import java.util.Arrays;
public class AStack<T> implements IStack<T>
{
    private Object[] entries;
    private int top;
    public AStack()
    {
        entries = new Object[10];
        top = 0;
    }
    public void push(T newItem)
    {
        if(top >= entries.length)
        {
            //resize
        }
        entries[top] = newItem;
        top++;
    }
    @SuppressWarnings("unchecked")
    public T peek()
    {
        if(isEmpty())
        {
            throw new EmptyStackException();
        }
        return (T)entries[top - 1];
    }
    @SuppressWarnings("unchecked")
    public T pop()
    {
        if(isEmpty())
        {
            throw new EmptyStackException();
        }
        T out = (T)entries[top - 1];
        entries[top - 1] = null;   //interest the garbage collector
        top--;
        return out;
    }
    public boolean isEmpty()
    {
        return top == 0;
    }
    public int size()
    {
        return top;
    }
    @Override
    public String toString()
    {
        if(isEmpty())
        {
              return "[]";
        }
        StringBuffer sb = new StringBuffer();
        sb.append("[");
        for(int k = top - 1; k > 0; k--)
        {
            sb.append(String.format("%s, ", entries[k]));
        }
        sb.append(String.format("%s]", entries[0]));
        return sb.toString();
    }
    private newSize(int n)
    {
        return n < 1000? 2*n : 3*n/2;
    }
    public static void main(String[] args)
    {
        IStack<String> s = new AStack<String>();
        s.push("A");
        s.push("B");
        s.push("C");
        System.out.println(s);
        System.out.println(s.size());
        s.pop();
        s.pop();
        s.pop();
        System.out.println(s);
        System.out.println(s.isEmpty());
    }
}