Java Classes and Objects.

Java is an Object-Oriented Programming Language and writing a program in Java is totally different from any other Object-Oriented Programming Language like C++ or Python. In Java, it is necessary to create a class before writing any solution to any programming question. 

Why it is important to create a Class in Java?

The property of the class is to combine all the properties and behaviors of an object in a program into a single unit.  A class is a template or blueprint that describes the behavior/states that an object of its type supports. 

What is an Object?

Any real-world entity with well-defined properties is called an Object. An Object will have a state, behaviors, and unique identity. An object can be tangible (physical entity) or intangible (cannot be touch or feel). 

There are three main characteristics of Objects, 

• State: Represents the data of an object. 
• Behavior: Represents the behavior of an object such as deposit, withdrawal, etc. 
• Identity: It is used internally by the JVM to identify each object uniquely. 

What is a Class?

A class defines its object properties and object behavior through methods. A class is a template for a collection of objects that share a common set of attributes and behavior. In the perspective of Object-Oriented programming, a class is a description of the object. It describes the data that each object possesses and it also describes the various behaviors of the object. 

In the above example, you can see a class name Car and with its three different objects Car1, Car2, and Car3 having their own behavior and identity. 

Example of class:
public class Car {
   String carColor;
   String carBrand;
   String fuelType;
}

Example of object:
Car objCar = new Car();

Attribute Declaration. 

Attributes / Fields are used to declare the properties of a class. These attributes are called instance variables. They can be intrinsic types like int, boolean, etc, or user-defined types. 

example: private double salary;

Method Declaration. 

Methods describe the responsibility of the class. These methods are common for all objects. Hence they are known as instance methods. A method always has a name and a return type and it can accept parameters. 

example:
public void calculateArea(int height) {
     int result = height*height;
  System.out.println(result);
}  

Read Input From User in Java.

In Java, Input can be read from the keyboard in many ways. One such way is to use the predefined/inbuilt class Scanner which is present in java.util package. Predefined/inbuilt classes in Java are organized in the form of packages. To use Scanner class, include package java.util using import statement as:

//import just a Scanner class
import java.util.Scanner;

//import the entire package java.util
import java.util.*;

Next, create a Scanner object to give input from the system from the given keyboard.

• Scanner sc = new Scanner(System.in);

import java.util.Scanner;

public class Main
{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		System.out.println("Enter Roll No: ");
		int rollNo = sc.nextInt(); //integer input
		
		System.out.println("Enter Student name: ");
		String name = sc.next();  //single word input
		
		System.out.println(rollNo+" : "+name);
	}
}

Output:

Enter Roll No:
112
Enter Student name:
John
112 : John

Like in the above example, the Scanner class has methods to get input of various data types. 

Methods	Data Type
nextInt()	Integer
nextFloat()	Float
nextDouble()	Double
nextLong()	Short
nextBoolean()	Boolean
next()	Single word (String without space)
nextLine()	String without space
* For next() :even if you try to input multiple words, the will takes the first word only as input

Important note: Reading String using nextLine().

Scanner sc = new Scanner(System.in);

System.out.println("Enter the Student name: ");
String name = sc.next();

System.out.println("Enter the Address: ");
String address = sc.nextLine();

System.out.println("Name: "+name+" Address: "+address);

When you will execute the above code, the address will not be taken as input because Scanner skips the nextLine() when it is used after the next() or nextXxx() (ex: nextInt()) method. It is because the next() reads only the String, not the new line, and nextLine() consumes the next line (\n) as input, hence no input is received. 

To overcome this problem, when using nextLine() after next() or nextXxx(), consume the new line by including the statement sc.nextLine(). See the below example to understand more. 

import java.util.Scanner;

public class Main
{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		System.out.println("Enter Roll No: ");
		int rollNo = sc.nextInt();
		
		//captures the new line 
		sc.nextLine(); 
		System.out.println("Enter Student Full Name: ");
		String name = sc.nextLine();
		
		System.out.println(rollNo+" : "+name);
	}
}

Output:

Enter Roll No: 
112
Enter Student Full Name: 
John Gupta
112 : John Gupta

Binary Search Algorithm in Java.

Given a sorted integer array arr[] of size n and a target element X, the task is to write a Java program to find and return the index of the target from the given array. Return -1 if the target element is not present in the array.

