import java.util.EmptyStackException;

public class ArrayStack
    implements Stack
{
    final static int DEFAULT_SIZE = 16;

    private Object [] stack;
    private int       top   = -1;
    private int       size;

    public ArrayStack( int sz )
    {
        size = sz;
        stack = new Object[ sz ];
    }

    public ArrayStack()
    {
        this( DEFAULT_SIZE );
    }

    public void push(Object o)
    {
        if ( top == size-1 )
            grow();
        stack[ ++top ] = o;
    }

    public Object pop()
        throws EmptyStackException
    {
        if ( isEmpty() )
            throw new EmptyStackException();
        else
        {
            return stack[ top-- ];
        }
    }

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

    public void clear()
    {
        top = -1;
    }
    
    private void grow()
    {
        Object [] old = stack;
        int oldSize = size;
        size *= 2;
        stack = new Object[ size ];
        System.arraycopy( old, 0, stack, 0, oldSize );
    }
}