java - 记住玩家对象的先前位置

Question

好的，所以我正在尝试创建一个程序，使用扫描仪，如果用户键入“上”、“下”、“左”或“右”，形状将相应移动。我让形状按照单词所说的那样移动，但是每当我输入新命令时，我都无法让程序记住在给出新命令之前形状在哪里。每次我进入新方向时，形状都会从 0,0 开始。这是我到目前为止所拥有的，我相信问题出在我的 y 坐标上：

 double location = 0;
 double prev = 0;     
 for (double i = -10; i <= 10; i++) // for the scale that was set 
 {
  String word = in.nextLine(); // new string: word = user typed

  if (word.equals("up")) // if word = up, move player up
  {
    location ++;
    prev -= location;
    StdDraw.clear();
    drawTarget(a);
    StdDraw.setPenColor(StdDraw.BLUE);
    StdDraw.filledCircle(prev, location, 0.5);

  }

 else if (word.equals("down")) // if word = down, move player down
  {
    location--;
    prev += location;
    StdDraw.clear();
    drawTarget(a);
    StdDraw.setPenColor(StdDraw.BLUE);
    StdDraw.filledCircle(prev, location, 0.5);

  }
 }