Java Arrays

Java array is an object which contains elements of a similar data type.

There are two types of arrays.

• Single Dimensional Array
• Multidimensional Array 

Multidimensional Array are two types.

• Two dimensional Array (2D)
• Three dimensional Array (3D)    

          

               

Java Program to illustrate how to declare & initialize

public class Testarray {
    public static void main(String[] args){
        //declaration
        int[] num = new int[5];
        //initialization
        num[0]=10;
        num[1]=20;
        num[2]=70;
        num[3]=40;
        num[4]=50;

        for(int i=0;i<num.length;i++)//length is the property of array that returns the size of the array
            System.out.println(num[i]);
    }
}

Array ( finding sum and average )

public class SumAverage {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        double[] number = new double[5];
        double sum = 0;

        System.out.print("Enter 5 numbers : ");

        for (int i=0; i<number.length; i++)
        {
            number[i] = input.nextDouble();
        }

        for (int i=0; i<number.length; i++)
        {
            sum = sum + number[i];
        }

        System.out.println("Sum is : "+sum);

        double avg = sum/ number.length;
        System.out.println("Avg is : "+avg);
    }
}

Array (Finding Maximum & Minimum)

public class MaxMin {
    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        double[] number = new double[5];

        System.out.print("Enter 5 number : ");
        for (int i = 0; i < number.length; i++) {
            number[i] = input.nextDouble();
        }

        double max = number[0];
        double min = number[0];

        for (int i = 1; i < number.length; i++) {
            if(max<number[i]) {
                max = number[i];
            }

            if (min>number[i]) {
                min = number[i];
            }
        }

        System.out.println("Maximum Number : "+max);
        System.out.println("Minimum Number : "+min);

    }
}

For-each Loop for Java Array

We can also print the Java array using for-each loop. The Java for-each loop prints the array elements one by one. It holds an array element in a variable, then executes the body of the loop.

The syntax of the for-each loop is given below:

                                for(data_type variable:array)

                                    { 

                                            //body of the loop  

                                     }

public class ForEachLoop {
    public static void main(String[] args) {
        int[] num = {10,20,30,40,50};
        int sum=0;

        for(int x:num)
        {
            System.out.println(x);
            sum+=x;
        }
        System.out.println("Sum is : "+sum);
    }
}

This program prompts the user to enter a day number (1-7) and then prints the corresponding day name. The program will display an error message if the user enters a number outside the valid range (1-7). 

•  Example 1:   Input -> Enter day number (1-7): 1          * Output -> Monday *
•  Example 2:   Input -> Enter day number (1-7): 3          * Output -> Wednesday*

public class DayOfWeek {
    public static void main(String[] args) {
        // Declare an array of weekdays
        String[] weekdays = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};

        // Create a Scanner object to read user input
        Scanner input = new Scanner(System.in);
        System.out.print("Enter day number (1-7): ");
        int dayNumber = input.nextInt();

        // Check if the day number is within the valid range
        if (dayNumber >= 1 && dayNumber <= 7) {
            System.out.println("Output-> " + weekdays[dayNumber - 1]);
        } else {
            // Print an error message for invalid day numbers
            System.out.println("Error: Please enter a valid day number between 1 and 7.");
        }
    }
}

         

Two Dimensional Array

 Example :  

public class TwoDarray {
    public static void main(String[] args) {

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

        num[0][0] = 10;
        num[0][1] = 20;
        num[0][2] = 30;
        num[1][0] = 40;
        num[1][1] = 50;
        num[1][2] = 60;

        for(int row=0; row<2; row++)
        {
            for(int col=0; col<3; col++)
            {
                System.out.print("  "+num[row][col]);
            }
            System.out.println();
        }
    }
}                     

Adding Two matrix example :

public class Matrix {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        //Declaring Array
        int[][] A = new int[2][3];
        int[][] B = new int[2][3];
        int[][] C = new int[2][3];

