我是13年级的学生，在我的计算机科学课程中，我决定用java制作一个小型聊天程序。我不得不从头开始学习网络方面的知识，而我的老师对此并不熟悉，所以他们很难回答我的问题。我有一个客户端程序和一个服务器程序，它们通过我创建的套接字进行通信，我的问题是我不理解尝试捕获对我的代码的影响。我真的很着迷于网络，这就是为什么我选择它，因为我很清楚这将是一个挑战，我会把我的代码和我遇到的错误留在下面。如果你能给我一些技巧，告诉我如何解决我的问题，并让我的代码"更好"，那将是非常棒的。此外，请考虑到我只知道java大约一年了，而且我还是一个新手，这是我关于堆栈溢出的第一个问题！。非常感谢！
客户代码：
/*
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* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package oclient;
import java.net.*;
import java.io.*;
import com.dosse.upnp.UPnP;
import java.util.Scanner;
/**
*
* @author sgmud
*/
public class CClient {
public Socket CSocket;
public PrintWriter out;
public BufferedReader in;
public int GetPort(){
Scanner scan = new Scanner(System.in);
System.out.println("please enter the port number");
int PortNum = scan.nextInt();
return PortNum;
}
public String GetAddress(){
Scanner scan = new Scanner(System.in);
System.out.println("Please enter the IP address of the host");
String Address = scan.nextLine();
return Address;
}
public void StartConnection(String ip, int port){
try{
CSocket = new Socket(ip, port);
out = new PrintWriter(CSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(CSocket.getInputStream()));
}catch(IOException e){
System.out.println("ERROR");
}
}
public void sendMessage(String msg){
//String Response;
out.println(msg); // outputs message
//try{
// Response = in.readLine();
//}catch(IOException e){
// System.out.println("ERROR");
//}
//System.out.println(Response);
//return Response;
}
public String receveMessage(){
String Response = "IfYouReadThisThetryCatchIsNotWorkingHowYouIntendItTo";
try{
Response = in.readLine();
}catch(IOException e){
System.out.println("Error");
}finally{
System.out.println(Response);
return Response;
}
}
public void convosation(){ // method will keep letting you send a message untill you stop
CClient client = new CClient();
while (true){
Scanner scan = new Scanner(System.in);
System.out.println("Type QUIT to end the convosation or press any key to send a message");
String qit = scan.nextLine();
if("QUIT".equals(qit)){
client.STOP();
}
else{
client.sendMessage(client.Message()); // Runs the send message method with the output from the Message method
client.receveMessage();
}
}
}
public String Message(){
Scanner scan = new Scanner(System.in);
System.out.println("please enter a mesasge to be sent");
String message = scan.nextLine();
return message;
}
public void STOP(){
try{
in.close();
out.close();
CSocket.close();
}catch(IOException e){
System.out.println("ERROR");
}
}
/**
*
*/
public static void main(String[] args){
CClient client = new CClient(); // Making a new client class
client.StartConnection(client.GetAddress(), client.GetPort()); // runs the startConnection method but runs the Get address and Get port method first so the Start connection method has the IP and Port number
client.convosation();
// client.STOP(); // runs the stop method which will terminate the server
}
}
服务器代码：
/*
* To change this license header, choose License Headers in Project Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package oserver;
import java.net.*;
import java.io.*;
import com.dosse.upnp.UPnP;
import java.util.Scanner;
/**
*
* @author sgmud
*/
public class SServer { // Server Class
public ServerSocket SSocket;
public Socket CSocket;
public PrintWriter out;
public BufferedReader in;
public int GetPort(){ // Gets port number for socket to be set listening to
Scanner scan = new Scanner(System.in);
System.out.println("please enter the port number");
int PortNum = scan.nextInt();
return PortNum;
}
public void start(int port) { // Starts the server with the collected port
try{
System.out.println("Server Started");
UPnP.openPortTCP(port);
SSocket = new ServerSocket(port);
CSocket = SSocket.accept();
System.out.println("Server Connected");
out = new PrintWriter(CSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(CSocket.getInputStream()));
String input;
while ((input = in.readLine()) != null){
if(".".equals(input)){
out.println("goodbye");
break;
}
else{
out.println(input);
}
}
}catch(IOException e){
System.out.println("ERROR");
}
}
public void stop(){ // Will close all connections
try{
//in.close();
out.close();
CSocket.close();
SSocket.close();
}catch(IOException e){
System.out.println("ERROR");
}
}
public static void main(String[] args){
SServer server = new SServer(); // Creat new server class
server.start(server.GetPort()); // Starts the server with the port number
}
}
错误(这是在客户端(：
run:
Please enter the IP address of the host
0.0.0.0
please enter the port number
6666
Type QUIT to end the convosation or press any key to send a message
h
please enter a mesasge to be sent
hello
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.io.PrintWriter.println(String)" because "this.out" is null
at oclient.CClient.sendMessage(CClient.java:54)
at oclient.CClient.convosation(CClient.java:94)
at oclient.CClient.main(CClient.java:125)
您的问题似乎不是try-catch，而是缺乏错误处理。
如果我们假设sendMessage中注释掉的代码实际上是可操作的：
try{
Response = in.readLine();
}catch(IOException e){
System.out.println("ERROR");
}
System.out.println(Response);
return Response;
如果在"try"中执行的代码出现异常，则会打印出ERROR，这就是您所要做的。
然后执行"向下翻页"，您尝试打印并返回Response的值。但"回应"从未被分配；try块内的分配从未完成。因此，响应仍然具有它在sendMessage条目中的值，该值可能为null。
如果要在sendMessage中捕获错误异常，则(a(需要编写代码来处理错误，(b(sendMessage需要一种指示失败的方法。也许您让函数返回response表示成功，null表示失败。然后，调用者必须意识到返回null是可能的。
try{
Response = in.readLine();
}catch(IOException e){
System.out.println("ERROR");
return null;
}
System.out.println(Response);
return Response;
或者，您可以不在那里捕获错误，但允许它传播出去。也就是说，sendMessage的调用者必须知道sendMessage可以抛出异常，并处理它，或者让它传播出去。在哪里处理异常是合适的决定，需要在更广泛的基础上考虑，而不仅仅是一种方法。
顺便说一句，Stack Overflow上的许多帖子都显示了相同的"打印并继续"异常处理模式。这很少是个好办法。