javasocket上的NullPointerException

Question

当我尝试创建socket连接时，我得到一个返回NullPointerException，当我尝试在主类上打印socket.isConnected()时，它返回true，但当我尝试在另一个方法上再次打印它时，它返回NullPointerException，这是我的代码

服务器

ServerSocket socketServer = null;
Socket socketConnect = null;

public static void main(String[] args) throws IOException {
    ChatServer cc = new ChatServer();
    cc.socketServer = new ServerSocket(2000);
    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new ChatServer().setVisible(true);
        }
    });
        cc.socketConnect = cc.socketServer.accept();
        System.out.println(cc.socketConnect.isConnected());
}

public void send(String msg) throws IOException {
    System.out.println(this.socketConnect.isConnected());
}

此代码将首先返回true，因为socket。isConnected（）正在处理main，但不处理send方法

Answer 1

实际上它是NullPointerException，因为 socketConnect从未初始化过，所以必须初始化它，您可以在main方法中使用这些变量，当然您必须将它们声明为 static

// static variable
public static ServerSocket socketServer = null;
public static Socket socketConnect = null;

public static void main(String[] args) throws IOException {
    ChatServer cc = new ChatServer();
    cc.socketServer = new ServerSocket(2000);
    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new ChatServer().setVisible(true);
        }
    });
    cc.socketConnect = cc.socketServer.accept();
    System.out.println(cc.socketConnect.isConnected());
    
    // instance
    socketServer = cc.socketServer;
    socketConnect = cc.socketConnect;
}

// now you can use socketConnect, cause you did init in main at the final from the main method
public void send(String msg) throws IOException {
    System.out.println(this.socketConnect.isConnected());
}

gl
对不起，我的英语不好