Java Array index out of bounds exception fix

Question

I am currently getting an array out of bounds exception while executing the line name.firstName = this.firstNames[rand.nextInt(NUM_NAMES)]; Normally I dont have issues finding the source of these exceptions however I have been stuck on this one for some time now. Any help is appreciated, class and stacktrace are pasted below:

public class NameGenerator {
    private static final int NUM_NAMES = 200;
    private static final File NAMES_FILE = new File("resources/info.dat");

    /** a random number generator */
    private Random rand;

    /** the array of first names */
    private String[] firstNames;
    /** the array of last names */
    private String[] lastNames;

    /**
     * Default Constructor
     */
    public NameGen() {
        super();
        this.rand = new Random();

        try {
            readFile();
        } catch (IOException exp) {
            this.first = new String[] { "foo" };
            this.last = new String[] { "bar" };
        }
    }

    /**
     * Read the names from the file
     */
    private void readNFiles() throws IOException {
        List<String> tempFirst = new ArrayList<String>();
        List<String> tempLast = new ArrayList<String>();

        Scanner scnr = new Scanner(NAMES_FILE);

        while (scnr.hasNext()) {
            tempFirst.add(scnr.next());
            tempLast.add(scnr.next());
        }

        scnr.close();

        int size = tempFirst.size();

        this.first = new String[size];
        tempFirst.toArray(this.firstNames);

        this.last = new String[size];
        tempLast.toArray(this.last);
    }

    /**
     * @return a generated name
     */
    public FullName generateName() {
        FullName name = new FullName();
        name.first = this.firstNames[rand.nextInt()];

        name.last = this.lastNames[rand.nextInt()];
        return name;
    }

    /**
     * Class describing a full name
     */
    public static final class FullName {
        /** the first name */
        public String firstName;
        /** the last name */
        public String lastName;
    }
}

Answer 1

Based on...

try {

    readNamesFiles();

} catch (IOException exp) {

    this.firstNames = new String[] { "John" };
    this.lastNames = new String[] { "Doe" };

}

There is no guarantee that your arrays will contain NUM_NAMES elements (you should be logging the exception at the very least).

So using something like name.firstName = this.firstNames[rand.nextInt(NUM_NAMES)]; has the potional to cause some serious issues, as you've discovered.

Instead, you should work with reality instead of assumptions, using something more like...

name.firstName = this.firstNames[rand.nextInt(this.firstNames.length)];

Answer 2

Here is a summary of your problematic code:

List<String> tempFirstNames = new ArrayList<String>(NUM_NAMES);
int size = tempFirstNames.size();

this.firstNames = new String[size];
FullName name = new FullName();
name.firstName = this.firstNames[rand.nextInt(NUM_NAMES)];

You are using rand.nextInt(NUM_NAMES) as the array index into firstNames . This will generate a number between 0 and NUM_NAMES . The problem is that there is no guarantee that the array firstNames will have a size of NUM_NAMES . As the @AngryProgrammer pointed out, you can use this instead:

name.firstName = this.firstNames[rand.nextInt(firstNames.length)];