错误java.lang.ArrayIndexOutOfBoundsException：我的代码为-1

Question

This is my code. There is an error showing on the terminal window. It says java.lang.ArrayIndexOutOfBoundsException: -1 at top--; and while (!stack.isEmpty()). Please help me solve this issue. I looked it up online but it did not help much. That is why I am asking you for help.

import java.util.*;

public class ArrayStack<T> implements StackADT<T>
{
    private final static int DEFAULT_CAPACITY = 100;
    private int top;  
    private T[] stack;

    public ArrayStack()
    {
        this(DEFAULT_CAPACITY);
    }    

    public ArrayStack(int initialCapacity)
    {
        top = 0;
        stack = (T[])(new Object[initialCapacity]);
    }    

    public void push(T element)
    {
        if (size() == stack.length) 
            expandCapacity();        
        stack[top] = element;
        top++;
    }    

    private void expandCapacity()
    {
        stack = Arrays.copyOf(stack, stack.length * 2);   
    }    

    public T pop() throws EmptyCollectionException
    {
        if (isEmpty())
            throw new EmptyCollectionException("stack");        
        top--;
        T result = stack[top];
        stack[top] = null; 

        return result;
    }

    public T peek() throws EmptyCollectionException
    {
        if (isEmpty())
            throw new EmptyCollectionException("stack");        
        return stack[top-1];
    }    

    public boolean isEmpty()
    {
         return stack.length == 0;
    }

    public int size()
    {
        return top;
    }

    public static void main(String[] args)
    {
        ArrayStack<Character> stack = new ArrayStack<Character>();
        String sentence = " ", word;
        Scanner in = new Scanner(System.in);
        System.out.println("Enter a sentence:");
        sentence= in.nextLine();
        System.out.println("Reversing each word:");
        Scanner sentenceScanner = new Scanner(sentence);
        while(sentenceScanner.hasNext())
        {
            word = sentenceScanner.next();
            for(int i= 0; i<word.length(); i++)        
            {
                stack.push(word.charAt(i));
            }

            while (!stack.isEmpty())   
            {
                System.out.print(stack.pop()); 
            } 
        }
    }
}

Answer 1

Your isEmpty() and pop() functions do not work together. Think about this: when you decrement top and set the index to null does the actual size of the list get adjusted? The fix will come by changing isEmpty() I am just using pop() as an example.

Answer 2

Your isEmpty() method is implemented incorrectly.

public boolean isEmpty()
{
     return stack.length == 0;
}

It is currently comparing the size or capacity of of the stack array. This is a constant, it does not change. The value of

stack.length

is always equal to the value you used to initialize the array

new Object[length];

You should be comparing to the number of elements in your stack.

public boolean isEmpty()
{
     return top == 0;
}