To Check Prime Number

package com.tutorial4u;

import java.util.Scanner;

public class PrimeNumber {

       public static void main(String[] args) {

             int temp ;

             boolean isPrimeNumber = true;

             System.out.print("Please enter a number to check prime : ");

             Scanner sc = new Scanner(System.in);

             int num = sc.nextInt();

             for(int i=2;i<num/2;i++){

                    temp = num%i;

                    if(temp==0){

                           isPrimeNumber = false;

                           break;

                    }

             }

             if(isPrimeNumber){

                    System.out.println(num +" is Prime Number");

             }else{

                    System.out.println(num +" is not Prime Number");

             }

       }

}

Output:

Please enter a number to check prime : 23

23 is Prime Number

Palindrome Number

package com.tutorial4u;

import java.util.Scanner;

public class PalindromeNumber {

       public static void main(String[] args) {

             int num, reverse=0,temp;

             System.out.print("Please enter any number to check palindrome : ");

             Scanner sc = new Scanner(System.in);

             num = sc.nextInt();

            

             temp = num;

             while(num!=0){

                    reverse = reverse*10;

                    reverse=reverse+num%10;

                    num= num/10;

             }

             if(temp == reverse){

                    System.out.println("Given number is palindrome");

             }else{

                    System.out.println("Given number is not palindrome");

             }

       }

}

Output:

Please enter any number to check palindrome : 121

Given number is palindrome

Even Or Odd Number Without (%,/) Operator

package com.tutorial4u;

import java.util.Scanner;

public class EvenOrOdd {

       public static void main(String[] args) {

             System.out.print("Enter a num to check even or odd : ");

             Scanner sc = new Scanner(System.in);

             int num = sc.nextInt();

             if((num&1) == 0){

                    System.out.println("Given number is Even ");

             }else{

                    System.out.println("Given number is Odd");

             }

       }

}

Output: 

Enter a num to check even or odd : 34

Given number is Even 

Object and Class

OOPS (Object-Oriented Programming System) is a programming paradigm that solves problems by organizing code around objects, which represent real-world entities. It uses real-world concepts to model problems and solutions.

Main OOP Concepts:

• Inheritance
• Polymorphism
• Encapsulation
• Abstraction

Class : 

A class is declared using the class keyword. It acts as a blueprint that defines the structure and behavior (data and methods) of objects. 

A class can contain:

• Fields (variables)
• Methods
• Constructors
• Blocks (static or instance)
• Nested classes and interfaces

Syntax:

class ClassName {  
    // fields  
    // methods  
}                                

Object :

An object is an instance of a class that represents a real-world entity. It has three main characteristics:

• State → represents data or properties of the object.
• Behavior → represents functionality (methods) of the object.
• Identity → represents a unique reference to the object in memory.

Ways to Create/Initialize an Object in Java :

1. Using new keyword

MyClass obj = new MyClass();   

2.Using Reflection (Class.forName(), getDeclaredConstructor().newInstance())

MyClass obj = (MyClass) Class.forName("MyClass").getDeclaredConstructor().newInstance();  

3.Using clone() method

MyClass obj2 = (MyClass) obj1.clone();   

4. Using Deserialization

(Object is recreated from a saved state).

For more details click 👉here 

A class is initialized in Java when:

1. An instance of the class is created using the new operator.
2. A static method of the class is invoked.
3. A static field of the class is accessed or assigned (unless it’s a compile-time constant).
4. Reflection is used (e.g., Class.forName("MyClass")).

Example :

class MyClass {
    static {
        System.out.println("Class is being initialized!");
    }

    MyClass() {
        System.out.println("Constructor called: Object created");
    }

    static void staticMethod() {
        System.out.println("Static method called");
    }
}

public class TestInitialization {
    public static void main(String[] args) throws Exception {
        // Class initialization by creating object
        MyClass obj1 = new MyClass();

        // Class initialization by calling static method
        MyClass.staticMethod();

        // Class initialization using reflection
        MyClass obj2 = (MyClass) Class.forName("MyClass").getDeclaredConstructor().newInstance();
    }
} 

Output:

Class is being initialized! Constructor called: Object created Static method called Constructor called: Object created

Explanation:

• The static block executes once when the class is initialized.
• Class is initialized when an object is created, a static method is called, or using reflection.
• Each new object triggers the constructor but does not re-run the static block.

StaticBlock Program

class StaticBlock
{
 /*public static void main (String[] args)
 {
  System.out.println("Main Method is executed");
 }*/
 static
 {
  System.out.println("static method is executed");
  System.exit(0);
 }