当我尝试发送到服务器时，为什么我的getter返回null

Question

我知道这个问题在这里被问了无数次，我已经在SO和其他来源中搜索了解决方案，但我就是无法解决这个错误。我的ClientPlayer类属性有一个getter和setter，当调用某个按钮时，会在GUI中调用setter，我希望在客户端连接并将对象发送到服务器后使用getter。在ClientController中调用方法"this.client.sendTCP(clientPlayer.getPlayerName())；"会返回nullPointerException。

错误："JavaFX应用程序线程"java.lang.IollegalArgumentException：对象不能为null。

我的猜测是，我创建了一个新的ClientPlayer实例，次数太多了，因此返回null，因为我的字符串playerName最初设置为none。然而，我不确定如何着手解决这个问题。我真的很感激任何帮助。

public class ClientPlayer implements Serializable {
public ClientPlayer() {
}
private String playerName;
public void setPlayerName(String playerName) {
this.playerName = playerName;
}
public String getPlayerName() {
return this.playerName;
}
}  

在我的GUI代码的相关部分下面，我设置了一个按钮将连接到服务器：

ClientPlayer clientPlayer = new ClientPlayer();  
clientToGame.setOnAction((ActionEvent w) -> {
clientPlayer.setPlayerName(clientNameText.getText());
clientController.connect();
window.setScene(lobbyScene);
});

这是我的客户端类的一部分，我试图在其中获取名称并发送到服务器：

public class ClientController() {
ClientPlayer clientPlayer = new ClientPlayer();
public void connect() {
if (client.isConnected()) {
Logger.getLogger(getClass().getName()).log(Level.INFO, "You are      already connected to :{0}", config.getHost());
return;
}
this.client.start();
try {
this.client.connect(5000, config.getHost(), config.getTCPPort());
System.out.println("Successfully connected to " + config.getHost());
this.client.sendTCP(clientPlayer.getPlayerName());
} catch (IOException ex) {
Logger.getLogger(getClass().getName()).log(Level.SEVERE, "Server connection failed: {0}", ex.getMessage());
throw new RuntimeException(ex);
}
MessageRegistry.registerMessages(client.getKryo());
this.client.addListener(listener);
}
}

Answer 1

在GUI中，创建一个ClientPlayer实例并在其上调用setPlayerName()。然后在ClientController中，创建新的ClientPlayer实例(在该实例上从未调用setPlayerName())并在其上面调用getPlayerName()。由于从未为该实例设置玩家名称，因此getPlayerName()当然会返回null。

您需要决定由谁负责"拥有"ClientPlayer实例。如果是ClientController的责任，则在ClientController中添加getClientPlayer()方法，然后进行

clientToGame.setOnAction((ActionEvent w) -> {
clientController.getClientPlayer().setPlayerName(clientNameText.getText());
clientController.connect();
window.setScene(lobbyScene);
});

并从GUI类中完全删除ClientPlayer。如果GUI类负责拥有它，则将对它的引用传递给connect()方法，并从控制器中删除ClientPlayer字段：

public class ClientController() {
// ClientPlayer clientPlayer = new ClientPlayer();
public void connect(ClientPlayer clientPlayer) {
if (client.isConnected()) {
Logger.getLogger(getClass().getName()).log(Level.INFO, "You are      already connected to :{0}", config.getHost());
return;
}
this.client.start();
try {
this.client.connect(5000, config.getHost(), config.getTCPPort());
System.out.println("Successfully connected to " + config.getHost());
this.client.sendTCP(clientPlayer.getPlayerName());
} catch (IOException ex) {
Logger.getLogger(getClass().getName()).log(Level.SEVERE, "Server connection failed: {0}", ex.getMessage());
throw new RuntimeException(ex);
}
MessageRegistry.registerMessages(client.getKryo());
this.client.addListener(listener);
}
}

当然还有

clientToGame.setOnAction((ActionEvent w) -> {
clientPlayer.setPlayerName(clientNameText.getText());
clientController.connect(clientPlayer);
window.setScene(lobbyScene);
});

Answer 2

丑陋但应该工作：

使ClientPlayer访问为静态。在您的gui类中：

public static ClientPlayer clientPlayer = new ClientPlayer();

然后您可以通过访问ClientController中的同一对象

...
this.client.sendTCP(MyGuiClass.clientPlayer.getPlayerName());
...

编辑：问题是，您不会像在GUI类中那样通过ClientController中的clientPlayer引用"同一"对象。您应该以某种方式将该对象移交或使用静态引用。您也可以将它交给ClientController的构造函数。

第2版：

你应该怎么做

ClientPlayer clientPlayer = new ClientPlayer();  
clientToGame.setOnAction((ActionEvent w) -> {
clientPlayer.setPlayerName(clientNameText.getText());
clientController.connect(clientPlayer);
window.setScene(lobbyScene);
});

在您的ClientController中：

public void connect(ClientPlayer player) {
this.clientPlayer = player;
if (client.isConnected()) {
Logger.getLogger(getClass().getName()).log(Level.INFO, "You are already connected to :{0}", config.getHost());
return;
}
this.client.start();
try {
this.client.connect(5000, config.getHost(), config.getTCPPort());
System.out.println("Successfully connected to " + config.getHost());
this.client.sendTCP(player.getPlayerName());
} catch (IOException ex) {
Logger.getLogger(getClass().getName()).log(Level.SEVERE, "Server connection failed: {0}", ex.getMessage());
throw new RuntimeException(ex);
}
MessageRegistry.registerMessages(client.getKryo());
this.client.addListener(listener);
}