Unit 5 Java Array

 

Array in Java

Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value. Java array is an object which contains elements of a similar data type. Additionally, The elements of an array are stored in a contiguous memory location. It is a data structure where we store similar elements. We can store only a fixed set of elements in a Java array.

declaration: An "array declaration" names the array and specifies the type of its elements.

Initialisation : Initialisation is when you provide an initial value for a variable.

Instantiation : Instantiation is where an object (class instance) is created.

Syntax to Declare an Array in Java

1.      dataType[] arr;  

2.      dataType []arr; 

3.      dataType arr[];                       data types : int, char float, string

Instantiation of an Array in Java

arr=new datatype[size];  

declaration, instantiation and initialization in same time

int a[]={33,3,4,5};  

Example

1.      class ArrExample{  

2.      public static void main(String[] args){  

3.      int a[]=new int[4];

4.      a[0]=1;

5.      a[1]=2;  

6.      a[2]=0;    

7.      a[3]=4; 

8.      for(int i=0;i<a.length;i++)

9.      System.out.println(a[i]);  

10.  }

11.  }  

Syntax of for-each loop

for (type var : array) {

   statements using var;

}

Java Program to print the array elements using for-each loop  

1.      class Arr1{  

2.      public static void main(String []args){  

3.      int arr[]={33,3,4,5};  

4.      for(int i:arr)  

5.      System.out.println(i);  

6.      }

}  

Types of Arrays in Java

Java mainly supports two types of arrays:

1. Single-Dimensional Array

2. Multi-Dimensional Array

1. Single-Dimensional Array (1D Array)

Definition

A single-dimensional array stores data in a linear form (either row-wise or column-wise).
It can be visualized as a single row of elements.

Characteristics

• Stores elements of the same data type

• Accessed using a single index

• Index starts from 0

• Size is fixed once declared

Declaration Syntax

datatype[] arrayName = new datatype[size];

Example: Array Input and Output Program

import java.util.Scanner;

public class ArrayInputExample {
    public static void main(String[] args) {
        int n;
        Scanner sc = new Scanner(System.in);

        System.out.print("How many numbers: ");
        n = sc.nextInt();

        int[] array = new int[n];

        System.out.println("Enter the elements of the array:");
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }

        System.out.println("Array elements are:");
        for (int i = 0; i < n; i++) {
            System.out.println(array[i]);
        }
    }
}

Explanation

• User enters number of elements

• Elements are stored using a for loop

• Elements are printed using another loop

2. Multi-Dimensional Array

Definition

A multi-dimensional array stores data in row and column format (matrix form).
It is also called an array of arrays.

Uses

• Matrix operations

• Table-like data storage

• Mathematical computations

Declaration Syntax

datatype[][] arrayName = new datatype[rows][columns];

Example: Matrix Addition Program

import java.util.Scanner;

class AddMatrix {
    public static void main(String[] args) {
        int row, col, i, j;
        Scanner in = new Scanner(System.in);

        System.out.println("Enter the number of rows:");
        row = in.nextInt();

        System.out.println("Enter the number of columns:");
        col = in.nextInt();

        int mat1[][] = new int[row][col];
        int mat2[][] = new int[row][col];
        int res[][] = new int[row][col];

        System.out.println("Enter the elements of matrix 1:");
        for (i = 0; i < row; i++) {
            for (j = 0; j < col; j++) {
                mat1[i][j] = in.nextInt();
            }
        }

        System.out.println("Enter the elements of matrix 2:");
        for (i = 0; i < row; i++) {
            for (j = 0; j < col; j++) {
                mat2[i][j] = in.nextInt();
            }
        }

        // Matrix addition
        for (i = 0; i < row; i++) {
            for (j = 0; j < col; j++) {
                res[i][j] = mat1[i][j] + mat2[i][j];
            }
        }

        System.out.println("Sum of matrices:");
        for (i = 0; i < row; i++) {
            for (j = 0; j < col; j++) {
                System.out.print(res[i][j] + "\t");
            }
            System.out.println();
        }
    }
}
For-Each Loop in Java Arrays
The for-each loop is used to traverse array elements one by one without using index values.
Syntax
for (dataType variable : array) {
    // loop body
}
Example: Printing Array Using For-Each Loop
class TestArray1 {
    public static void main(String[] args) {
        int arr[] = {33, 3, 4, 5};

        for (int i : arr) {
            System.out.println(i);
        }
    }
}
Advantages
Simple and readable
No risk of index out-of-bounds error
Best for printing or reading elements
Arrays Class in Java
The Arrays class belongs to the java.util package and provides static methods
for array manipulation.
·        length is not a method of Arrays class
·        It is a property of array
Array Length Property
Example
public class Main {
    public static void main(String[] args) {
        String[] fruits = {"apple", "banana", "orange", "kiwi"};
        System.out.println(fruits.length);
    }
}
Output
4
equals()
Checks whether two arrays are equal or not.
Two arrays are equal if they have the same
length and same elements in the
same order.
Example: equals()
import java.util.Arrays;
public class Main {
    public static void main(String[] args) {
        String[] stuG = {"Simran", "Pratima", "Aakirti"};
        String[] stuB = {"Raj", "Mousam", "Sishir"};
        System.out.println(Arrays.equals(stuG, stuB));
    }
}
Output
false
compare()
Compares two arrays element by element (lexicographically).

Returns:
·        0 if both arrays are equal
·        Negative value if first array is smaller
·   Positive value if first array is larger
Example: compare()
import java.util.Arrays;
public class Main {
    public static void main(String[] args) {
        String[] stuG = {"Simran", "Pratima", "Aakriti"};
        String[] stuB = {"Raj", "Mousam", "Sishir"};
        System.out.println(Arrays.compare(stuG, stuB));
    }
}
sort()
Sorts the elements of an array in ascending order.

For numbers → smallest to largest

For strings → alphabetical order
Example: sort()
import java.util.Arrays;
public class Main {
    public static void main(String[] args) {
        String[] books = {"COA", "Java", "Maths", "OS", "English"};
        Arrays.sort(books);
        for (String i : books) {
            System.out.println(i);
        }
    }
}
fill()
Fills all elements of an array with a single specified value.

Used to initialize or reset array values.
Example: fill()
import java.util.Arrays;
public class Main {
    public static void main(String[] args) {
        String[] fruits = {"Banana", "Orange", "Apple", "Mango"};
 
        Arrays.fill(fruits, "Kiwi");
 
        for (String i : fruits) {
            System.out.println(i);
        }
    }
}
copyOf()
Creates a new array by copying elements from an existing array.

The original array remains unchanged.
Example: copyOf()
import java.util.Arrays;
public class ExampleCopyOf {
    public static void main(String[] args) {
        int[] a1 = {1, 2, 3, 4, 5};
        int[] a2 = Arrays.copyOf(a1, a1.length);
 
        System.out.println("Original Array: " + Arrays.toString(a1));
        System.out.println("Copied Array: " + Arrays.toString(a2));
    }
}
deepEquals()
Compares multi-dimensional arrays completely.

Checks all rows, columns, and nested elements.
Example: deepEquals()
import java.util.Arrays;
public class ExampleDeepEquals {
    public static void main(String[] args) {
        int[][] arr1 = {{1, 2}, {3, 4}};
        int[][] arr2 = {{1, 2}, {3, 4}};
        int[][] arr3 = {{5, 6}, {7, 8}};
        System.out.println(Arrays.deepEquals(arr1, arr2));
        System.out.println(Arrays.deepEquals(arr2, arr3));
    }
}
Output
true
false