Custom exception for string that is too long

Question

This is my first time working with Java exceptions. I have made a program that works to specifications, but I am wondering if there is anything I've done which is unconventional or not recommended. The application program does specify that there be line breaks between each message.

My driver is:

import java.util.Scanner;
public class ReadStrings {

    public static void main(String[] args) throws StringTooLongException{
        String str1;
        final int MAX_STRING_LENGTH = 20;
        Scanner input = new Scanner(System.in);
        System.out.println("Enter strings, enter DONE when finished:\n");
        str1 = input.nextLine();
        if(str1.equalsIgnoreCase("done"))
            System.exit(0); //exit on first prompt
        do{
            try
            {
                if(str1.length()>MAX_STRING_LENGTH)
                    throw new StringTooLongException();
                System.out.println("\nEnter a string, enter DONE when finished:\n");
                str1 = input.nextLine();            
            }
                catch(StringTooLongException e)
            {
                    System.out.println(e.getMessage());
                    str1 = input.nextLine();
            }
        }while(!str1.equalsIgnoreCase("done"));
        input.close();
    }
}

And my custom exception class is:

public class StringTooLongException extends Exception {
    public StringTooLongException()
    {
        super("\nString has too many characters\n\nPlease try again:\n");       
    }
}

Thanks for taking a look. Any advice is appreciated.

Answer 1

if(str1.equalsIgnoreCase("done"))
        System.exit(0); //exit on first startup

Do not use System.exit() (somewhere else then in a catch block in main) It terminates the JVM immediately. This is usually not intended.
just write a return instead.

         if(str1.length()>MAX_STRING_LENGTH)
                throw new StringTooLongException();
            System.out.println("\nEnter a string, enter DONE when finished:\n");
            str1 = input.nextLine();         

This will result in an endless loop when the user enters a long string. The check is done (and the exception thrown) before she gets a chance to enter a new one.

You should move the if behind the input request and delete the initial input request (along wit the if there) before the loop.

---

Since you do nothing else in the loop the checking the string and requesting new one some may argue that you use the Exception as flow control for which exceptions are not supposed to be used.
I'd agree to that argument in this particular case as long as the "happy path" also repeats the loop...

Answer 2

        String str1;

Delete that and change

        str1 = input.nextLine();

to

        String str1 = input.nextLine();

instead.

        if(str1.equalsIgnoreCase("done"))
            System.exit(0); //exit on first prompt
        do{
            try
            {
                if(str1.length()>MAX_STRING_LENGTH)
                    throw new StringTooLongException();
                System.out.println("\nEnter a string, enter DONE when finished:\n");
                str1 = input.nextLine();            
            }
                catch(StringTooLongException e)
            {
                    System.out.println(e.getMessage());
                    str1 = input.nextLine();
            }
        }while(!str1.equalsIgnoreCase("done"));

This seems the long way around. Why not

        while (!str1.equalsIgnoreCase("done")) {
            if (str1.length() > MAX_STRING_LENGTH)
            {
                System.out.println("\nString has too many characters\n\nPlease try again:");
            }

            System.out.println("\nEnter a string, enter DONE when finished:\n");
            str1 = input.nextLine();
        }

It's uncommon to throw an exception in a try block that you catch immediately. And it's totally unnecessary in this case. The only thing that you do differently is to print a message.

While not unknown, it is uncommon to put a try/catch in a loop. It's extra overhead that you don't need in this case.

It's unclear why you exit before the loop. You can just use a regular while loop and exit normally. You also close input in that case.

Answer 3

You could make StringTooLongException a subclass of RuntimeException instead of Exception to make it an unchecked exception, in the spirit similar to the unchecked IndexOutOfBoundsException and ArithmeticException. However, this depends on the API you are willing to present to your users, and whether you think that this exception must be handled specially over others.