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Args Array

Every program you run, Java or not, can be given a list of words on its command line.

ls                            # No words on the command line
ls drills                     # One word on the command line
ls drills solutions           # Two words on the command line
ls drills solutions whatever  # Three words on the command line
* It's up to each program to interpret what those words mean - filenames? options? something else?

Each word on the command line (also known as command-line parameters or arguments) is separated from the others by one or more spaces.

The shell - the program running in the terminal which receives and executes your commands - puts all words on the command line into an array which gets passed to the program you're running.

shell

A computer program that allows a user to interact with the system, primarily by executing other programs at the user's command. When you work in a terminal or console, the shell is your command-line interface (CLI) with the operating system.

  • The java command places the words it receives (other than its own name java and the name of the main class it's running) into a Java String array which it passes to our main method.

The args Array

  • We could actually name main's string array parameter something other than args.
public static void main(String[] stuffFromTheCommandLine) { ... }
  • But then all the other Java programmers will point and laugh at us.

Drill

In main add: 

System.out.println(args.length);
Save ShowMe.java, return to the terminal, and run it both with and without arguments. 
java drills.ShowMe
java drills.ShowMe cat dog frog

Treat args just like any other array.

  • Check its length field.

  • Iterate over it with a for loop.

  • Pass it to another method.

Drill

In main, add a for loop below the existing code. * Open a new line. * Type for and hit Ctrl-Space. * Select for - iterate over an array from the list and hit Enter. Eclipse creates a for loop for the nearest array it sees, which happens to be args. * In the for loop body add: 

System.out.println(i + ": " + args[i]);
* Save ShowMe.java, return to the terminal, and run it both with and without arguments. 
java drills.ShowMe
java drills.ShowMe cat dog frog

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