        //Getting input for A matrix
        System.out.println("Enter elements for A matrix : ");
        for (int row = 0; row < 2; row++) {
            for (int col = 0; col < 3; col++) {
                System.out.printf("A[%d][%d] = ",row,col);
                A[row][col] = input.nextInt();
            }
        }

        System.out.println();
        //Getting input for B matrix
        System.out.println("Enter elements for B matrix : ");
        for (int row = 0; row < 2; row++) {
            for (int col = 0; col < 3; col++) {
                System.out.printf("B[%d][%d] = ",row,col);
                B[row][col] = input.nextInt();
            }
        }

        System.out.println();
        //Printing A matrix
        System.out.print("A = ");
        for (int row = 0; row < 2; row++) {
            for (int col = 0; col < 3; col++) {
                System.out.print("\t "+A[row][col]);
            }
            System.out.println();
        }

        System.out.println();
        //Printing B matrix
        System.out.print("B = ");
        for (int row = 0; row < 2; row++) {
            for (int col = 0; col < 3; col++) {
                System.out.print("\t "+B[row][col]);
            }
            System.out.println();
        }

        System.out.println();
        //Adding A and B matrix
        System.out.print("A + B = ");
        for (int row = 0; row < 2; row++) {
            for (int col = 0; col < 3; col++) {
                C[row][col] = A[row][col] + B[row][col];
                System.out.print("\t "+C[row][col]);
            }
            System.out.println();
        }
    }
}

                                         

Diagonal Matrix

• Diagonal elements: The main diagonal elements of a square matrix are the elements where the row index equals the column index. For an $n \times n$ matrix, these elements are at positions (0,0), (1,1), (2,2), ..., (n-1, n-1).
• Upper Triangle: The upper triangle of a square matrix includes all the elements on and above the main diagonal.
• Lower Triangle: The lower triangle of a square matrix includes all the elements on and below the main diagonal.
• $n\times n$ matrix:  An $n \times n$ matrix is a square matrix that has the same number of rows and columns.
• Example of a 3×3 Matrix:

Java Program to Calculate Sum of Diagonal, Upper Triangle, and Lower Triangle Elements of a 3x3 Matrix:

public class DiagonalMatrix {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        int[][] A = new int[3][3];
        int sumofdiagonal=0,sumofupper=0,sumoflower=0;


        //Getting input for A matrix
        System.out.println("Enter elemets for 3*3 matrix : ");
        for (int row = 0; row < 3; row++) {
            for (int col = 0; col < 3; col++) {
                System.out.printf("A[%d][%d] = ",row,col);
                A[row][col] = input.nextInt();
            }
        }

        //Diagonal,Upper,Lower
        for (int row = 0; row < 3; row++) {
            for (int col = 0; col < 3; col++) {

                if(row==col)
                {
                    sumofdiagonal = sumofdiagonal + A[row][col];
                }
                if(row<col)
                {
                    sumofupper = sumofupper + A[row][col];
                }
                if(row>col)
                {
                    sumoflower = sumoflower + A[row][col];
                }
            }
        }
        System.out.println("Sum of Diagonal matrix : "+sumofdiagonal);
        System.out.println("Sum of Upper elements : "+sumofupper);
        System.out.println("Sum of Lower elements : "+sumoflower);

    }
}

Java Program to Initialize and Print a Jagged Array

This Java program demonstrates how to initialize and print a jagged array. A jagged array is an array of arrays where each "row" can have a different length. This program creates a jagged array with 4 rows of increasing lengths, initializes its elements with consecutive integers, and then prints the array.

