Java Programming

// Hi, i would like to ask on how can i limit my user input to a Single Character??, here is my code.

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import java.util.Scanner;
public class calc_scan {
public static void main(String [] args)
{
Scanner input = new Scanner(System.in);
double num1=0,num2=0, ans=0;
char choice2;
System.out.println("What would you like to do? ");
System.out.println("A - Addition");
System.out.println("S - Subtraction");
System.out.println("M - Multiplication");
System.out.println("D - Division");


System.out.print("Enter Choice: ");
choice2 = input.next().charAt(0);


if(choice2=='A' || choice2=='a') //Addition
{
System.out.print("Enter first number: ");
num1 = input.nextDouble();
System.out.print("Enter second number: ");
num2 = input.nextDouble();

ans = num1+num2;
System.out.print("The answer is "+ans);
}
else if(choice2=='S' || choice2=='s') //Subtraction
{
System.out.print("Enter first number: ");
num1 = input.nextDouble();
System.out.print("Enter second number: ");
num2 = input.nextDouble();

ans = num1-num2;
System.out.print("The answer is "+ans);
}
else if(choice2=='M' || choice2=='m') // Multiplication
{
System.out.print("Enter first number: ");
num1 = input.nextDouble();
System.out.print("Enter second number: ");
num2 = input.nextDouble();

ans = num1*num2;
System.out.print("The answer is "+ans);
}
else if(choice2=='D' || choice2=='d') //Division
{
System.out.print("Enter first number: ");
num1 = input.nextDouble();
System.out.print("Enter second number: ");
num2 = input.nextDouble();

if (num2==0)
{
System.out.println("Cannot be divide by Zero");
}
else
{
ans = num1/num2;
System.out.print("The answer is "+ans);
}

}
else
{
System.out.println("Invalid Option");
}

}
}
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

In this code, whenever i tried to type a String of characters like ("Annnnnnnnnn") it is still follows the operation for Addition, what i would like to do is that the user can only enter a Single Character and not a String, if the user tried to enter the required character for the if else condition plus a string of characters , the system should not accept it.


example
user tried to input a string of characters like "Annnn"

the output should be "Invalid"

what would i like is that, the if else condition will only work if the user tried to input a Single Only Character

example
input: "A"

the output should be the answer for addition

now if the user tried to input "Annn"

the output should be invalid.....