4.3. Command-Line Arguments Tutorial¶

Let’s try to use the args parameter in Java!

Create a directory for this tutorial called cs1302-cla, then change into it.
Create a src and bin directory, then create the .java file for a class called ArgTester and place ArgTester.java in the src directory.
```
cs1302-cla
├── bin
└── src
    └── ArgTester.java
```
In ArgTester.java, add the declaration for the ArgTester class.

In your ArgTester class, add the following main method:

public static void main(String [] args) {
    System.out.println("arguments:");
    for (int i = 0; i < args.length; i++) {
        String arg = args[i];
        System.out.printf("args[%d] = %s\n", i, arg);
    } // for
} // main

Take a few minutes to carefully read through the code above. Try and understand what it’s doing. Note: Up until this point, you’ve always typed String[] args as a parameter to main but you’ve probably never used it. That parameter is a reference to an array containing the command-line arguments.
Compile the ArgTester class, specifying bin as the destination directory.
```
javac -d bin ArgTester.java
```
Run ArgTester as usual using the java command. Here is what the command looks like with the expected program output, assuming you are running it from the cs1302-cla directory:
```
java -cp bin ArgTester
```
```
arguments:
```
As you can see, no iterations of the for-loop were executed. This is expected as the args array would be empty in this scenario (there are no command-line arguments provided).
Now try the following command:
```
java -cp bin ArgTester one two three
```
What happened when you ran it? It looks like the for-loop iterated. The array referred to by args is not empty. That’s right, we’ve used the args array for something! Here’s the expected output:
```
java -cp bin ArgTester one two three
```
```
arguments:
args[0] = one
args[1] = two
args[2] = three
```
In Java, the args array of a standard main method is used to capture command-line arguments and make them available to the program.
Now that we see how to access the command-line arguments in our code, let’s see how different command-line arguments are parsed. Try the following commands:
```
java -cp bin ArgTester "one two" three
```
```
java -cp bin ArgTester --help "some topic"
```
```
java -cp bin ArgTester --string "my \"awesome\" day"
```
That’s it! The rest is purely in the realm of code. We’ve shown you how command-line arguments are passed into a Java program (i.e., via the args array). Now, experiment by adding some command-line options of your own.