Java 为什么我在填充后得到空对象?
我从网站上得到一些输入流(即) 我有xml解析器,在一个类中,我通过解析这个流来生成自己的对象xml。如果我在那里检查一下,一切都很好。在我试图通过getter从另一个类获取这个对象之后,我得到了null指针异常,我不明白为什么 头等舱Java 为什么我在填充后得到空对象?,java,android,nullpointerexception,Java,Android,Nullpointerexception,我从网站上得到一些输入流(即) 我有xml解析器,在一个类中,我通过解析这个流来生成自己的对象xml。如果我在那里检查一下,一切都很好。在我试图通过getter从另一个类获取这个对象之后,我得到了null指针异常,我不明白为什么 头等舱 public class Connection { private final String URLSTRING = "http://blabla.html"; private Xml xml; private Context context; private
public class Connection {
private final String URLSTRING = "http://blabla.html";
private Xml xml;
private Context context;
private String result;
private boolean connection;
public Xml getXml() {
return xml;
}
public void setXml(Xml xml) {
this.xml = xml;
}
public String getResult() {
return result;
}
public void setResult(String result) {
this.result = result;
}
public boolean getConnection() {
return connection;
}
public void setConnection(boolean connection) {
this.connection = connection;
}
public Connection(Context context) {
this.context = context;
}
public void connect() {
ConnectivityManager connMgr = (ConnectivityManager)
context.getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
if (networkInfo != null && networkInfo.isConnected()) {
new DownloadWebpageTask().execute(URLSTRING);
connection = true;
} else {
connection = false;
}
}
private class DownloadWebpageTask extends AsyncTask<String, Void, String> {
@Override
protected String doInBackground(String... urls) {
// params comes from the execute() call: params[0] is the url.
try {
return downloadUrl(urls[0]);
} catch (IOException e) {
return "Unable to retrieve web page. URL may be invalid.";
}
}
// onPostExecute displays the results of the AsyncTask.
@Override
protected void onPostExecute(String result) {
result = "";
}
}
private String downloadUrl(String myurl) throws IOException {
InputStream is = null;
try {
URL url = new URL(myurl);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setDoInput(true);
// Starts the query
conn.connect();
int response = conn.getResponseCode();
//Log.d(DEBUG_TAG, "The response is: " + response);
is = conn.getInputStream();
String contentAsString ="aa";
XmlParser xmlParser = new XmlParser();
xml = xmlParser.getParsedXml(is);
return contentAsString;
} finally {
if (is != null) {
is.close();
}
}
}
尝试设置TextView时出现错误。
谢谢你的建议。
编辑:logcat
FATAL EXCEPTION: main
java.lang.NullPointerException
at com.example.blackess.connectiontest.MainActivity$1.onClick(MainActivity.java:48)
at android.view.View.performClick(View.java:2485)
at android.view.View$PerformClick.run(View.java:9080)
at android.os.Handler.handleCallback(Handler.java:587)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:130)
at android.app.ActivityThread.main(ActivityThread.java:3687)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:507)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
at dalvik.system.NativeStart.main(Native Method)
这是一个并发性问题 方法
connect()
:
不等待下载WebPagetTask
异步任务完成。它立即返回
你能做的就是做一个倾听者。将其放入连接中
public interface ConnectionListener {
public void onGotXML(XMl xml);
}
将其添加到构造函数中:
ConnectionListener listener;
public Connection(Context context, ConnectionListener listener) {
this.context = context;
this.listener = listener;
}
在postExecute
@Override
protected void onPostExecute(String result) {
listener.onGotXML(this.xml);
}
然后在活动中实现此侦听器:
Connection con;
TextView textView;
public void onCreate(){
...
textView = (TextView) findViewById(R.id.textView2);
con = new Connection(context, new Connection.ConnectionListener(){
@Override
public void onGotXML(XMl xml){
if(xml != null){
textView.setText(xml.getDate());
}else{
Toast.makeText(getApplicationContext(),
"no connection", Toast.LENGTH_LONG).show();
}
}
});
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
con.connect();
}
});
}
我的英语不好,如果你有问题,请评论 根据,您应该在UI线程中调用
downloadWebPagetTask
,换句话说,就是View.OnClickListener.onClick()
并让您的下载WebPagetAsk
类更新按钮的文本
我同意@InMystry提供实际错误将有助于人们在我编写textView.setText(xml.getDate());时,帮助人们了解
MainActivity
第48行的内容。在第一个类中,即使在调试模式下,mainActivity对象中的xml中也有所有值。xml为null。我应该怎么做才能让该任务完成加载?现在只有Toast位于错误的位置,但我可以修复它。完美的非常感谢。
@Override
protected void onPostExecute(String result) {
listener.onGotXML(this.xml);
}
Connection con;
TextView textView;
public void onCreate(){
...
textView = (TextView) findViewById(R.id.textView2);
con = new Connection(context, new Connection.ConnectionListener(){
@Override
public void onGotXML(XMl xml){
if(xml != null){
textView.setText(xml.getDate());
}else{
Toast.makeText(getApplicationContext(),
"no connection", Toast.LENGTH_LONG).show();
}
}
});
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
con.connect();
}
});
}