While Statement

1. Initialization
2. Condition
3. Task
4. Updation

.
.
int cnt = 1; //Initialization

while(cnt < 5) //Condition
{
 SOP("Hello"); //Task
 cnt++; //Updation
}

.
.

While loop becomes infinite loop if the updating has no effect on while loop condition

Print the numbers divisible by 4 till N
Approach 1

import java.util.Scanner;

public class displayWhile {
    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        int count = scn.nextInt();
        int cnt = 4;

        while(cnt < count){
            if(cnt % 4 ==0)
             System.out.println(cnt);

            cnt++;
        }
    }
}

Approach 2

import java.util.Scanner;

public class displayWhile {
    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        int count = scn.nextInt();
        int cnt = 4;

        while(cnt < count){
            System.out.println(cnt + " ");
            cnt+=4;
        }
    }
}

Input

40

Output

4 8 12 16 20 24 28 32 36 

Approach2 is faster and efficient since count in while jumps in multiples of 4

Printing perfect Square

import java.util.Scanner;

public class PerfectSquare {
    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        int n = scn.nextInt();
        int cnt = 1;

        while(cnt*cnt < n){
            System.out.println(cnt*cnt + " ");
            cnt++;
        }
    }
}

Input

30

Output

1 4 9 16 25

Printing last character in string

import java.util.Scanner;

public class PrintIntval {
    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        int number = scn.nextInt();
        Character[] arrChar = new Character[]{};

        String strCount = String.valueOf(number);
        int index = strCount.length()-1;

        while(index >= 0 ) {
            System.out.println(strCount.charAt(index));
            index--;
        }
    }
}

Input

1365

Output

5
6
3
1

Approach 2

import java.util.Scanner;

public class PrintIntval {
    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        int number = scn.nextInt();

        while(number> 0 ) {
            System.out.println(number%10);
            number = number/10;
        }
    }
}

Input

1365

Output

5
6
3
1

-----------------------------------------------------
 n           n>0           n%10         n=n/10   
----------------------------------------------------
1365          Y             5            136
136           Y             6            13
13            Y             3            1
1             Y             1            0
0             N - Loop Stops 

Now how will you address if input is 0 por negative and you should get output?

To address above.. .

public class PrintIntval {
    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        int number = scn.nextInt();

        if(number < 0)
            number *= -1;

        if(number == 0){
            System.out.println(number);
        }else {
            while (number > 0) {
                System.out.println(number % 10);
                number = number / 10;
            }
        }
    }
}

Input

-1365

Output

5
6
3
1