import java.util.EmptyStackException;

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

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

    public ArrayStack( int cap )
    {
        capacity = cap;
        stack = new Object[ cap ];
    }

    public ArrayStack()
    {
        this( DEFAULT_CAPACITY );
    }

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

    public Object pop()
        throws EmptyStackException
    {
        if ( isEmpty() )
            throw new EmptyStackException();
        else
        {
            Object ret = stack[ top ];
            stack[ top-- ] = null;  // para recogida de basura
            return ret;
        }
    }

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

    public void clear()
    {
        for ( int i = 0; i < top; i++ )
            stack[ i ] = null;  // para recogida de basura
        top = -1;
    }
    
    private void grow()
    {
        Object [] old = stack;
        int oldCapacity = capacity;
        capacity *= 2;
        stack = new Object[ capacity ];
        System.arraycopy( old, 0, stack, 0, oldCapacity);
    }
}