Example:

Input: arr[] = {1, 2, 3, 4, 5, 6, 8} X = 4
Output: Element found at index 3

Input: arr[] = {3, 5, 10, 11, 12} x = 6
Output: Element not found in the array

Binary search and Linear search are the two most popular search algorithms and binary search is more efficient because its time complexity is less than linear search. But the important thing to note is that binary search is only applicable to a sorted array. 

Binary Search Algorithm.

The Binary Search Algorithm is based on the idea of comparing the target value with the middle element of the array. If the target value is equal to the middle value of the array then searching is complete. If they are not equal and the target value is smaller than the middle value, we will continue searching on the left side of the array. If the target value is larger than the middle value, we will continue searching on the right side of the array. 

Steps for Binary Search Algorithm:

• Initialize the left and right pointer as low = 0, high = n-1. 

• While low <= high, compare the middle element arr[mid] of the array with target value x. 

• If the middle element matches with target arr[mid] = x, return the index of the middle element mid.

• If the target is less than the middle element arr[mid] > x, continue searching the left half. (high = mid - 1)

• If the target is larger than the middle element arr[mid] < x, continue searching the right half. (low = mid + 1)

• If the target element is not found, return -1.

Java Program for Binary Search Using Iteration.

// Java code to search the target element using binary search
public class BinarySearch {

    public static int binarySearch(int[] arr, int target) {
        int left = 0, right = arr.length - 1;

        while (left <= right) {
            int mid = left + (right - left) / 2;

            if (arr[mid] == target) {
                return mid;
            } else if (arr[mid] < target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }

        return -1; // Element not found
    }

    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9};
        int target = 5;

        int result = binarySearch(arr, target);

        if (result != -1) {
            System.out.println("Element found at index " + result);
        } else {
            System.out.println("Element not found in the array");
        }
    }
}

Output:

Element found at index 4

Time Complexity: Binary search has a time complexity of O(log n), where n is the number of elements in the array. This is because, with each comparison, the size of the remaining array is halved.

Space Complexity: Binary search has a space complexity of O(1). It doesn't require additional memory that scales with the size of the input.

Java Program for Binary Search Using Recursion.

// Java code for Binary Search to target element using Recursion
public class BinarySearch {

    public static int binarySearchRecursive(int[] arr, int target, int left, int right) {
        if (left <= right) {
            int mid = left + (right - left) / 2;

            if (arr[mid] == target) {
                return mid;
            } else if (arr[mid] < target) {
                return binarySearchRecursive(arr, target, mid + 1, right);
            } else {
                return binarySearchRecursive(arr, target, left, mid - 1);
            }
        }

        return -1; // Element not found
    }

    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9};
        int target = 5;

        int result = binarySearchRecursive(arr, target, 0, arr.length - 1);

        if (result != -1) {
            System.out.println("Element found at index " + result);
        } else {
            System.out.println("Element not found in the array");
        }
    }
}

Output:

Element found at index 4

Time Complexity: O(log n) where n is the size of the given array. 

Space Complexity: O(log n) because extra stack space is required to perform Recursion.

Java Program for Binary Search Using Built-in Function.

In Java, you can use the Arrays.binarySearch() method to perform a binary search on an array. This method is part of the java.util package and is designed to work with arrays that are already sorted.

Java Code:

// Binary Search Built-in Function in Java to search 
// an element in sorted array
import java.util.Arrays;

public class BinarySearch {

    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9};
        int target = 6;

        // Sorting the array (required for binary search)
        Arrays.sort(arr);

        // Using binarySearch method
        int result = Arrays.binarySearch(arr, target);

        if (result >= 0) {
            System.out.println("Element found at index " + result);
        } else {
            System.out.println("Element not found in the array");
        }
    }
}

Output:

Element found at index 5

Time Complexity: The Arrays.binarySearch() method has a time complexity of O(log n), similar to a standard binary search algorithm.

Space Complexity: The Arrays.binarySearch() method has a space complexity of O(1). It doesn't require additional memory that scales with the size of the input.

Linear Search Algorithm in Java.

Given an integer array of size n and a target element X, the task is to write a Java program to search and return the index of the target element from the given array. Return -1 if the target element is not present in the given array.

