Simple Java code using (implementing ArrayList as a stack) gives an error when running

I finally managed to write my first Java assignment. The point of the assignment was to implement an ArrayList with Strings which was supposed to behave like a stack.

However, when I run the code in Eclipse I get some kind of error saying:

Exception in thread "main" øv2.Stack@c723704
java.lang.ArrayIndexOutOfBoundsException: -1
at java.util.ArrayList.elementData(Unknown Source)
at java.util.ArrayList.get(Unknown Source)
at øv2.Stack.peek(Stack.java:30)
at øv2.Stack.main(Stack.java:47)

I am not sure why this is happening, here is my code:

public class Stack {

    private ArrayList<String> stringList = new ArrayList<String>();

    RandomStringGenerator rsg = new RandomStringGenerator();

    private void push(String i){
        stringList.add(i);
        }

    private String Pop(){
        if (stringList.size() >= 1){
            String positionInArray = stringList.get(stringList.size() - 1);
            stringList.remove(stringList.size() - 1);
            return positionInArray;
        }

        else {
            return null;
        }

    }

    private String peek(int i){
        if (i >= 0 && i >= (stringList.size() - 1)){
            return stringList.get(stringList.size() - 1 - i);
        }
        else{
            return null;
        }
    }

    private int getSize(){
        return stringList.size();
    }

    public static void main (String [] args){
        Stack stack = new Stack();
        stack.push("1");
        stack.push("2");
        stack.push("3");
        System.out.println(stack);
        System.out.println(stack.peek(3));
        System.out.println(stack.Pop());
        System.out.println(stack);
    }

    }


import java.util.Random;

public class RandomStringGenerator {


    private int randomNumber;

    private Random randomNumberGenerator = new Random();

    public String randomStringGenerator(){
        int randomNumber = randomNumberGenerator.nextInt();
        String randomString = Integer.toString(randomNumber);
        return randomString;
    }

}

Thank you for your time!