Check If A String Is A Palindrome By Using Stacks

I'm supposed to use stacks (implemented with an array) to check to see if a string is a palindrome. I've finished all my classes and methods, but I'm getting an ArrayIndexOutOfBoundsException when I try to run my demo program.Here are my classes:

public interface Stack {
// Creates an empty stack
public void initializeStack()
// Returns true if the stack is empty, returns false otherwise
public boolean isEmpty();
// The stack can never be full, so always return false
public boolean isFullStack();

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