java - 如何从不同的类访问变量

Question

我有 2 个类，分别是 Game.java 和 KeyInput.java。如何从 Game.java 访问 int x 和 int y 并在 KeyInput.java 中使用？

游戏.java

public class Game extends JFrame {

  int x, y;

  //Constructor
  public Game(){
    setTitle("Game");
    setSize(300, 300);
    setResizable(false);
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.addKeyListener(new KeyInput());

    x = 150;
    y = 150;
  }

  public void paint(Graphics g){
    g.fillRect( x, y, 15, 15);
  }

  public static void main(String [] args){
    new Game();
  } 
}

KeyInput.java

public class KeyInput extends KeyAdapter {

  public void keyPressed(KeyEvent e) {
    int keyCode = e.getKeyCode();

    if(keyCode == e.VK_W)
      y--; //Error here saying "y cannot be resolved to a variable"
  }
}

Answer 1

您遇到的问题与范围有关。有很多方法可以解决这个问题，例如使用静态变量或将指针传递给包含您要访问的变量的对象。我给你两个。

静态:不推荐，但适用于小程序。你只能有一组 x 和 y。如果您有两个 Game 实例，它们将共享相同的值。

public class Game extends JFrame {
  //make this static and public so it can be accessed anywhere.
  public static int x, y;
  ...
  }
  ...
  public void keyPressed(KeyEvent e){
    int keyCode = e.getKeyCode();

    if(keyCode == e.VK_W)
      Game.y--; //Static access
  }

传入方式:

public class KeyInput extends KeyAdapter {
  Game game; //need a pointer to the original class object that holds x and y. Save it here
  public KeyInput(Game g){ //get the object pointer when this class is created.
    this.game = g;
  }

  public void keyPressed(KeyEvent e){
    int keyCode = e.getKeyCode();

    if(keyCode == e.VK_W)
       game.y--; //now a local variable we can access
  }
}


public class Game extends JFrame {
  //make these public
  public int x, y;

  //Constructor
  public Game(){
    setTitle("Game");
    setSize(300, 300);
    setResizable(false);
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.addKeyListener(new KeyInput(this)); //pass the pointer in here 
...