CMSC 206 (Introduction to Computing)

Assignment#0 (ungraded)

No due date since it will not be submitted, but you should complete this assignment by the end of the first week of class.

The purpose of this assignment is to get you started in writing programs in Java.  As such, you should complete this assignment by the end of the first week of classes.  Also, since we are just starting out, this assignment is ungraded.  In fact, you will not even be submitting this assignment.  It is purely for your benefit.  Because of this, you are welcome to work with other students in the class to complete this assignment.

Introduction

Write a program that reads a positive integer n from the command line and prints out the squares of all numbers 1 through n.

Here are some example traces of running this program:

> java Assignment0 4
   1  4  9  16

> java Assignment0 -4
   Error:  input "-4" is not a valid positive integer
   Usage:  java Assignment0 <positive integer>

> java Assignment0 3.14
   Error:  input "3.14" is not a valid positive integer
   Usage:  java Assignment0 <positive integer>

> java Assignment0 CMSC206
   Error:  input "CMSC206" is not a valid positive integer
   Usage:  java Assignment0 <positive integer>

> java Assignment0
   Error:  no input given
   Usage:  java Assignment0 <positive integer>

Notice that the bulk of the program will consist of error checking the input.  What do you think should happen if the user specifies more than one input?

Getting Started

The first thing you should try to run is the basic HelloWorld program, given below:

class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

Copy this code into a file called HelloWorld.java and the compile it using the following command:

> javac HelloWorld.java

This command should produce the file HelloWorld.class, which is the compiled Java program.  You can then run the HelloWorld program by:

> java HelloWorld
Hello World!

Implementation Hints

Here is a rough structure you can use for your program:
I recommend you first get the basic output of the program working (printing out the squares of 1..n) and then add in the error checking afterward.

The input arguments are stored as strings in the array args, as shown in the HelloWorld program above.  You can test the number of input arguments based on the length of this array.

To convert a String variable str that contains an integer in text form to an int variable, you can use the following code:
int n;   // why do we need to declare n outside of the try-catch statement?
try {
    n = Integer.parseInt(str);  // This line extracts the integer from the String variable str and stores it in n. 
                                // When first developing your program without error checking, this is the only line you need.
} catch (Exception e) {
    // code that is here is executed if the conversion fails, which is useful for error checking
}

Java contains multiple output streams, each with different purposes.  System.out should be used for normal output, and System.err should be used for errors:
System.out.println("This is a normal output message.");
System.err.println("This is an error message.");

Instead of writing multiple lines that print error messages to the screen, you can put each error message in a function and just call that function when you want the appropriate error message printed.

To exit your program before reaching the end of main(), you can use the following code:
System.exit(1);  // the number (in this case 1) denotes a status code.  By convention, exiting with a code of 0 indicates normal termination, and anything else denotes some error.  Different errors can use different status codes.