Java command line

It's a basic concept, but it threw me at first. How do the command line and a Java program you wrote in an IDE interact?

First, the command line:
     a) Save your java file in a text editor as YourFile.java. Read this for more details.
     b) Get to the command line. If you're using Win8, for example, go to the Start menu and either type in "cmd" or use search and type in "command".
     c) On the command line itself, first change to the directory in which you saved your file. (See page I referred to above.)
     d) Set the path. The page referred to above addresses this but it didn't work for me. This is what worked for me. Proceed with caution.
     e) Compile by entering this at the command prompt - javac YourJavaFileName.java
     f) Run the interpreter by entering this at the next prompt - java YourJavaFileName command parameters
     ex. java InvestmentFile calculate .04 10 5000
    //calculate is the command;
   //.04, 10 and 5000 are the arguments

Entering the command: Just make sure the user knows which command to enter in the beginning and follow up in case they still enter the wrong thing.

if(user enters right command)
{//code}
else
{//tell them what to do or use a try-catch block instead}


Now, how do the parameters in the methods connect to the arguments on the command line?

Let's assume in the example above that .04 represents the interest rate, 10 represents the number of years and 5000 represents the amount invested.

Let's say you have a method that has these 3 things as parameters. Here is how you'd normally write the method:

public double calculateROI(double rate, int yrs, double money)
{
    //code
}
 But for the command line, here is how you write the method:
public double calculateROI(args[0], args[1], args[2])
{
    //code
}

Look familiar? Yes, finally, we see a point for those args in the main method.  Here is where exception handling comes in. If the first parameter in your method is the interest rate and, obviously, you want the first argument written on the command line to be the interest rate, you have to account for handling the error of someone not doing that. Otherwise, if the number of years is entered as the first argument, you clearly won't get the right result.

This isn't unlike dealing with parameters and arguments normally. Just make sure the user enters the arguments in the right order, after you've made sure they've entered the right command.