class GFG
{
public static void main (String[] args)
{
int [] arr;
arr = new int [ 5 ];
arr[ 0 ] = 10 ;
arr[ 1 ] = 20 ;
arr[ 2 ] = 30 ;
arr[ 3 ] = 40 ;
arr[ 4 ] = 50 ;
for ( int i = 0 ; i < arr.length; i++)
System.out.println( "Element at index " + i +
" : " + arr[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
You can also access java arrays using foreach loops
Arrays of Objects
An array of objects is created just 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 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];
class Student
{
public int roll_no;
public String name;
Student( int roll_no, String name)
{
this .roll_no = roll_no;
this .name = name;
}
}
public class GFG
{
public static void main (String[] args)
{
Student[] arr;
arr = new Student[ 5 ];
arr[ 0 ] = new Student( 1 , "aman" );
arr[ 1 ] = new Student( 2 , "vaibhav" );
arr[ 2 ] = new Student( 3 , "shikar" );
arr[ 3 ] = new Student( 4 , "dharmesh" );
arr[ 4 ] = new Student( 5 , "mohit" );
for ( int i = 0 ; i < arr.length; i++)
System.out.println( "Element at " + i + " : " +
arr[i].roll_no + " " + arr[i].name);
}
}
Output:
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
What happens if we try to access element outside the array size?
JVM throws ArrayIndexOutOfBoundsException to indicate that array has been accessed with an illegal index. The index is either negative or greater than or equal to size of array.
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
Multidimensional arrays are arrays of arrays with each element of the array holding the reference of other array. 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
class multiDimensional
{
public static void main(String args[])
{
int arr[][] = { { 2 , 7 , 9 },{ 3 , 6 , 1 },{ 7 , 4 , 2 } };
for ( int i= 0 ; i< 3 ; i++)
{
for ( int j= 0 ; j < 3 ; j++)
System.out.print(arr[i][j] + " " );
System.out.println();
}
}
}
Output:
2 7 9 3 6 1 7 4 2
Passing Arrays to Methods
Like variables, we can also pass arrays to methods.For example, below program pass array to method sum for calculating sum of array's values.
class Test
{
public static void main(String args[])
{
int arr[] = { 3 , 1 , 2 , 5 , 4 };
sum(arr);
}
public static void sum( int [] arr)
{
int sum = 0 ;
for ( int i = 0 ; i < arr.length; i++)
sum+=arr[i];
System.out.println( "sum of array values : " + sum);
}
}
Output :
sum of array values : 15
Returning Arrays from Methods
As usual, a method can also return an array. For example, below program returns an array from method m1.
class Test
{
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()
{
return new int []{ 1 , 2 , 3 };
}
}
Output:
1 2 3
Class Objects for Arrays
Every array has an associated Class object, shared with all other arrays with the same component type.
class Test
{
public static void main(String args[])
{
int intArray[] = new int [ 3 ];
byte byteArray[] = new byte [ 3 ];
short shortsArray[] = new short [ 3 ];
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());
}
}
Output:
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 any 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.
Array Members
Now as you know that arrays are object of a class and direct superclass of arrays is 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 clone method in class Object and throws no checked exceptions.
Cloning of arrays
- 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.
classTest{publicstaticvoidmain(String args[]){intintArray[] = {1,2,3};intcloneArray[] = intArray.clone();System.out.println(intArray == cloneArray);for(inti =0; i < cloneArray.length; i++) {System.out.print(cloneArray[i]+" ");}}}Output:
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.
classTest{publicstaticvoidmain(String args[]){intintArray[][] = {{1,2,3},{4,5}};intcloneArray[][] = intArray.clone();System.out.println(intArray == cloneArray);System.out.println(intArray[0] == cloneArray[0]);System.out.println(intArray[1] == cloneArray[1]);}}Output:
false true true
Related Article:
Jagged Array in Java
For-each loop in Java
Arrays class in Java
Reference: Arrays by Oracle
This article is contributed by Nitsdheerendra and Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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Java How To Create An Array
Source: https://www.geeksforgeeks.org/arrays-in-java/
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