taking in command prompt arguments and constructing objects

Question

I need to take in command prompt arguments, and use them to construct an object. I am taking a number of people and the boat size.

public RiverCrossingPuzzle(int numEach, int boatSize)

Creates a RiverCrossingPuzzle with numEach number of cannibals and numEach number of missionaries with a boat that can carry up to boatSize number of missionaries/cannibals. For example, the input can be "-n", "4", "-b", "3"

Should I do it like this:

this.numEach = Integer.parseInt(args[2]);
this.boatSize = Integer.parseInt(args[4]);

Or like this:

int numEach = Integer.parseInt(args[2]);
int boatSize = Integer.parseInt(args[4]);
this.boatSize = boatSize;
this.numEach = numEach;

Answer 1

I would read the and parse the command line arguments before calling the constructor.

public static void main (String[] args)
{
    int numEach = 0; // or some other default value that makes sense
    int boatSize = 0; // or some other default value that makes sense
    if (args.length > 1)
        numEach = Integer.parseInt(args[1]);
    if (args.length > 3)
        boatSize = Integer.parseInt(args[3]);
    RiverCrossingPuzzle puzzle = new RiverCrossingPuzzle (numEach, boatSize);
}

You should also add exception handling in case the command line arguments cannot be parsed at int.

BTW, based on your "-n", "4", "-b", "3" example, you should be reading args[1] and args[3].

Answer 2

Something you might want to take a look into, also, is that of your program arguments. You are expecting a user to start the program with the arguments of -n 4 -b 3, that could not always be the case. The user could start the program with -n and -b the other way around. You always have to assume that the user will try to break your application (even though this is probably just an exercise, it's a good habit to get in to). Now how could you resolve this? The most simple way would be to go through and see where the indicators -n and -b are in the program arguments, as such:

public static void main(String[] args) {
    int nIndex = -1; //Just some value that clearly will never resolve in an array, suggesting it has not been found
    int bIndex = -1;
    for(int i = 0; i < args.length; i++)
        if (args[i].equals("-n"))
            nIndex = i + 1; //The next index will correspond to the value of -n
        else if (args[i].equals("-b"))
            bIndex = i + 1; //The next index will correspond to the value of -b
    int boatSize, numEach;
    if(nIndex != -1 && bIndex != -1 && nIndex < args.length && bIndex < args.length) {
        /*Check that they were both parsed AND that neither of them are outside of the array range
         *(such as starting with "-n 5 -b" and no value for b
         */
        boatSize = Integer.parse(args[bIndex]);
        numEach = Integer.parse(args[nIndex]);
    }

}

Answer 3

I would follow the try to parse command line approach because it also handles invalid input:

public static void main (String[] args){

    RiverCrossingPuzzle puzzle;
    try{
        int numEach= Integer.parseInt(args[1]);
        int boatSize= Integer.parseInt(args[3]);
        puzzle = new RiverCrossingPuzzle (numEach, boatSize);
    }catch(NumberFormatException e){
        e.printStackTrace(); // maybe log it?
        System.out.println("Invalid command line arguments passed in!! You should pass arguments such as : -n NUMBER -b NUMBER");
        return; 
   }
}