how to fix iterating through this arraylist?

Question

I'm using two buttons to iterate back and forth in an arraylist that contains 12 months, when i reach the last element and press the prevButton the app crashes, this also happens when i go back to the first element and hit the nextButton, how do i fix my if statment?

I have tried to do it with >= and <= but it didn't work

nextButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if (i < months.size()) {
                    Month month = months.get(i);
                    monthTextView.setText("" + month);
                    i++;
                }
            }
        });


        prevButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if (i >= 0) {
                    Month month = months.get(i);
                    monthTextView.setText("" + month);
                    i--;
                }
            }

        });

Answer 1

with the last click you are changing the i value anyway to -1 or months.size() by adding/subtracting 1 after accessing last/first element

let us take a look closer on this fragment:

if (i >= 0) { // let us assume that i = 0, we are getting inside
    Month month = months.get(i); // you are assigning first element of months, everything is fine
    monthTextView.setText("" + month); // some data operation - irrelevant
    i--; // anyway you are updating i to [i = -1]
}

// one "next" click later...

if (i < months.size()) { // condition is fine because i = -1
    Month month = months.get(i); // oooops tried to access months[-1] - ERROR
    monthTextView.setText("" + month);
    i++;
}

The same is happening in the second scenario

What you should do - you should update i value only if current value is strongly > 0 or < month.size() - 1

if (i >= 0) {
    Month month = months.get(i);
    monthTextView.setText("" + month);
    i = (i > 0) ? (i - 1) : 0; // for example like this
}

Answer 2

when you reach the last or first index the final value will be -1 or maxsize of it but when you click the opposite button back the condition is still pass, for example when you already at -1 the next button condition still pass since -1 is < than month.size() your condition should be something like if i-1 <= 0 and then set i

Edit: I think the person who posted this means that when ( i == months.size() - 1 ) and you say next button, it works fine right then because it gets the last month but then i increases by one, that is, i == months.size() is true now. Then, when you press the prevButton, it is only checked if i >= 0 and not i < months.size(). Thus, the array index is out of bounds now. You should check the index bounds in both button methods.