Example:

Input: arr[] = {9, 1, 3, 5, 7}  X = 3
Output: Element found at index 2

Input: arr[] = {1, 2, 3, 4, 5, 7, 9}  X = 6
Output: Element not found in the array

This problem can be easily solved by using any search algorithm. In Java, we have two searching algorithms Linear Search and Binary Search, and in this article, we will understand the workings of Linear Search Algorithm and how to use it to find a target element in the given array.

Linear Search Algorithm.

Linear search is a simple searching algorithm that finds the position of a target value within a list. It sequentially checks each element of the list until a match is found or the entire list has been searched.

Step-by-step Algorithm:

Step 1: Start from the first element of the array.

Step 2: Compare the target value with the current element:

• If it matches, return the index.
• If it doesn't match, move to the next element.

Step 3: Repeat step 2 until a match is found or the end of the array is reached.

Step 4: If the end of the array is reached and no match is found, return -1 to indicate that the element is not present in the array.

Pseudo Code for Linear Search Algorithm.

function linearSearch(arr, target):
    for i from 0 to length of arr - 1:
        if arr[i] equals target:
            return i
    return -1

Java Program for Linear Search Algorithm.

// Java code for linear search algorithm to find and
// return the index of searching element
public class LinearSearch {

    public static int linearSearch(int[] arr, int target) {
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == target) {
                return i;
            }
        }
        return -1;
    }

    public static void main(String[] args) {
        int[] arr = {4, 2, 7, 1, 9, 5};
        int target = 7;

        int result = linearSearch(arr, target);

        if (result != -1) {
            System.out.println("Element found at index " + result);
        } else {
            System.out.println("Element not found in the array");
        }
    }
}

Output:

Element found at index 2

• Time Complexity: In the worst case, linear search has a time complexity of O(n), where n is the number of elements in the array. This is because, in the worst scenario, we may have to go through the entire array to find the element.

• Space Complexity: Linear search has a constant space complexity of O(1). It doesn't require additional memory that scales with the size of the input.

Linear search is straightforward and effective for small datasets or unordered lists. However, for large datasets or lists where the elements are sorted, more efficient algorithms like binary search are preferred.

Arrays Data Structure in Java.

An Array is a reference data type that can hold a fixed number of the same type of data. Array elements are store contiguously and the length of the array is decided when the array is created. We cannot change the array length after creation. 

Array in Java is very different than the array present in other programming languages like C/C++. Some key points to note about Array in Java. 

• In Java, arrays are dynamically allocated. What does it mean? It means that we can allocate memory to an array at run time using the heap. 
• Arrays are objects in Java and we can use several array properties on them like we can use object property length to know the size of the array. 
• We can specify the size of the array using int and short data type only. 

An array in Java can hold primitive data like int, char, long, etc, and non-primitive data types like object references. 

int mark[] = {10, 20, 30, 40 ,50}; //primitive datatype

Employee empList[] = new Employee[3]; //non-primitive datatype

empList[0] = new Employee();
empList[1] = new Employee();
empList[2] = new Employee();

The values that we store in an array are called elements and each element can be accessed by using their index. 

Declaring an Array in Java. 

The two common ways of declaring an array in Java is:

datatype[] arrayName;
        or
datatype arrayName[];

//You can declare array of several different datatype:
int[] arrayInt;
long[] arrayLong;
short[] arrayShort;
float[] arrayFloat;
double[] arrayDouble;
boolean[] arrayBoolean;
char[] arrayChar;
String[] arrayString;

Creating an Array in Java.

An array is created when we allocate memory to an array. 

arrayInt = new int[10]; //creating an array

Initializing an Array in Java.

It is a process of assigning values to each element of the array. 

arrayInt[0] = 10;   //initializing first element
arrayInt[1] = 15;   //initializing second element
arrayInt[2] = 20;   //initializing third element
arrayInt[3] = 25;   //so on..
arrayInt[4] = 30;

Accessing Array Elements in Java. 

We can access any array elements using their index value.

System.out.println(arrayInt[0]); //15
System.out.println(arrayInt[1]); //20
System.out.println(arrayInt[4]); //30

System.out.println(arrayInt[8]); //0
because our array length is 10 but the index 8 is not initialize with any value so by default it assign with 0. 

