How can I stop this out of bounds exception in Java

Question

Hi I wanted to know how to write up a try and catch block to stop from getting the below error.

java.lang.IndexOutOfBoundsException: Index: 1, Size: 1

I have this method which takes a sentence and splits it into an ArrayList. I then use that to store values into a hashmap, where index 1 is the key and the words after become the value. I use the below method to split the user input into an array.

private Scanner reader;

    /**
     * Create a new InputReader that reads text from the text terminal.
     */
    public InputReader()
    {
        reader = new Scanner(System.in);
    }

    public ArrayList<String> getInput() 
    {
        System.out.print("> ");                // print prompt
        String inputLine = reader.nextLine().trim().toLowerCase();

        String[] wordArray = inputLine.split(" ");  // split at spaces

        // add words from array into ArrayList
        ArrayList<String> words = new ArrayList<String>();
        for(String word : wordArray) {
            words.add(word);
        }
        return words;
    }

}

and the below method uses the class above to detect user input. So when the user types in write they can write into a hashmap but if they press return before they type in a key and value I get the out of bounds exception. So How can i rewrite the below method to avoid this?

 public void start()
    {

        boolean finished = false;


            printWelcome();
            while(!finished) {
                ArrayList<String> input = reader.getInput();

                if(input.contains("shutdown")) {
                    finished = true;
                }

                if (input.contains("load")) {
                    System.out.println();
                    instruct.readAndFill();
                    System.out.println();
                }            

                if (input.contains("write")) {
                    String key = input.get(1);
                    String value = "";
                    for(int i=2; i<input.size(); i++) {
                        value = value + " " + input.get(i);
                    }
                    instruct.mapWrite(key, value);
                }
            } 
            instructorGoodBye();
        }

Sorry if i wasn't clear enough, or if my code is not up to scratch i have only been learning java for about 2 months now.

Answer 1

basically if the user types in write key value on one line it is fine but if they hit return after write then the error happens.

So, fundamentally what you are missing is error checking. Your program is taking input from a user, and assuming it is valid. This is always a bad idea.

Instead, you should validate what you get from the user. One way you can do this, for your "write" block, is to make sure the elements you expect to be there, are actually there.

To start, I would rewrite your loop as follows:

while(!finished) {
    List<String> input = reader.getInput();
    if(input.size() == 0) {
        throw new IllegalArgumentException("Must specify command, one of 'shutdown', 'load', 'write'");
    }

    final String command = input.remove(0).toLowerCase();
    // TODO: Make sure command is one of the valid commands!

Note the changes:

1. Assigning to List instead of ArrayList is just a good general practice.
2. Checking the input to make sure it has more than zero elements
3. Taking the first element, since we don't want to have to do List.contains(). Consider the input garbage garbage garbage write, clearly we don't want this to invoke the "write" command, it should be considered invalid input.

Finally, we use this to rewrite the conditions on executing our commands:

if(command.equals("write")) {
    // Make sure the user put the right stuff in here
    // Since we removed the command from the input already, just make sure what is left is 
    if(input.size() <= 1) {
        throw new IllegalArgumentException("Must specify correct data");
    }
    String key = input.remove(0);
    String value = String.join(" ", input); // Java 8
    instruct.mapWrite(key, value);
}