Java中引发异常

Question

I just started teaching myself java, and am currently learning how to throw exceptions. The online tutorial I'm viewing says that at the SECOND bold line (list.get(i)) an exception could be caused if the value of I is less than 0 or too large. I understand how it could be too large, but how could the value be less than 0? In what situations would this occur?

private List<Integer> list;
    private static final int SIZE = 10;

    public ListOfNumbers () {
        list = new ArrayList<Integer>(SIZE);
        for (int i = 0; i < SIZE; i++) {
            list.add(new Integer(i));
        }
    }

    public void writeList() {
        PrintWriter out = new PrintWriter(**new FileWriter("OutFile.txt")**);

        for (int i = 0; i < SIZE; i++) {
            out.println("Value at: " + i + " = " + **list.get(i)**);
        }
        out.close();
    }
}

All taken directly from http://docs.oracle.com/javase/tutorial/essential/exceptions/handling.html near the bottom

Answer 1

One example would be if you were reading off the list backwords, by starting at the end and decrementing your index.

Say you didn't set the correct condition to end your loop...This would result in a negative index and an out of bounds exception.

In this specific example getting a negative index would not be possible the way you currently have your code set up.

Example:

for(int i = list.size(); i>= -1; i--){
    list.get(i);
}

Answer 2

In your loop, it can't.

But if you changed:

for (int i = 0; i < SIZE; i++) {

to:

for (int i = -1; i < SIZE; i++) {

then i will be less than 0, so list.get(i) will throw an exception.

Answer 3

As written, your code will never encounter a negative index. In fact, even if the list is empty, you will never run into a case in which you will get a negative index, nor will it ever exceed the maximum capacity of your list.

This particular exception ( IndexOutOfBoundsException ) happens if you do something silly, like use a negative index to pull a value out of a list:

list.get(-1);

...or better yet, if you step off the edge of the list (which is far more common):

list.get(SIZE); // invalid - you can only get up to SIZE - 1