I am looking at starting a program on the command line as normal. (i.e java myprogram)
However I know it's possible to type
java myprogram configfile.txt
so it takes this string and later on in the code I can use it to load a file.
How do I capture this input in the program using something like a constructor if possible.
public static void main(String[] args) { ... }
args array contains the parameters from the command line. In your example the filename would be args[0]. Beware however that if you hard code args[0] into your code, then don't pass a parameter, you will get an ArrayIndexOutOfBoundsException because args[0] doesn't exist.
You can expand this to include multiple parameters if you need to. For example
java myprogram configfile.txt configfile2.txt configfile3.txt
Those filenames would be args[0] args[1] and args[2] respectively.
If your program can run with or without arguments, you can find out if you have some by checking the length of the array is > 0. Something like
if (args.length > 0) { myfile = args[0]; }
else { myFile = someDefaultFilename; }
or if you can have multiple parameters you can loop through the entire array
for (String s : args) { //Do something with each parameter }
if (args.length == x) with x as the expected size.args if you use a for each loop. args won't be null and if it is empty, then this loop will be skipped without any problem.