Custom exception example.

ExceptionTest.java

/*
 * Custom exception example.
 * © G J Barnard 2013 - Attribution-NonCommercial-ShareAlike 3.0 Unported - http://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_GB.
 */
package myexception;

/**
 * Demonstrate how custom exceptions can be defined and employed.
 */
public class ExceptionTest
{
    private int x;

    /**
     * @param args - Not used.
     */
    public static void main(String[] args)
    {
        ExceptionTest us = new ExceptionTest();

        /*
         * The purpose here is to demonstrate how you can define your own exceptions for invalid program situations where
         * perhaps you have not performed validation.  By specifying that a certain input can be between 1 and 100 then
         * any number outside of that range is invalid and should be treated as such.  Exceptions do not have to be reported
         * to the user or with the full stack trace.  They can just be handled internally.  The beauty of using them comes
         * with logging where full stack traces can assist you in fixing a sitaution that a user has encountered.
         */
        int guesses[] =
        {
            -2, 1, 100, 105, 42
        };
        System.out.println("Guessing the value of X...");
        for (int i = 0; i < guesses.length; i++)
        {
            System.out.print("Trying: " + guesses[i]);
            try // Only put in the try block what you need.
            {
                if (us.guessX(guesses[i]))
                {
                    System.out.println(" - Correct");
                }
                else
                {
                    System.out.println(" - Incorrect");
                }
            }
            catch (MyException ex)
            {
                System.out.println();
                // Note that we just report here, there is no re-throw and program execution continues - there is no crash.
                System.err.println(ex.getReason());
                // Here you show a single message to the user as above and log the stack trace for them to send to you.
                System.err.println("Stack trace...");
                ex.printStackTrace(System.err);
            }
        }
    }

    /**
     * Set the value to guess.
     */
    public ExceptionTest()
    {
        x = 42;
    }

    /**
     * Checks to see if the guess is correct.
     *
     * @param guess The guess.
     * @return If the guess is correct.
     * @throws MyException If the guess is outside the valid range.
     */
    public boolean guessX(int guess) throws MyException
    {
        boolean retr = false;

        if ((guess < 1) || (guess > 100))
        {
            throw new MyException("Guess " + guess + " must be between 1 and 100.");
        }
        else if (guess == x)
        {
            retr = true;
        }

        // I dislike methods that return all over the place, better to have a single clean point of return.  Except the exception!
        return retr;
    }
}

MyException.java

/*
 * Custom exception example.
 * © G J Barnard 2013 - Attribution-NonCommercial-ShareAlike 3.0 Unported - http://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_GB.
 */
package myexception;

/**
 * Define your own exception.
 */
public class MyException extends Throwable
{
    private static final long serialVersionUID = -7131266248952296430L;
    private String reason;

    /**
     * Constructor
     * @param reason The reason for the exception.
     */
    public MyException(String reason)
    {
        super("MyException - " + reason);
        this.reason = reason;
    }
    
    /**
     * Prevent use of no parameter constructor.
     */
    private MyException()
    {        
    }

    /**
     * Gets the reason for the exception.
     * @return String
     */
    public String getReason()
    {
        return "MyException::getReason() - " + reason; // Prefixed with the method name to understand how printStackTrace() works.
    }

    /**
     * Override default method.
     * @return String
     */
    @Override
    public String toString()
    {
        return "MyException::toString() - " + reason; // Prefixed with the method name to understand how printStackTrace() works.
    }
}