System.out.println(arrayInt[11]); //Error
We will get "ArrayIndexOutOfBoundsException" because our array length is 10 and we are trying to access index value 11 which is out of its range.

Java program to create integer array and access each element using their index value.  

public class ArrayExample
{
	public static void main(String[] args) {
		//declare an integer 
		int[] array;
		
		//allocating memory to the array 
		array = new int[8];
		
		array[0] = 10;  //initializing first element
		array[1] = 20;  //initializing second element
		array[2] = 30;
		array[3] = 40;
		System.out.println("Element at index 0: "+array[0]); //accessing first element
		System.out.println("Element at index 1: "+array[1]); //accessing second element
		System.out.println("Element at index 2: "+array[2]);
		System.out.println("Element at index 3: "+array[3]);
		System.out.println("Element at index 6: "+array[6]);
	}
}

Output:

Element at index 0: 10
Element at index 1: 20
Element at index 2: 30
Element at index 3: 40
Element at index 6: 0

You can also access array elements using a loop.

public class Main
{
	public static void main(String[] args) {
		//declare an integer 
		int[] array;
		
		//allocating memory to the array 
		array = new int[5];
		
		array[0] = 10;  //initializing first element
		array[1] = 20;  //initializing second element
		array[2] = 30;
		array[3] = 40;
		array[4] = 50;
		
        System.out.println("Length of Array: "+array.length);  
        
		for(int i = 0; i < array.length; i++) {
		    System.out.println("Element at index "+i+":"+array[i]);
		}
	}
}

Output:

Length of Array: 5
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

An array of Objects. 

An array can also store objects similarly like it stores primitive data types. 

Example:
Student studentList[] = new Student[3];

The array variable studentList[] does not hold an array of Student objects, instead, it holds an array of Student reference variables. To make these variables hold the object, we have to create the object as:

studentList[0] = new Student();
studentList[1] = new Student();
studentList[2] = new Student();

Example of Array of Object in Java.

import java.util.*;

class Student {
    public int stud_roll_no;
    public String studen_Name;
    Student(int rollno, String name) {
        this.stud_roll_no = rollno;
        this.studen_Name = name;
    }
}

public class Main
{
	public static void main(String[] args) {
	    Scanner sc = new Scanner(System.in);
	    
	    //allocating memory to object of type student
		Student studentList[] = new Student[3];
		
		//initializing elements of the array
		for(int i = 0; i < studentList.length; i++) {
		    System.out.println("Enter the rollNo: ");
		    int rollno = sc.nextInt();
		    
		    System.out.println("Enter the name: ");
		    String name = sc.next();
		    
		    studentList[i] = new Student(rollno, name);
		}
		
		//accessing elements of the array 
		for(int i = 0; i < studentList.length; i++) {
		    System.out.println(studentList[i].studen_Name+" Roll no is: "+studentList[i].stud_roll_no);
		}
	}
}

Output:

Enter the rollNo: 
101
Enter the name: 
Rahul
Enter the rollNo: 
102
Enter the name: 
Mohit
Enter the rollNo: 
103
Enter the name: 
Kajal
Rahul Roll no is: 101
Mohit Roll no is: 102
Kajal Roll no is: 103

Multidimensional Arrays in Java.

A multidimensional array is also called an Array of Arrays in which an array holds another array. Each row of the array can contain different lengths. The dimension of the array is decided by the number of square brackets ([ ]) you are adding. These are also known as Jagged Array. 

• int[] arr; // 1D array
• int[][] arr; // 2D array
• int[][][] arr; //3D array  

Example to 2D Array in Java.

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

Output:

1 2 3
4 5 6
7 8 8

Array as a Function Argument.

Java passes "objects by reference" and not by values. The array is passed as a reference and not as a copy of individual elements so any change done using array reference will be reflected in the original array. 

