Android（java）学习笔记116：PC_Phone通信程序报错

1. 首先我写的程序代码如下：

 package com.himi.udpsend;

 import java.net.DatagramPacket;
 import java.net.DatagramSocket;
 import java.net.DatagramSocketImpl;
 import java.net.InetAddress;
 import java.net.SocketException;
 import java.net.UnknownHostException;

 import android.app.Activity;
 import android.content.Context;
 import android.net.wifi.WifiInfo;
 import android.net.wifi.WifiManager;
 import android.os.Bundle;
 import android.util.Log;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.widget.Button;
 import android.widget.EditText;

 public class MainActivity extends Activity implements OnClickListener {
     private EditText edittext;
     private Button  btnsend;
     private WifiInfo wifiInfo; //类Wifiinfo，WifiInfo获取各种Wifi各种信息
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);

         edittext = (EditText)findViewById(R.id.edit);
         btnsend = (Button)findViewById(R.id.btnsend);

         btnsend.setOnClickListener(this);

         WifiManager wman = (WifiManager) getSystemService(Context.WIFI_SERVICE);

         if(! wman.isWifiEnabled()) {
             wman.setWifiEnabled(true);
         }
         Log.e("ip", intToIp(wifiInfo.getIpAddress()));//获得本机ip

     }

     public String intToIp(int i) {
         return (i & 0xff) +"."+
           ((i>>8) & 0xff )+"."+
           ((i>>16) & 0xff )+"."+
           ((i>>24) & 0xff );

     }

     public void onClick(View v) {

         try {
             // 创建发送端socket对象
             DatagramSocket ds = new DatagramSocket();

             // 创建数据并打包
             String edit_content = edittext.getText().toString();
             byte[] bys = edit_content.getBytes();
             int length = bys.length;

             String ip = intToIp(wifiInfo.getIpAddress());
             InetAddress hostip = InetAddress.getByName(ip);

             int port = 10086;
             DatagramPacket dp = new DatagramPacket(bys, length, hostip, port);

             //发送数据包
             ds.send(dp);
             //关闭套接字，回收资源
             ds.close();

         } catch (Exception e) {
             // TODO 自动生成的 catch 块
             e.printStackTrace();
         }

     }

 }

2. 报错如下：

06-26 02:37:29.270: E/Trace(841): error opening trace file: No such file or directory (2)
06-26 02:37:30.400: D/AndroidRuntime(841): Shutting down VM
06-26 02:37:30.400: W/dalvikvm(841): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
06-26 02:37:30.540: E/AndroidRuntime(841): FATAL EXCEPTION: main
06-26 02:37:30.540: E/AndroidRuntime(841): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.himi.udpsend/com.himi.udpsend.MainActivity}: java.lang.NullPointerException
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.os.Looper.loop(Looper.java:137)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.app.ActivityThread.main(ActivityThread.java:5041)
06-26 02:37:30.540: E/AndroidRuntime(841):     at java.lang.reflect.Method.invokeNative(Native Method)
06-26 02:37:30.540: E/AndroidRuntime(841):     at java.lang.reflect.Method.invoke(Method.java:511)
06-26 02:37:30.540: E/AndroidRuntime(841):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
06-26 02:37:30.540: E/AndroidRuntime(841):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
06-26 02:37:30.540: E/AndroidRuntime(841):     at dalvik.system.NativeStart.main(Native Method)
06-26 02:37:30.540: E/AndroidRuntime(841): Caused by: java.lang.NullPointerException
06-26 02:37:30.540: E/AndroidRuntime(841):     at com.himi.udpsend.MainActivity.onCreate(MainActivity.java:41)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.app.Activity.performCreate(Activity.java:5104)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
06-26 02:37:30.540: E/AndroidRuntime(841):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
06-26 02:37:30.540: E/AndroidRuntime(841):     ... 11 more
06-26 02:37:34.910: I/Process(841): Sending signal. PID: 841 SIG: 9

原因：空指针异常，这时就要查看自己的程序中是否存在空指针。

4. 解决办法是：

 package com.himi.udpsend;

 import java.net.DatagramPacket;
 import java.net.DatagramSocket;
 import java.net.DatagramSocketImpl;
 import java.net.InetAddress;
 import java.net.SocketException;
 import java.net.UnknownHostException;

 import android.app.Activity;
 import android.content.Context;
 import android.net.wifi.WifiInfo;
 import android.net.wifi.WifiManager;
 import android.os.Bundle;
 import android.util.Log;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.widget.Button;
 import android.widget.EditText;

 public class MainActivity extends Activity implements OnClickListener {
     private EditText edittext;
     private Button  btnsend;
     private WifiInfo wifiInfo;
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);

         edittext = (EditText)findViewById(R.id.edit);
         btnsend = (Button)findViewById(R.id.btnsend);

         btnsend.setOnClickListener(this);

         WifiManager wman = (WifiManager) getSystemService(Context.WIFI_SERVICE);

         if(! wman.isWifiEnabled()) {
             wman.setWifiEnabled(true);
         }//没有正确使用Log（）
         //Log.e("ip", intToIp(wifiInfo.getIpAddress()));//获得本机ip

     }

     public String intToIp(int i) {
         return (i & 0xff) +"."+
           ((i>>8) & 0xff )+"."+
           ((i>>16) & 0xff )+"."+
           ((i>>24) & 0xff );

     }

     public void onClick(View v) {

         try {
             // 创建发送端socket对象
             DatagramSocket ds = new DatagramSocket();

             // 创建数据并打包
             String edit_content = edittext.getText().toString();
             byte[] bys = edit_content.getBytes();
             int length = bys.length;

             String ip = intToIp(wifiInfo.getIpAddress());
             InetAddress hostip = InetAddress.getByName(ip);

             int port = 10086;
             DatagramPacket dp = new DatagramPacket(bys, length, hostip, port);

             //发送数据包
             ds.send(dp);
             //关闭套接字，回收资源
             ds.close();

         } catch (Exception e) {
             // TODO 自动生成的 catch 块
             e.printStackTrace();
         }

     }

 }

附加：

DatagramSocket：套接字用来UDP通信

DatagramPacket：用来包装UDP通信数据包

InetAddress：是Java对IP地址的封装