Validating input using java util Scanner

Question

I m taking user input from System in using a java util Scanner  I need to validate the input for things like    It must be a non-negative number It must be an alphabetical letter     etc   What s the best way to do this

User · Answer

For checking Strings for letters you can use regular expressions for example:

someString.matches("[A-F]");

For checking numbers and stopping the program crashing, I have a quite simple class you can find below where you can define the range of values you want. Here

    public int readInt(String prompt, int min, int max)
    {
    Scanner scan = new Scanner(System.in);

    int number = 0;

    //Run once and loop until the input is within the specified range.
    do 
    {
        //Print users message.
        System.out.printf("\n%s > ", prompt);

        //Prevent string input crashing the program.
        while (!scan.hasNextInt()) 
        {
            System.out.printf("Input doesn't match specifications. Try again.");
            System.out.printf("\n%s > ", prompt);
            scan.next(); 
        }

        //Set the number.
        number = scan.nextInt();

        //If the number is outside range print an error message.
        if (number < min || number > max)
            System.out.printf("Input doesn't match specifications. Try again.");

    } while (number < min || number > max);

    return number;
}

User · Answer

One idea   try       int i   Integer parseInt myString       if  i  lt  0               Error  negative input         catch  NumberFormatException e           Error  not a number      There is also  in commons-lang library the CharUtils class that provides the methods isAsciiNumeric   to check that a character is a number  and isAsciiAlpha   to check that the character is a letter

User · Answer

Here s a minimalist way to do it   System out print  Please enter an integer      while  scan hasNextInt    scan next    int demoInt   scan nextInt

User · Answer

what i have tried is that first i took the integer input and checked that whether its is negative or not if its negative then again take the input  Scanner s new Scanner System in        int a s nextInt        while a lt 0            System out println  please provide non negative integer input         a s nextInt              System out println  the non negative integer input is   a     Here  you need to take the character input first and check whether user gave character or not if not than again take the character input              char ch   s findInLine      charAt 0       while  Charcter isLetter ch             System out println  please provide a character input         ch s findInLine      charAt 0             System out println  the character  input is   ch

User · Answer

If you are parsing string data from the console or similar  the best way is to use regular expressions  Read more on that here  http   java sun com developer technicalArticles releases 1 4regex   Otherwise  to parse an int from a string  try Integer parseInt string   If the string is not a number  you will get an exception  Otherise you can then perform your checks on that value to make sure it is not negative   String input  int number  try       number   Integer parseInt input       if number  gt  0                System out println  You positive number is     number           catch  NumberFormatException ex         System out println  That is not a positive number         To get a character-only string  you would probably be better of looping over each character checking for digits  using for instance Character isLetter char    String input for int i   0  i lt input length    i         if  Character isLetter input charAt i               System out println  This string does not contain only letters           break           Good luck

User · Answer

Overview of Scanner hasNextXXX methods  java util Scanner has many hasNextXXX methods that can be used to validate input  Here s a brief overview of all of them    hasNext   - does it have any token at all  hasNextLine   - does it have another line of input  For Java primitives   hasNextInt   - does it have a token that can be parsed into an int  Also available are hasNextDouble    hasNextFloat    hasNextByte    hasNextShort    hasNextLong    and hasNextBoolean   As bonus  there s also hasNextBigInteger   and hasNextBigDecimal   The integral types also has overloads to specify radix  for e g  hexadecimal   Regular expression-based   hasNext String pattern  hasNext Pattern pattern  is the Pattern compile overload    Scanner is capable of more  enabled by the fact that it s regex-based  One important feature is useDelimiter String pattern   which lets you define what pattern separates your tokens  There are also find and skip methods that ignores delimiters   The following discussion will keep the regex as simple as possible  so the focus remains on Scanner     Example 1  Validating positive ints  Here s a simple example of using hasNextInt   to validate positive int from the input   Scanner sc   new Scanner System in   int number  do       System out println  Please enter a positive number         while   sc hasNextInt              System out println  That s not a number             sc next       this is important            number   sc nextInt      while  number  lt   0   System out println  Thank you  Got     number     Here s an example session      Please enter a positive number    five   That s not a number    -3   Please enter a positive number    5   Thank you  Got 5   Note how much easier Scanner hasNextInt   is to use compared to the more verbose try catch Integer parseInt NumberFormatException combo  By contract  a Scanner guarantees that if it hasNextInt    then nextInt   will peacefully give you that int  and will not throw any NumberFormatException InputMismatchException NoSuchElementException   Related questions   How to use Scanner to accept only valid int as input  How do I keep a scanner from throwing exceptions when the wrong type is entered   java       Example 2  Multiple hasNextXXX on the same token  Note that the snippet above contains a sc next   statement to advance the Scanner until it hasNextInt    It s important to realize that none of the hasNextXXX methods advance the Scanner past any input  You will find that if you omit this line from the snippet  then it d go into an infinite loop on an invalid input   This has two consequences    If you need to skip the  garbage  input that fails your hasNextXXX test  then you need to advance the Scanner one way or another  e g  next    nextLine    skip  etc   If one hasNextXXX test fails  you can still test if it perhaps hasNextYYY    Here s an example of performing multiple hasNextXXX tests   Scanner sc   new Scanner System in   while   sc hasNext  exit          System out println          sc hasNextInt       int      sc nextInt             sc hasNextLong       long      sc nextLong               sc hasNextDouble       double      sc nextDouble             sc hasNextBoolean       boolean      sc nextBoolean               String      sc next              Here s an example session      5    int  5   false    boolean  false   blah    String  blah   1 1    double  1 1   100000000000    long  100000000000   exit     Note that the order of the tests matters  If a Scanner hasNextInt    then it also hasNextLong    but it s not necessarily true the other way around  More often than not you d want to do the more specific test before the more general test     Example 3   Validating vowels  Scanner has many advanced features supported by regular expressions  Here s an example of using it to validate vowels   Scanner sc   new Scanner System in   System out println  Please enter a vowel  lowercase     while   sc hasNext   aeiou           System out println  That s not a vowel         sc next      String vowel   sc next    System out println  Thank you  Got     vowel     Here s an example session      Please enter a vowel  lowercase    5   That s not a vowel    z   That s not a vowel    e   Thank you  Got e   In regex  as a Java string literal  the pattern   aeiou   is what is called a  character class   it matches any of the letters a  e  i  o  u  Note that it s trivial to make the above test case-insensitive  just provide such regex pattern to the Scanner   API links   hasNext String pattern  - Returns true if the next token matches the pattern constructed from the specified string  java util regex Pattern   Related questions   Reading a single char in Java   References   Java Tutorials Essential Classes Regular Expressions regular-expressions info Character Classes     Example 4  Using two Scanner at once  Sometimes you need to scan line-by-line  with multiple tokens on a line  The easiest way to accomplish this is to use two Scanner  where the second Scanner takes the nextLine   from the first Scanner as input  Here s an example   Scanner sc   new Scanner System in   System out println  Give me a bunch of numbers in a line  or  exit      while   sc hasNext  exit          Scanner lineSc   new Scanner sc nextLine         int sum   0      while  lineSc hasNextInt              sum    lineSc nextInt              System out println  Sum is     sum       Here s an example session      Give me a bunch of numbers in a line  or  exit     3 4 5   Sum is 12   10 100 a million dollar   Sum is 110   wait what    Sum is 0   exit   In addition to Scanner String  constructor  there s also Scanner java io File  among others     Summary   Scanner provides a rich set of features  such as hasNextXXX methods for validation  Proper usage of hasNextXXX nextXXX in combination means that a Scanner will NEVER throw an InputMismatchException NoSuchElementException  Always remember that hasNextXXX does not advance the Scanner past any input  Don t be shy to create multiple Scanner if necessary  Two simple Scanner is often better than one overly complex Scanner  Finally  even if you don t have any plans to use the advanced regex features  do keep in mind which methods are regex-based and which aren t  Any Scanner method that takes a String pattern argument is regex-based    Tip  an easy way to turn any String into a literal pattern is to Pattern quote it

[java] Validating input using java.util.Scanner

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