我正在为我的Android手机编写一段代码，该代码将使用WifiDevices在该区域中创建personal scan的列表。
我在创建列表时收到java.lang.NullPointerException。
我的代码如下：
public static synchronized void addDevice(DeviceInformation device, View v, Context con, boolean bool, int type) throws IOException {
Log.v("addDevice", "Called");
if (bool) {
TableLayout tb = (TableLayout) v.findViewById(R.id.DeviceList);
LayoutParams layout = new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);
TableRow tr = new TableRow(con);
TextView tv = new TextView(con);
System.out.println(v.toString());
tv.setLayoutParams(layout);
tr.setLayoutParams(layout);
String message;
Log.v("addDevice","Device Timeout");
switch(type) {
case 1:
computerEnd = true;
break;
case 2:
raspberryEnd = true;
break;
case 3:
flyportEnd = true;
break;
}
if (computerEnd && raspberryEnd && flyportEnd) {
if (rowCounter > 0) {
message = "No More Devices";
} else {
message = "No Devices Found";
}
tv.setText(message);
tv.setTextColor(Color.WHITE);
if (rowCounter % 2 == 0) {
tr.setBackgroundColor(Color.DKGRAY);
} else {
tr.setBackgroundColor(Color.GRAY);
}
tv.setVisibility(1);
tr.addView(tv);
tb.addView(tr); //This is line number 131
}
} else {
TableLayout tb = (TableLayout) v.findViewById(R.id.DeviceList);
LayoutParams layout = new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);
TableRow tr = new TableRow(con);
TextView tv = new TextView(con);
tv.setLayoutParams(layout);
tr.setLayoutParams(layout);
Log.v("addDevice", "Received");
String textToDisplay = device.getDeviceTypeString() + "\n" + device.getIPAddress(); //Write the text to display
tv.setText(textToDisplay);
tv.setTextColor(Color.WHITE);
Drawable img;
if (device.getDeviceType() == 1) {
img = con.getResources().getDrawable(R.drawable.pc);
} else if (device.getDeviceType() == 2) {
img = con.getResources().getDrawable(R.drawable.raspberry);
} else {
img = con.getResources().getDrawable(R.drawable.flyport);
}
img.setBounds(0,0,70,45);
tv.setCompoundDrawables(null, null, img, null);
tv.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
//empty
}
});
if (rowCounter % 2 == 0) {
tr.setBackgroundColor(Color.DKGRAY);
} else {
tr.setBackgroundColor(Color.GRAY);
}
rowCounter++;
Log.v("Result", "Device Added");
}
}
我的Logcat错误：
05-11 05:25:07.500: E/AndroidRuntime(30710): FATAL EXCEPTION: Thread-20873
05-11 05:25:07.500: E/AndroidRuntime(30710): java.lang.NullPointerException
05-11 05:25:07.500: E/AndroidRuntime(30710): at com.example.devicecontrolpanel.DeviceManagerWindow.addDevice(DeviceManagerWindow.java:131)
05-11 05:25:07.500: E/AndroidRuntime(30710): at com.connection.NetworkScanListenerRaspberry.run(NetworkScanListenerRaspberry.java:62)
05-11 05:25:07.500: E/AndroidRuntime(30710): at java.lang.Thread.run(Thread.java:864)
这是达到此特定代码的方式：
此代码调用Thread，然后调用此方法。
public void searchDevice(View view) throws IOException, InterruptedException
{
try
{
Thread ComputerListener = new Thread(new NetworkScanListenerComputer(getApplicationContext(),view));
ComputerListener.start();
}
catch(Exception e)
{
Log.v("Exception:","Exception from SearchDevice Method"+e.toString());
}
}
这是Thread的代码：
package com.connection;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import android.content.Context;
import android.util.Log;
import android.view.View;
import com.example.devicecontrolpanel.DeviceManagerWindow;
public class NetworkScanListenerComputer implements Runnable
{
View thisView;
Context thisContext;
MulticastSocket socket = null;
DatagramPacket inPacket = null;
byte[] inBuf;
public NetworkScanListenerComputer(Context con, View v)
{
thisView = v;
thisContext = con;
try
{
socket = new MulticastSocket(WifiConstants.COMPUTER_RECV_PORT);
socket.joinGroup(InetAddress.getByName(WifiConstants.COMPUTER_NETWORK_ADDR));
inBuf = new byte[1024];
inPacket = new DatagramPacket(inBuf, inBuf.length);
socket.setSoTimeout(1*60*1000);
}
catch(Exception ioe)
{
Log.v("Exeception:","Computer Listener Exception"+ioe);
}
}
@Override
public void run()
{
try
{
while(true)
{
System.out.println("Listening...");
socket.receive(inPacket);
System.out.println("Received");
String msg = new String(inBuf, 0, inPacket.getLength());
DeviceInformation device = new DeviceInformation(1, msg, inPacket.getAddress().toString());
DeviceManagerWindow.addDevice(device, thisView, thisContext, false, 1);
Log.v("Received:","Received Computer From :" + inPacket.getAddress() + " Msg : " + msg);
//System.out.write(inPacket.getData(),0,inPacket.getLength());
System.out.println();
Thread.sleep(2000);
}
}
catch(Exception e)
{
Log.v("Exception:","During Receiving Computer: "+e.toString());
try
{
DeviceManagerWindow.addDevice(null, thisView, thisContext, true, 1);
}
catch (IOException e1)
{
Log.v("Exception:", "Computer End Error: " +e1);
}
}
finally
{
socket.close();
}
}
}
我正在传递应用程序的view和context，并再次重定向到static method，因为静态方法无法使用它们。
我想出了问题...它说... 只有创建视图层次结构的原始线程才能触及其视图。
创建视图的线程（DeviceManagerWindow.java）只能访问R.id.deviceList。那么如果我想访问它，我该怎么办？
答案 0 :(得分：1)
嗯，你应该做的第一件事是调试，以获得你的空指针异常的确切位置。但是如果你仔细看看你的代码，那么有两个非常可能的空指针异常，第一个可能是：
TableLayout tb = (TableLayout) v.findViewById(R.id.DeviceList);
因为View v未找到任何带有id ==“DeviceList”的视图
或在这里：
if(device.getDeviceType()==1)
因为设备在您的方法“null”中传递。 所以，看看这两个。 祝你好运
答案 1 :(得分：1)
问题似乎是您尝试从后台UI更新Thread。您可以使用runOnUiThread()，但我认为更容易使Thread成为AsyncTask。然后，您可以执行doInBacground()中的所有网络内容并更新UI中的onPostExecute()
如果这是一个单独的文件而不是内部类，那么您可以将context传递给构造函数，以便更新UI。或者您可以返回一个值，告诉调用Activity做某事或不做事。我不确定，但看起来你可能会一次调用这个。如果是这样的话，我会建议在UI做某事的时候调用一个任务让所有设备都在后台运行，然后从那里将它们添加到列表中。另一件事是你的loop看起来可能有while true Thread。如果是这种情况，我会将其更改为侦听要更改的值的内容。希望这有帮助