//VERSION 0.3 with Calculator 0.3, 8-June-2008
public class Engine
{
    private int acc = 0;
    private String op = "=";
    private boolean start = true;

    public Engine()
    {
    }

    public void start()
    {
        start = true;
    }

    public void stop()
    {
        start = false;
    }

    public boolean started()
    {
        return start;
    }

    public void operation( String op )
    {
        this.op = op;
    }

    public int calculate( int n )
    {
        if ( op.equals( "+" ) )
            acc += n;
        else if ( op.equals( "-" ) )
            acc -= n;
        else if ( op.equals( "*" ) )
            acc *= n;
        else if ( op.equals( "/" ) )
            acc /= n;
        else if ( op.equals( "%" ) )
            acc %= n;
        else if ( op.equals( "=" ) )
            acc = n;
      //else if ( op.equals( "CE" ) )
      //   acc =
      else if ( op.equals( "CA" ) )
         acc = acc;
      //else if ( op.equals( "M+" ) )
      //   acc =
      //else if ( op.equals( "MC" ) )
      //   acc =
      //else if ( op.equals( "MRC" ) )
      //   acc =

        return acc;
    }
}

private int acc = 0;