Example of an array as a function argument in Java(Sort an Array). 

class SortArray {
    public void sort(int[] newArr) {
        
        int n = newArr.length;
        
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < n-1; j++) {
                if(newArr[j] > newArr[j+1]) {
                    int temp = newArr[j];
                    newArr[j] = newArr[j+1];
                    newArr[j+1] = temp;
                }
            }
        }
    }
}
public class Main
{
	public static void main(String[] args) {
		
		SortArray obj = new SortArray();
		
		int arr[] = {20, 12, 50, 65, 15, 10};
		obj.sort(arr);
		
		for(int i : arr) {
		    System.out.print(i+" ");
		}
	}
}

Output:

10 12 15 20 50 65

The above example program is to sort all the array elements in ascending order. Similarly, we can perform several different operations on an array, like searching an element in an array using different searching methods or sorting the array elements in ascending or descending order. 

Units of Computer Memory Measurement

The computer only understands the language of 0 and 1 which is known as binary digits. A bit is a binary digit that can hold only one value either 0 or 1.  A bit is a very small unit to work on so we group 8 bits together which is known as bytes. Similarly, 1 Kilobyte of memory is equal to 1024 Bytes. 

Computer Memory Measurement

1 Bit	Binary Digit
8 Bits	1 byte
1024 Bytes	1 Kilobyte
1024 Kilobytes	1 Megabyte
1024 Megabytes	1 Gigabyte
1024 Gigabytes	1 Terabyte
1024 Terabytes	1 Petabyte
1024 Petabytes	1 Exabyte
1024 Exabytes	1 Zettabyte
1024 Zettabytes	1 Yottabyte
1024 Yottabytes	1 Brontobyte
1024 Brontobytes	1 Geopbyte
1024 Geopbytes	1 Saganbyte
1024 Saganbytes	1 Pijabyte
Alphabyte	1024 Pijabytes
Kryatbyte	1024 Alphabytes
Amosbyte	1024 Kryatbytes
Pectrolbyte	1024 Amosbytes
Bolgerbyte	1024 Pectrolbytes
Sambobyte	1024 Bolgerbytes
Quesabyte	1024 Sambobytes
Kinsabyte	1024 Quesabytes
Rutherbyte	1024 Kinsabytes
Dubnibyte	1024 Rutherbytes
Seaborgbyte	1024 Dubnibytes
Bohrbyte	1024 Seaborgbytes
Hassiubyte	1024 Bohrbytes
Meitnerbyte	1024 Hassiubytes
Darmstadbyte	1024 Meitnerbytes
Roentbyte	1024 Darmstadbytes
Coperbyte	1024 Roentbytes

Do While Loop in Java.

Java do-while loop is a control flow statement and is very similar to a while loop. It is also used to iterate a block of code repeatedly until the boolean condition is true. 

The only difference between the java do-while loop and the java while loop is that the do-while loop executes the block of code at least once even if the boolean condition is false but the while loop executes the block of code only when the boolean condition is true. 

Example: 1 Write a Java Program to print "Hello" four times using a do-while loop. 

public class Main
{
	public static void main(String[] args) {
		
		int i = 1;
		
		do {
		    System.out.println("Hello");
		    i++;
		} while(i < 5);
	}
}

Output:

Hello
Hello
Hello
Hello

Example: 2

public class Main
{
	public static void main(String[] args) {
		
		int i = 1;
		
		do {
		    System.out.println("Hello");
		    i++;
		} while(i > 5);
	}
}

Output:

Hello

Note: In the above second example, "Hello" is printed only one time because the boolean condition is false and when the boolean condition is false in the do-while loop, the block of code executes at least once. 

While loop in Java.

Java while loop is a control flow statement used to iterate a block of code repeatedly until the boolean condition present in the while loop is true. It is mainly used when we don't know the number of iteration in advance. The loop gets automatically stops when the boolean condition returns false. 

Example: Write a Java Program to sum all numbers from 1 to 10 using a while loop.

public class Main
{
	public static void main(String[] args) {
		
		int i = 1, sum = 0;
		
		while(i <= 10) {
		    sum = sum + i;
		    //increment value of i for next iteration
		    i++;
		}
		System.out.println("Sum: "+sum);
	}
}

Output:

Sum: 55

Example: Write a Java Program to print all-natural numbers from 1 to 10 using a while loop. 

public class Main
{
	public static void main(String[] args) {
		
		int i = 1;
		
		while(i <= 10) {
		    System.out.print(i+" ");
		    i++;
		}
	}
}

Output:

1 2 3 4 5 6 7 8 9 10