我的PrintList函数的ArrayIndexOutOfBoundsException我不知道为什么

Question

I am writing a really simple program which automatically extends the array when the user reaches the limit of the current array.

The problem is that I am getting a java.lang.ArrayIndexOutOfBoundsException when I run my PrintList method and I really don't know why. It's working perfectly if I use a random number, which is bigger than the array (eg 500), but if I use

    for (int i = 0; i < stringArray.length; i++)

or

    for (int i = 0; i <= stringArray.length; i++)

I get a nasty exception. How do I deal with this and why am I getting it in the first place?

Thanks a lot for your help!

Here's the source code of my program:

public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);
    int index = 0;
    String[] randomString = new String[1];

    while (index <= randomString.length) {

        out.println("Enter your string");
        String input = keyboard.next();
        randomString[index] = input;
        PrintArray(randomString);
        index++;

        if (index >= randomString.length) {
            ExtendArray(randomString);
            continue;
        }
    }
}

public static void ExtendArray(String[] stringArray) {

    String[] secondArray = new String[stringArray.length * 2];
    // Copy first array into second array
    for (int i = 0; i < stringArray.length; i++) {
        stringArray[i] = secondArray[i];
        stringArray = secondArray;
    }
}

public static void PrintArray(String[] stringArray) {
    for (int i = 0; i < stringArray.length; i++) {
        out.println(" " + stringArray[i]);
    }
}

Answer 1

Java does not work in the methods you are trying to employ. Everything in Java is passed by value, unless it is a data point in an object. What you are trying to employ is a pass by reference, which is not possible in Java.

What you are trying to do is an already existing data structure called a Vector: http://docs.oracle.com/javase/7/docs/api/java/util/Vector.html

I would suggest doing this: (not sure if it will work properly, as my current PC doesn't have dev tools):

public static String[] ExtendArray(String[] stringArray) {

    String[] secondArray = new String[stringArray.length * 2];
    // Copy first array into second array
    for (int i = 0; i < stringArray.length; i++) {
        secondArray[i] = stringArray[i];
    }

    return secondArray;
}

then calling it like so in main:

randomString = ExtendArray(randomString);

Relating to vectors, this is how it works in a Vector class:

public void incrementCount(int count){
    int increment = (count * 2);
    Object newElementData [] = new Object[increment];
    for(int i = 0; i < count; i++)
    {
        newElementData[i] = elementData[i];
    }

    elementData = new Object[increment];

    elementData = newElementData;
}

In this case, elementData is the original array, newElementData is a temp array that acts to up the bounds.

Answer 2

You cant get error on your PrintArray method, you get the error on the line before!

randomString[index] = input;

Because if you do this

index <= randomString.length

The last iteration is out of bounds, String of length 10 has values on 0-9. You have to change the while cycle to

index < randomString.length

Also your ExtendArray method is NOT functional!

Answer 3

You are supposed to swap out the randomString array for a new array with double length. You create a new array and copy the contents of the old one to the new one, but don't do anything with the new array.

I suppose you want the ExtendArray method to return the new array, and set the randomString variable to be the new array.

Answer 4

You need to return your second array from ExtendArray function:

public static String[] ExtendArray(String[] stringArray) {

    String[] secondArray = new String[stringArray.length * 2];
    // Copy first array into second array
    for (int i = 0; i <= stringArray.length; i++) {
        stringArray[i] = secondArray[i];
    }
    return secondArray;
}

and in your main:

randomString = ExtendArray(randomString);

also your while condition should be:

while (index < randomString.length)