validate digit in java, error using length

Question

I tried to validate digit and make this function.

public int digitCheck(String string)
    {
        int flag=0;

        for(int i=0;i<string.length();i++)
        {
            if(!Character.isDigit(string.charAt(i)))    
                flag++; 
        }       
        return flag;
    }

But it's always said an error on length on for

for(int i=0;i<string.length();i++)

Does anyone know why there's an error on length?

Answer 1

You forgot to check for null, and I think you want a boolean function like this.

public static boolean digitCheck(String string) {
  if (string != null) { // <--- Add This
    for (int i = 0; i < string.length(); i++) {
      if (!Character.isDigit(string.charAt(i)))
        return false;
    }
    return true; // we only got here if there were characters, and they're all digit(s).
  }
  return false; 
}

Answer 2

Checking for null and using Java's Pattern class is another solution.

Here is a code sample:

public static final Pattern DIGIT_PATTERN = java.util.regex.Pattern.compile("[0-9]*");

public boolean digitCheck(String str) {
    if ( str == null ) {
        return true; // Assume Null is valid digit string;
    }
    return DIGIT_PATTERN.matcher(str).matches();
}