validating string input java
boolean quit = false;
do{ // change your while by this
// your own stuff here
// then after all your program stuff
boolean choiceIsOK = false;
do{
String userinput = sc.next();
char choice = userinput.toLowerCase().charAt(0);
switch(choice){
case 'y':
// case y, do nothing, you could even remove that case.
choiceIsOK = true;
break;
case 'n':
// case n, do something here
choiceIsOK = false;
quit = true;
break;
default:
// error or warning
System.out.println("Type Y or N to respectively continue or quit");
break;
}
}while(!choiceIsOK);
}while (!quit);