Input validation with Java

In this article I will be showing you how to perform validation on user input in Java. This article assumes that you have a basic understanding of Java and the Scanner class, and that you already have a compiler or IDE installed. I will be using JGrasp but you can use your favorite enviroment.

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The purpose of user validation is to make sure that the user inputs a value that is both the correct type and a valid value. For example: the user should not be allowed to center a negative number for their age. The user should also not be allowed to enter a String such as "Hello" for their age. The age must be a positive integer, or float, depending on the use case.

Code to read an int

// must have imported the scanner class
// import java.util.*;
public static int read_int(Scanner stdin, String prompt){
  int i = -1;
  while( i < 0 ){
    System.out.print(prompt);
 
    String input = stdin.nextLine();
 
    try{
      i = Integer.valueOf(input);
    } catch (NumberFormatException e) {
      i = -1;
    }
  }
  return i;
}// end read_int function

Code to read a float

// must have imported the scanner class
// import java.util.*;
public static float read_float(Scanner stdin, String prompt){
   float i = -1;
   while( i < 0 ){
      System.out.print(prompt);
 
      String input = stdin.nextLine();
 
      try{
         i = Float.valueOf(input);
      } catch (NumberFormatException e) {
         i = -1;
      }
   }
   return i;
}// end read_float function

On line 3 the function accepts a scanner object in addition to a string.

On line 4 we set a variable i which will hold our data value returned by the user. We initailize it at an invalid value so that our while loop will run.

On line 5 we continue looping until i is greater than 0. This could easily be changed if we wanted the integer to be in a different range.

On line 6 we ask our user for input by printing the prompt.

On line 8 we accept input as a string using the scanner. Note that this MUST be .nextLine() we cannot use .nextInt() or .nextFloat() or we will get an error.

On line 11 we attempt to convert the input string into an integer inside a try/catch block.

On line 13 we set the variable i to an invalid value so the while loop will continue to run. Line 13 will only be executed if the input String cannot be converted to an integer.

Using the read methods

// sets up a new scanner object
Scanner stdin = new Scanner(System.in);
 
// calls the read_int function and passes in the scanner object and a text prompt
int age = read_int(stdin, "Enter your age: ");
 
// outputs the users age to the console
System.out.println("Your age is: "+age);

Full code example

import java.util.*;
public class Demo {
    public static void main(String[] args) {
        Scanner stdin = new Scanner(System.in);
 
        float f = read_float(stdin, "Enter a float");
 
        System.out.println(f);
    }
 
    public static float read_float(Scanner stdin, String prompt) {
        float i = -1;
        while (i < 0) {
            System.out.print(prompt);
 
            String input = stdin.nextLine();
 
            try {
                i = Float.valueOf(input);
            } catch (NumberFormatException e) {
                i = -1;
            }
        }
        return i;
    }
}