Error when the user enters values not in String Array

You can convert it into a List and use contains, e.g.:

String[] alpha = {"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"}; List<String> list = Arrays.asList(alpha); System.out.println(list.contains("a")) 

If you want case insensitive comparison then you can use toLowerCase().

You could use this:-

if (! ArrayUtils.contains( alpha, "[i-dont-exist]" ) ) { try{ throw new Exception("Not Found !"); }catch(Exception e){} } 

Documentation Here

if the purpose to check the existence of the elements inside collections its more reasonable to use a set since the access time is constanct

so Instead

 Set<String> set = new HashSet<>();//if not java 8 make it HashSet<String> set.put("a") // do this for all the strings you would like to check 

Then to check if a string exists in this set

if(set.contains(str)) //str is the string you want to make sure it exists in the collections 

You could use a character array than that one . Example is given here https://www.javatpoint.com/java-string-tochararray. Then you could use indexof method to see if the user entered value is valid as explained here How can I check if a single character appears in a string?

Use this approach if you would like to stick with array instead of other data structures.

1. Create a method that return true if the user input is present in the array, otherwise return false

   public static boolean isValid(String input) { String[] alpha = {"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"}; for(String s: alpha) { if(input.equals(s)) { //use input.equalsIgnoreCase(s) for case insensitive comparison return true; //user input is valid } } return false; //user input is not valid } 

2. In the calling method, simply pass the user input to isValid

   //write some codes here to receive user input..... if(!isValid(input)) System.out.println("Oops, you have entered invalid input!");