How to determine if a String has non-alphanumeric characters?

One approach is to do that using the String class itself. Let's say that your string is something like that:

String s = "some text";
boolean hasNonAlpha = s.matches("^.*[^a-zA-Z0-9 ].*$");

one other is to use an external library, such as Apache commons:

String s = "some text";
boolean hasNonAlpha = !StringUtils.isAlphanumeric(s);

Use this function to check if a string is alphanumeric:

public boolean isAlphanumeric(String str)
{
    char[] charArray = str.toCharArray();
    for(char c:charArray)
    {
        if (!Character.isLetterOrDigit(c))
            return false;
    }
    return true;
}

It saves having to import external libraries and the code can easily be modified should you later wish to perform different validation checks on strings.

You have to go through each character in the String and check Character.isDigit(char); or Character.isletter(char);

Alternatively, you can use regex.

Using Apache Commons Lang:

!StringUtils.isAlphanumeric(String)

Alternativly iterate over String's characters and check with:

!Character.isLetterOrDigit(char)

You've still one problem left: Your example string "abcdefà" is alphanumeric, since à is a letter. But I think you want it to be considered non-alphanumeric, right?!

So you may want to use regular expression instead:

String s = "abcdefà";
Pattern p = Pattern.compile("[^a-zA-Z0-9]");
boolean hasSpecialChar = p.matcher(s).find();