Example Output:

                                0

                                1 2

                                3 4 5 

                                6 7 8 9  

Program:

public class OutputProgram {
    public static void main(String[] args) {

        int[][] number = new int[4][];
        int k = 0;

        // Initializing jagged array with different row lengths
        number[0] = new int[1];
        number[1] = new int[2];
        number[2] = new int[3];
        number[3] = new int[4];

        // Assigning consecutive integers to the jagged array
        for (int i = 0; i < 4; i++) {
            for (int j = 0; j < i + 1; j++) {
                number[i][j] = k;
                k++;
            }
        }

        // Printing the jagged array
        for (int i = 0; i < 4; i++) {
            for (int j = 0; j < i + 1; j++) {
                System.out.print(number[i][j] + "  ");
            }
            System.out.println();
        }
    }
}

Java Program to Sort an Array in Ascending and Descending Order

public class AscDsc {
    public static void main(String[] args) {
        int[] number = {2, 1, 45, 34, 67, 0};

        // Sorting Method
        Arrays.sort(number);

        System.out.println("Ascending: ");
        for (int i = 0; i < number.length; i++) {
            System.out.print(number[i] + " ");
        }

        System.out.println("\nDescending: ");
        for (int i = number.length - 1; i >= 0; i--) {     //dsc sorting logic
            System.out.print(number[i] + " ");
        }
    }
}

Array List

Java program demonstrating basic ArrayList operations: adding elements, adding collections, checking if empty, getting, setting, finding index, removing, clearing, and comparing equality of ArrayLists.

public class AddRemove {
    public static void main(String[] args) {
        // Declaring Array List
        ArrayList<Integer> number1 = new ArrayList<>();

        // Adding Elements individually
        number1.add(10);
        number1.add(20);
        number1.add(30);

        // Adding a collection of elements using addAll
        number1.addAll(Arrays.asList(40, 50, 60));

        // Printing Array List
        System.out.print("Array List number1 contains: ");
        for (int x : number1) { // Using for-each loop
            System.out.print(" " + x);
        }
        System.out.println();
        System.out.println("Size of Array List number1: " + number1.size()); // See array size

        // Check if an element exists using contains
        System.out.println("Array List number1 contains 20: " + number1.contains(20));

        // Get an element at a specific index using get
        System.out.println("Element at index 2: " + number1.get(2));

        // Set a new value at a specific index using set
        number1.set(2, 35);
        System.out.print("After setting value at index 2, Array List number1 contains: ");
        for (int x : number1) { // Using for-each loop
            System.out.print(" " + x);
        }
        System.out.println();

        // Get the index of a specific element using indexOf
        System.out.println("Index of element 50: " + number1.indexOf(50));

        // Check if the Array List is empty using isEmpty
        System.out.println("Is the Array List number1 empty? " + number1.isEmpty());

        // Removing Elements
        number1.remove(2);
        System.out.print("After removing, Array List number1 contains: ");
        for (int x : number1) { // Using for-each loop
            System.out.print(" " + x);
        }
        System.out.println();
        System.out.println("After removing, Size of Array List number1: " + number1.size());

        // Declaring another Array List for comparison
        ArrayList<Integer> number2 = new ArrayList<>(Arrays.asList(10, 20, 40, 50, 60));

        // Comparing two Array Lists using equals
        boolean areEqual = number1.equals(number2);
        System.out.println("Are number1 and number2 equal? " + areEqual);

        // Clear all elements in the Array List using clear
        number1.clear();
        System.out.println("After clearing, is the Array List number1 empty? " + number1.isEmpty());
    }
}

Explanation:

• Add Individual Elements: Adds elements (10, 20, 30) to number1 using the add method.
• Add All: Uses addAll to add a collection of elements (40, 50, 60) to number1.
• Printing Elements: Uses a for-each loop to iterate over and print the elements in number1.
• Contains Method: Checks if number1 contains a specific element (20) and prints the result.
• Get Method: Retrieves the element at index 2 in number1 and prints it.
• Set Method: Updates the element at index 2 to a new value (35) and prints the updated list.
• IndexOf Method: Finds the index of a specific element (50) in number1 and prints it.
• IsEmpty Method: Checks if number1 is empty and prints the result.
• Remove Element: Removes the element at index 2 in number1 and prints the updated list.
• Equals Method: Declares another ArrayList (number2) with specific elements and uses equals
•  to compare it with number1. Prints whether the two lists are equal.
• Clear Method: Clears all elements from number1 and checks if it is empty.