The Demo Series: Console I/O, Errors, and Exception Handling

This week I’m covering exception handling when it comes to user errors, specifically with console input. I’ll be using a Scanner class object to read in an int and will implement a try-catch-finally block to handle exceptions, and print a nicer message out to the console instead of the typical stack trace.

Here’s the program below. Again, this time I’m actually using “pre” tags to put the code in a block, which preserves formatting. Unfortunately I can’t go in and adjust the CSS to change the “pre” class to increase width via “display: inline” or something similar.

Here’s a basic implementation:

/*
 *			Programmer: The Dude
 * 			Project Name: ExceptionBasics.java
 * 			Description: Basics of Exception Handling
 * 
 */

//	Imports and packages

import java.util.Scanner;                // for scanner object 
import java.util.InputMismatchException; // This is the ExceptionType we need in the catch block

public class ExceptionBasics 
{

	public static void main(String[] args) 
	{
		//	Implementing a try-catch block to handle bad user input
		//	Setting up vars and a conditional to repeat the input prompt if user enters bad input
		int i = 0; 
		boolean done = false; //for the do-while
		
		do{
		
			try	
			{
				Scanner input = new Scanner(System.in);
				System.out.print("Enter an integer: \t\t");
				i = input.nextInt();
				done = true;
			} 
			catch	(InputMismatchException e) 
			{
				System.out.println("You didn't enter an integer");
			}
			finally	//	This is a block to put potential cleanup code whether exception occurs or not
			{
				//	Demo output
				System.out.println("This is the finally block & executes whether exception occurs or not");
			}
		
		} while(!done);
		
		System.out.println("You entered: " + i);
		
	}

}

So that is a very basic demo of exception handling. Below is a slightly more in-depth demo that uses two catch blocks to handle two types of possible user errors. One is the InputMismatchException imported from the java.util package, and the other is a programmed class called IntegerOutOfBoundsException which extends Exception. Here’s the simple class declaration for that.

//  This is the class that we need to catch the 
//  2nd type of exception, any number entered 
//  which is outside the given range.
public class IntegerOutOfRangeException extends Exception {}

And here’s the complete implementation.

/*
 *	Programmer: The Dude
 * 	Project Name: CustomExceptionExample.java
 * 	Description: Using throws and try-catch-catch example
 * 
 */

//	Imports and packages

import java.util.Scanner;
import java.util.InputMismatchException;


public class CustomExceptionExample {

	public static void main(String[] args) {
		/*This example will throw a InputMismatchException
		 when you enter a non-integer*/
		
		//Variable declarations and objects
		int i;
		Scanner input = new Scanner(System.in);
		System.out.print("Enter a number between 1 and 10:  ");
		// Try-catch for 2 exception types. 
		try{
			i = input.nextInt();
			if(i  10)throw new IntegerOutOfRangeException();
		} catch(InputMismatchException ex) {
			System.out.println("You did not enter an integer");
			System.out.println(ex);
		} catch(IntegerOutOfRangeException ex) {
			System.out.println("Your value is not in the specified range!");
			System.out.println(ex);
		}
	}
}