Question

我收到一个错误，表示为Java Null Point Exception。我试过调试，发现错误只是持久存在。我认为printwriter或客户端对象存在问题。

是否有任何初始化错误？

我甚至关闭了我打开的每个插座。

是不是我在其他地方放了几行？代码是：

    package com.example.temp;

    public class MainActivity extends Activity {
        private Socket client;
        private PrintWriter printwriter;
        private EditText textField;
        private Button button;
        private String messsage;

        StringBuilder sb = new StringBuilder();
        public final static String extra = "com.example.temp.MESSAGE";  
        protected static final long TIME_DELAY = 5000;
        TextView mTextView;
        Handler handler=new Handler();  
        int count =0; String data ="";

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            mTextView = (TextView) findViewById(R.id.text_id);
            messsage= mTextView.getText().toString();
            new Asynctask().execute(messsage);  
            handler.post(updateTextRunnable);       
        }

        Runnable updateTextRunnable = new Runnable() {
            public void run() {
                if (count < 5) {
                    WifiManager mainWifiObj;
                    mainWifiObj = 
                        (WifiManager) getSystemService(Context.WIFI_SERVICE);

                    class WifiScanReceiver extends BroadcastReceiver {
                        public void onReceive(Context c, Intent intent) {
                        }
                    }

                    WifiScanReceiver wifiReciever = new WifiScanReceiver();
                    registerReceiver(wifiReciever, new IntentFilter(
                        WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));                
                    List<ScanResult> wifiScanList = 
                        mainWifiObj.getScanResults();
                    for (ScanResult result : wifiScanList) {
                        if (result.SSID.equals("Khosla ka Ghosla")) {
                            sb.append(result.level);
                        }               
                        count++;
                        mTextView.setText("getting called " +count + sb);                   
                    } else {
                }                                
                handler.postDelayed(this, TIME_DELAY);
            }
        };  

        public class Asynctask extends AsyncTask<String, Void, Void> {
            private static final String IP_ADDRESS = "192.168.0.8";
            private static final int DEST_PORT = 4444;

            private EditText mTextField;

            protected Void doInBackground(String... messages) {
                String message = messages[0];
                Socket client = null;
                try {
                    client = new Socket(IP_ADDRESS, DEST_PORT); // connect to server
                } catch (UnknownHostException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }

                // Write to server.
                try {
                    printwriter = 
                        new PrintWriter(client.getOutputStream(), true);
                    printwriter.write(messsage); // write the message to output stream
                }
                catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    try {                   
                        if(client !=null){
                            printwriter.flush();
                            printwriter.close();
                            client.close();
                        }
                    } catch (IOException e) { 
                        e.printStackTrace();
                    }
                }
                return null;
            }
        }
    }

当我运行当前代码时，错误出现在以下行中：printwriter.write(messsage)。你能指导我吗？

提前致谢

Answer 1

您应该在finally块中执行此操作。

if (printwriter != null) {
  printwriter.flush();
  printwriter.close();
}

Answer 2

您确实需要调试代码。我可以看到几个可能的问题

String message = messages [0];

您确定该消息是否已正确创建？最有可能的不是问题，因为您没有收到索引超出范围的异常。但无论如何都要另外看。

client = new Socket（IP_ADDRESS，DEST_PORT）; //连接到服务器

运行此行代码后“客户端”的价值是多少？它是空的吗？

printwriter = new PrintWriter(client.getOutputStream(), true)

您确定是否已创建了该打印作品？

Java.lang.NullPointerException代码中的错误

2 个答案: