How to use Scanner to accept only valid int as input

Question

I m trying to make a small program more robust and I need some help with that   Scanner kb   new Scanner System in   int num1  int num2   0   System out print  Enter number 1      num1   kb nextInt     while num2  lt  num1        System out print  Enter number 2          num2   kb nextInt         Number 2 has to be greater than number 1 Also I want the program to automatically check and ignore if the user enters a character instead of a number  Because right now when a user enters for example r instead of a number the program just exits

User · Answer

the condition num2  lt  num1 should be num2  lt   num1 if num2 has to be greater than num1 not knowing what the kb object is  I d read a String and then trying Integer parseInt   and if you don t catch an exception then it s a number  if you do  read a new one  maybe by setting num2 to Integer MIN VALUE and using the same type of logic in your example

User · Answer

I see that Character isDigit perfectly suits the need  since the input will be just one symbol  Of course we don t have any info about this kb object but just in case it s a java util Scanner instance  I d also suggest using java io InputStreamReader for command line input  Here s an example   java io BufferedReader reader   new java io BufferedReader new java io InputStreamReader System in    try     reader read      catch Exception e      e printStackTrace      reader close

User · Answer

Try this       public static void main String   args                Pattern p   Pattern compile     d              Scanner kb   new Scanner System in           int num1          int num2   0          String temp          Matcher numberMatcher          System out print  Enter number 1              try                       num1   kb nextInt                       catch  java util InputMismatchException e                        System out println  Invalid Input                               return                    while num2 lt num1                        System out print  Enter number 2                  temp   kb next                numberMatcher   p matcher temp               if  numberMatcher matches                                  num2   Integer parseInt temp                              else                               System out println  Invalid Number                                    You could try to parse the string into an int as well  but usually people try to avoid throwing exceptions   What I have done is that I have defined a regular expression that defines a number   d means a numeric digit  The   sign means that there has to be one or more numeric digits  The extra   in front of the  d is because in java  the   is a special character  so it has to be escaped

User · Answer

This should work   import java util Scanner   public class Test       public static void main String    args  throws Throwable           Scanner kb   new Scanner System in            int num1          System out print  Enter number 1              while  true              try                   num1   Integer parseInt kb nextLine                     break                catch  NumberFormatException nfe                    System out print  Try again                             int num2          do               System out print  Enter number 2                  while  true                  try                       num2   Integer parseInt kb nextLine                         break                    catch  NumberFormatException nfe                        System out print  Try again                                  while  num2  lt  num1

User · Answer

What you could do is also to take the next token as a String  converts this string to a char array and test that each character in the array is a digit   I think that s correct  if you don t want to deal with the exceptions

User · Answer

Use Scanner hasNextInt        Returns true if the next token in this scanner s input can be interpreted as an int value in the default radix using the nextInt   method  The scanner does not advance past any input    Here s a snippet to illustrate   Scanner sc   new Scanner System in   System out print  Enter number 1      while   sc hasNextInt    sc next    int num1   sc nextInt    int num2  System out print  Enter number 2      do       while   sc hasNextInt    sc next        num2   sc nextInt      while  num2  lt  num1   System out println num1         num2     You don t have to parseInt or worry about NumberFormatException  Note that since the hasNextXXX methods don t advance past any input  you may have to call next   if you want to skip past the  garbage   as shown above   Related questions    How do I keep a scanner from throwing exceptions when the wrong type is entered   java

[java] How to use Scanner to accept only valid int as input

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