Develop Reference

How to use java scanner with string validation?

I have the following code:
String f_name = "";
System.out.println(ANSI_PURPLE + "What is your first name?");
System.out.print(ANSI_RESET + " Type your name here (use only latin characters) > ");
while(!sc.hasNext("[A-Za-z]*")) {
System.out.println(ANSI_RED + " ERROR: Invalid option! ");
System.out.print(ANSI_RESET + " Type your name here (use only latin characters) > ");
f_name = sc.next().toUpperCase();
}
System.out.println("First Name = " + f_name);
The issue with the above code is that it would store what has previously added.
for example:
What is your first name?
Type your name here (use only latin characters) > 123
ERROR: Invalid option!
Type your name here (use only latin characters) > c
First Name = 123
How to fix so that the validation of latin characters will still work, redirect the user to same question if there is a mistake and store the correct value?
CORRECT ANSWER TO MY QUESTION:
...
while(!sc.hasNext("[A-Za-z]*")) {
System.out.println(ANSI_RED + " ERROR: Invalid option! ");
System.out.print(ANSI_RESET + " Type your name here (use only latin characters) > ");
sc.next();
}
f_name = sc.next().toUpperCase();
System.out.println("First Name = " + f_name);

When sc.hasNext("[A-Za-z]*") returns true, that means the next input you read will be the one you want. So you need to read f_name in after the loop ends.
You still need sc.next() inside the loop to move past bad input; otherwise you will have an infinite loop.
By the way, perhaps you want to use + instead of * in your regular expression. * means "zero or more", and + means "one or more". I assume you want one or more characters to be input.
while (!sc.hasNext("[A-Za-z]+")) {
System.out.println(ANSI_RED + " ERROR: Invalid option!");
System.out.print(ANSI_RESET + " Type your name here (use only latin characters) > ");
sc.next();
}
String f_name = sc.next().toUpperCase();

How can I make sure that the user did not enter his/her entire name in the First Text Field named as "First Name"

This question says ask for the 'First Name' and the 'Last Name' from the user and then show the message Welcome with the full name of the user . also make sure that the user does not enter his/her full name in the first Text Field which asks for First Name only
I thought that if the user enters his/her full name in the first text field , we can know that from the fact that he/she entered a space or (' ') or not . If not we can simply show the message Welcome + full name . However it didn't work the way I thought it would ... Can somebody help me with itenter image description here

If I understand you the below will work accomplish what you need by ignoring the data after the space and asking the user for their last name.
code:
public static void main(String[] args) {
// Properties
Scanner keyboard = new Scanner(System.in);
String firstName, lastName
// Ask the user for their first name
System.out.println("What is your first name? ");
System.out.print("--> "); // this is for style and not needed
firstName = keyboard.next();
// Ask the user for their last name
System.out.println("What is your last name? ");
System.out.print("--> "); // this is for style and not needed
lastName = keyboard.next();
// Display the data
System.out.println("Your first name is : " + firstName);
System.out.println("Your last name is : " + lastName);
}

There is actually a few ways you can do this, but if I understand your question correctly a simple way would be below, which is from http://math.hws.edu/javanotes/c2/ex6-ans.html and helped me understand Java more when I was learning it, you just would alter it to your needs.
code:
public class FirstNameLastName {
public static void main(String[] args) {
String input; // The input line entered by the user.
int space; // The location of the space in the input.
String firstName; // The first name, extracted from the input.
String lastName; // The last name, extracted from the input.
System.out.println();
System.out.println("Please enter your first name and last name, separated by a space.");
System.out.print("? ");
input = TextIO.getln();
space = input.indexOf(' ');
firstName = input.substring(0, space);
lastName = input.substring(space+1);
System.out.println("Your first name is " + firstName + ", which has "
+ firstName.length() + " characters.");
System.out.println("Your last name is " + lastName + ", which has "
+ lastName.length() + " characters.");
System.out.println("Your initials are " + firstName.charAt(0) + lastName.charAt(0));
}
}
edit:
If this doesn't make sense I can give a better explanation with a better example with more detail.
More notes on similar problems.
https://www.homeandlearn.co.uk/java/substring.html

The problem with your code is, that you check every single charackter and then do the if/else for every single charackter. which means if the last charackter is not a whitespace it will at the end process the else tree.
The solution is to just check once:
if(fn.contains(' '){
//Do what you want to do, if both names were entered in the first field
}else{
//Everything is fine
}

Prevent user from entering whitespace in Java

I want the user to only enter his age. So I did this program :
Scanner keyb = new Scanner(System.in);
int age;
while(!keyb.hasNextInt())
{
keyb.next();
System.out.println("How old are you ?");
}
age = keyb.nextInt();
System.out.println("you are" + age + "years old");
I found how to prevent user from using string by using the while loop with keyb.hasNextInt(), but how to prevent him from using the whitespace or from entering more input than his age ?
For example I want to prevent this kind of typing "12 m" or "12 12"
Also, how can I clear all existing data in the buffer ? I'm facing an infinite loop when I try to use this :
while(keyb.hasNext())
keyb.next();

