Checked Exceptions

Checked exceptions must be declared on the method and must be caught (or allowed to raise up) by the caller. Here's an example that creates a checked exception (NoTicketsAvailableException) by extending Exception. The buyTicket() method in Theatre throws this exception when there are no tickets to sell.

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package org.megha.blog.example.part9;

/** An exception thrown when no tickets are available */
public class NoTicketsAvailableException extends Exception {
}

package org.megha.blog.example.part9;

/**
 * A theatre that sells tickets.
 */
public class Theatre {

	int availableTickets;

	/** creates a new theatre with a fixed number of tickets */
	public Theatre(int availableTickets) {
		this.availableTickets = availableTickets;
	}

	/**
	 * Sells a ticket to the buyer.
	 *
	 * @return a unique ticket id representing the ticket sold
	 * @throws NoTicketsAvailableException when all tickets are sold out
         */
	public int buyTicket() throws NoTicketsAvailableException {
		if (availableTickets == 0) {
			throw new NoTicketsAvailableException();
		}
		return availableTickets--;
	}

	public static void main(String args[]) {
		// create a new threatre with only 3 seats/tickets
		Theatre theatre = new Theatre(3);

		for (int i = 1; i <= 5; i++) {
			try {
				int ticketId = theatre.buyTicket();
				System.out.println("Ticket #" + i + " is " + ticketId);
			} catch (NoTicketsAvailableException e) {
				System.out.println("Could not buy ticket #" + i);
			}
		}
	}
}

Unchecked Exception

An example that shows how/where an un-checked exception can be thrown. IllegalArgumentException is a sub-class of RuntimeException.

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package org.megha.blog.example.part8;

public class MyMath {

	/**
	 * Calculates the average of the numbers provided.
	 *
	 * @param numbers the numbers to average over.
	 * @throws IllegalArgumentException if no numbers a specified
	 */
	public static double average(int... numbers) {
		// note the use of var-args above. java collapses all the arguments into an array
		if (numbers.length == 0) {
			throw new IllegalArgumentException("no numbers to average over!");
		}
		int sum = 0;
		for(int i : numbers) {
			sum = sum + i;
		}
		return ((double) sum) / numbers.length;
	}

	public static void main(String args[]) {

		// averaging works over any number of arguments
		System.out.println(MyMath.average(1, 2, 3, 7, 9, 10));
		System.out.println(MyMath.average(55, 77));
		System.out.println(MyMath.average(3));

		// but when it's called with no arguments, an unchecked exception is thrown.
		// ofcourse, we know about it already, so we have a try/catch block in place.
		// unchecked/runtime exceptions usually happen when some assumed condition, or
		// an api contract is not met by the programmer. eg passing in a null, where
		// null is not expected, or trying to lookup into an array beyond it's bounds.
		try {
			System.out.println(MyMath.average());
		} catch (IllegalArgumentException e) {
			System.out.println("Exception caught: " + e);
		}
		System.out.println("still running... :)");
	}
}