Stack help. Why will my peekAtTop() not work??

I am new to Data Structures and am trying to write my own code for a Stack. Everything seems to be working but my peekAtTop() method. I keep getting errors. To test this i just threw in a few strings and displayed them as i popped them off. Let me know if you see anything obvious that I am missing.

import java.util.ArrayList;

public class Stack<T>
{
private ArrayList<T> elements;
private int size;

public Stack()
{
size = 0;
elements = new ArrayList();
}

public void push (T addElement)
{

elements.add(size, addElement);
size++;
}

public T pop()
{
size -=2;

return elements.get(++size);
}

public boolean isEmpty()
{
if(size == 0)
return true;
else
return false;
}

public T peekAtTop()
{

return elements.get(size);

}
}

What errors do you get and which lines cause them?

Quote:

---

Originally Posted by Fubarable

What errors do you get and which lines cause them?

---

import java.util.*;
public class StackTester {

public static void main(String[] args)
{
System.out.print("This is to test a stack");
System.out.println(" ");


Stack<String> stack = new Stack<String>();

System.out.print(stack.isEmpty());

//add to stack
stack.push("poop");
stack.push("Doop");
stack.push("loop");
stack.push("moop");

System.out.println(stack.peekAtTop());
int count=0;
while(stack.isEmpty() != true)
{
System.out.println(stack.pop());
count++;
System.out.println(count+" removed!");

}


}


}
that is my tester program.

Here is the error.


This is to test a stack
trueException in thread "main" java.lang.IndexOutOfBoundsException: Index: 4, Size: 4
at java.util.ArrayList.RangeCheck(ArrayList.java:547)
at java.util.ArrayList.get(ArrayList.java:322)
at Stack.peekAtTop(Stack.java:39)
at StackTester.main(StackTester.java:20)

If there are n elements on the stack, size == n but there is no element #n (that would be the n+1st element); use stack.get(n-1) instead.

kind regards,

Jos

In other words, arrays etc use zero based indexing: 0 to length-1.