You want to get the whole line. Use nextLine and check that for digits e.g.
String possibleAge = "";
do {
System.out.println("How old are you ?");
possibleAge = keyb.nextLine();
} while (!possibleAge.matches("\\d+"))

Your problem is that the default behaviour of Scanner is to use any whitespace as the delimiter. This includes spaces. This means that a 3 a is in fact three tokens, not one. You can change the delimiter to a new line so that a 3 a becomes a single token, which will then return false for hasNextInt.
I've also added an initial question, because in your example the first input was taken before asking any questions.
Scanner keyb = new Scanner(System.in);
keyb.useDelimiter("\n"); // You can try System.lineSeparator() but it didn't work in IDEA
int age;
System.out.println("How old are you?");
while(!keyb.hasNextInt())
{
keyb.next();
System.out.println("No really. How old are you?");
}
age = keyb.nextInt();
System.out.println("You are " + age + " years old");

String age = "11";
if (age.matches(".*[^0-9].*")) {
System.out.println("Invalid age");
} else {
System.out.println("valid age");
}
If age contains other then digits then it will print invalid age.

How do I define the end of a string in Java?

I have a string that a user inputs their name in [Last, First Middle] format and I need to change it to [First Middle Last] format.
I've defined the last name as LFM.substring(0, commaSpace) . commaSpace being the name for the ", " in the input of the LFM (Last, First Middle) user input.
Then I needed to define firstMiddle . My question to you is, how could I define the end of the string LFM so I can have firstMiddle be LFM.substring(commaSpace, (end of string) ); ? That way I can just print firstMiddle + last .
ALL OF MY CURRENT CODE:
(IT'S REALLY MESSY, SORRY)
System.out.println();
System.out.println("This program will separate and convert a name in [Last, First, Middle] format to [First Middle Last].");
System.out.println();
System.out.print("Please enter a name in [Last, First Middle] format. ");
Scanner userInput = new Scanner(System.in);
String lineSeparator = System.getProperty("line.separator");
String LFM, first, middle, last, firstMiddle;
int commaSpace, end, lastLength;
userInput.useDelimiter(lineSeparator);
LFM = userInput.nextLine();
commaSpace = LFM.indexOf(",");
last = LFM.substring(0, commaSpace);
lastLength = last.length();
firstMiddle = LFM.substring(commaSpace, //?);
first = LFM.substring(commaSpace + firstMiddle.length());
System.out.println(firstMiddle + (" ") + last);

Use replaceAll or replaceFirst functions since it accepts regex as first argument.
string.replaceAll("^(\\w+),\\s*(\\w+)\\s+(\\w+)$", "$2 $3 $1");
DEMO

Trouble Adding String Length

I've been learning Java and for some reason, I'm having a brain fart on how to add two strings together. In the program, I successfully have the program state what the length of the First and Last names are independently. However I would like to have the program also state how many characters there are in the name.
I know that I need to assign the string lengths to an integer variable that can be added together, but I'm just blanking at the moment.
My source code is as follows, thank you for your help!
import java.util.Scanner;
/*
* //Program Name: stringwk7
* //Author's Name: MrShiftyEyes
* //Date: 05-12-2013
* //Description: Prompt for a user name; print initials;
* // print out the reverse name, find the length of the names
*/
/**
*
* #author MrShiftyEyes
*/
public class stringwk7{
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
// Create first name and last name StringBuffers
Scanner input = new Scanner(System.in);
System.out.print("What is your first name? ");
StringBuffer firstName = new StringBuffer(input.nextLine());
System.out.print("What is your last name? ");
StringBuffer lastName = new StringBuffer(input.nextLine());
// Declare two string variables using StringBuffer variables
String fName = new String(firstName);
String lName = new String(lastName);
// Display the users initials
System.out.println("Your initials are: " + firstName.charAt(0) + " " + lastName.charAt(0));
// Displays the user's name in reverse
System.out.println("Your name in reverse is: " + lastName.reverse() + " " + firstName.reverse());
// Length of name
System.out.println("The length of my first name is " + firstName.length());
System.out.println("The length of my last name is " + lastName.length());
// Insert a goodbye into the firstName string and display to user
firstName = firstName.reverse(); // Change firstName back to the initial value
System.out.println(firstName.insert(0, "See ya later, "));
}
}

you just mean the first and last name together?
int fullNameLength = lastName.length() + firstName.length();
System.out.println("The length of my full name is " + fullNameLength);

(firstName + lastName).length()