Current path and inputting files in Java

Current path and inputting files in Java

I'm writing a fairly simple Java program, and some options I require from
the user give them the ability to read from a list of text for data
comparisons.
What I would like to do is to give them the ability to simply define the
file if it is in the current working path, and assume so unless they
provide a separate path to their file. I'd like to use the same approach
to generate a output file wherever they define.
So I start by checking if the file even exists (which does not work if you
do not define a full path):
File f1 = new File(inputFile);
if(f1.exists())
System.out.println("Exists");
else if(!f1.exists())
{
System.out.println("Cannot find file " + inputFile);
System.exit(1);
}
So I could create the file with new File(path/inputFile), but I can break
that fairly easily.
I want them to be able to do any of the following:
program.exe -m -1 inputFile.txt outputFile.txt
program.exe -m -1 C:\Users\tmp\Desktop\inputFile.txt outputFile.txt
program.exe -m -1 inputFile.txt C:\outputFile.txt
Any suggestions on where I might make my next step?