How to encrypt and decrypt the string using 2d array?

I actually manage to encrypt and decrypt the plain string using 2d array.

Below is my encrypt code:

import java.util.Scanner;
public class Secret {
public static void  main(String[] args) {
Scanner x = new Scanner(System.in);
System.out.println("Enter a sentence that need to be encrypt : ");
String y = x.nextLine();
int len = y.length();
System.out.println("The length of the sentences is : " + len);
System.out.print("Enter number of column : " );
int arr = x.nextInt();	
char [] [] z = new char [6] [arr];
// fill the character into array //
int pos = 0;
for (int i = 0; i<z.length; i++) {
for (int j = 0; j<z[i].length; j++) {
z[i][j] = y.charAt(pos);
pos++;
}
}
// output the encrypted text by reading the array downward, column by
column.//
for (int j = 0; j<z[arr].length; j++) {
System.out.print(z[0][j] + "" + z[1][j] + "" + z[2][j] + "" + z[3][j] + "" + z[4][j] 
+ "" + z[5][j]);}	
}
}

Input:

Enter a sentence that need to be encrypt : 
Hello World.
The length of the sentences is : 12
Enter number of column : 2

Output:

HloWrdel ol. 

But my desired output is something like this HloWrdel*ol... which means that the space is replace by "*" and if have extra empty array print "." instead.

Right now, my array size is depend on string length so when I input arr = 3 ( 6 X 3 = 18 ) it show the error like this

Enter a sentence that need to be encrypt : 
Hello World.
The length of the sentences is : 12
Enter number of column : 3
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 12
at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:48)
at java.base/java.lang.String.charAt(String.java:711)
at Secret.main(Secret.java:33)

Therefore, my question is how to come out the desired output like this HloWrdel*ol...? The space becomes "*" and there is no limit for entering the array size. When I enter the array size (column) that is more than the string length, the character of the string automatically fill up the array and the extra empty array will print "." instead.

For decryption code:

    import java.util.Scanner;
public class Secret1 {
public static void main(String[] args) {
Scanner x = new Scanner(System.in);
System.out.println("Enter the sentences that need to decrypt : ");
String str = x.nextLine();
int len = str.length();
System.out.println("The number of length is " + len);
System.out.println("Enter the number of rows : " );
int arr = x.nextInt();
char [][] z = new char [arr][6];
// fill character into array //
int pos = 0;
for(int i = 0; i<z.length; i++) {
for(int j = 0; j<z[i].length; j++) {
z[i][j] = str.charAt(pos);
pos++ ;
}
}
//output the decrypted text by reading the array from left to right, row by
row.//
for (int i = 0; i<z.length; i++) {
System.out.print(z[i][0]);
}
for (int i = 0; i<z.length; i++) {
System.out.print(z[i][1]);
}
for (int i = 0; i<z.length; i++) {
System.out.print(z[i][2]);
}
for (int i = 0; i<z.length; i++) {
System.out.print(z[i][3]);
}
for (int i = 0; i<z.length; i++) {
System.out.print(z[i][4]);
}
for (int i = 0; i<z.length; i++) {
System.out.print(z[i][5]);
}
}
}

Input:

Enter the sentences that need to decrypt : 
HloWrdel ol.
The number of length is 12
Enter the number of rows : 
2

Output:

Hello World.

But my desired output is from encrypted text HloWrdel*ol... into plain text Hello World.. How can I get such output?

Besides, the decryption code is also depend on string length so when I enter the array size more than string length, it show like this:

Enter the sentences that need to decrypt : 
HloWrdel ol.
The number of length is 12
Enter the number of rows : 
3
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 12
at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:48)
at java.base/java.lang.String.charAt(String.java:711)
at Secret1.main(Secret1.java:147)

Is it possible the array size that I have enter is not limited to the string length but print nothing for the extra empty array after filling into the array?

Inside your innermost loop (the for (int j = 0; j<z[i].length; j++)), check if pos >= len, and if so, skip attempting to read and fill the 2D element with '.'

if (pos>=len)
{
z[i][j]='.';
continue;
}
//your normal code

And then after you fill the element, check if you've just filled it with space

if (z[i][j]==' ')
z[i][j]='*';