问题
I am working on a program that is supposed to take one required command line argument and one optional command line argument. The first argument is the name of an output file where data will be written to, and the second is a number that will be used to calculate the data to be written to the output file. If the user does not enter a number, then it should just use a default value to calculate the data. For example, if the user entered command line arguments "Foo.csv 1024" the program would use 1024 to calculate the data and write it to Foo.csv, but if the user only used the command line argument "Foo.csv" then the program would use a default value of 2048 to calculate the data and write it to Foo.csv. I am creating/running this program using the Intellij IDE. How would I do this? Any advice/suggestions would be much appreciated.
回答1:
Your program seems to be simple, so the solution is also simple for this particular case. You can test how many arguments were passed to the program checking the argument args of your main function:
public static void main(String[] args){...}
args is an array that contains the arguments passed to your program. So if your program is called prog and you run it with prog Foo.csv 1024, then args will have:
args[0] = "Foo.csv";
args[1] = "1024";
With this, you know which arguments were passed to your program and by doing args.length, you can know how many they were. For the example above, args.length=2 If the user didn't indicate the last argument ("1024"), then you would have args.length=1 with the following in args:
args[0] = "Foo.csv";
So your program would be something like:
public static void main(String[] args){
//The default value you want
int number = 2048
if(args.length==2){
number = Integer.parseInt(args[1]);
}
//'number' will have the number the user specified or the default value (2048) if the user didn't specify a number
}
To supply arguments to your program you must run it on a console or some kind of terminal. Using IntelliJ (or any other IDE) it's also possible to run the program with arguments, but you specify those in the Run Settings.
If you want a more complex treatment of arguments, usually what you want is done by argument parsers. These are usually libraries that take you argv and help you reading arguments to your program. Among other things, these libraries usually support optional arguments, arguments supplied via flags, type checking of arguments, creating automatic help pages for your commands etc. Consider using an argument parser if your argument requirements are more complex or if you just want to give a professional touch to your program :) For java i found this particular library: http://www.martiansoftware.com/jsap/doc/
来源：https://stackoverflow.com/questions/42753849/java-making-an-optional-command-line-argument