实际上我以编程方式添加一些按钮没有任何问题。之后我需要重新创建按钮并删除旧按钮。我也为每个按钮设置了OnClickListener。我的问题是，当我回想起funcrion refresh（）并重新创建按钮，按钮成功添加，但当我点击按钮，程序崩溃！ ：|
Main Class:
package com.example.mysqlex;
import com.example.mysqlex.DbHandler;
import com.example.mysqlex.DbHelper;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.database.sqlite.SQLiteDatabase;
import android.graphics.Typeface;
import android.graphics.drawable.Drawable;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup.LayoutParams;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.ScrollView;
import android.widget.TextView;
import android.widget.Toast;
import android.content.Context;
@SuppressLint("ResourceAsColor")
public class Main extends Activity {
static String Totalresult;
static LinearLayout Llayout,mainRel;
static TextView titleTest;
private static Context context;
private Typeface font;
private DbHandler dbHandler=new DbHandler(this);
public DbHelper dbHelper=new DbHelper(this);
private SQLiteDatabase database;
public static boolean haveNetworkConnection() {
boolean haveConnectedWifi = false;
boolean haveConnectedMobile = false;
ConnectivityManager cm = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo[] netInfo = cm.getAllNetworkInfo();
for (NetworkInfo ni : netInfo) {
if (ni.getTypeName().equalsIgnoreCase("WIFI"))
if (ni.isConnected())
haveConnectedWifi = true;
if (ni.getTypeName().equalsIgnoreCase("MOBILE"))
if (ni.isConnected())
haveConnectedMobile = true;
}
return haveConnectedWifi || haveConnectedMobile;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
mainRel=(LinearLayout) findViewById(R.id.mainRel);
Typeface font = Typeface.createFromAsset(getAssets(),"fonts/yekan2.TTF");
TextView title=(TextView) findViewById(R.id.title);
title.setTypeface(font);
context=this;
if(dbHandler.getSettings("catlist", "value")=="null"){
Toast.makeText(getApplicationContext(), "اطلاعاتی ذخیره نشده است.لطفا نوسازی کنید...", 5000).show();
}else{
refresh("home");
}
}
public void refresh(String refreshType){
dbHandler=new DbHandler(context);
dbHelper=new DbHelper(context);
if(refreshType=="button"){
if(dbHandler.getSettings("catlist", "value")=="null"){
dbHandler.insertSettings("catlist", Totalresult, "");
}else{
dbHandler.updateSettings("catlist", Totalresult, "");
}
}else if(refreshType=="home"){
Totalresult=dbHandler.getSettings("catlist", "value");
}
Typeface font = Typeface.createFromAsset(context.getAssets(),"fonts/yekan2.TTF");
String[] childs=Totalresult.split("/.../<br />");
mainRel.removeAllViews();
for(int i=0; i<childs.length; i++){
String[] parts=childs[i].split("<br />");
final String[] values=parts[1].split("->");
final String[] id=parts[0].split("->");
Button btn = new Button(context);
LinearLayout Llayout = new LinearLayout(context);
btn.setBackgroundResource(R.drawable.cat_drawable);
btn.setHeight(110); btn.setTextSize(24); btn.setTypeface(font); btn.setTextColor(R.color.white);
btn.setId(i);
btn.setOnClickListener(new OnClickListener() {
public void onClick(View arg0) {
Intent goToContent = new Intent(Main.this, Content.class);
goToContent.putExtra("catid", id[1]);
goToContent.putExtra("pageTitle", values[1]);
startActivity(goToContent);
}
});
LayoutParams btnLp = new LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT);
LinearLayout.LayoutParams LLp = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT);
LLp.setMargins(0, 5, 0, 5);
btn.setLayoutParams(btnLp);
Llayout.setLayoutParams(LLp);
btn.setText(values[1]);
Llayout.addView(btn);
mainRel.addView(Llayout);
}
}
public void refresh_btn(View v) {
if(haveNetworkConnection()){
SigninActivity.classReferrer="Main";
new SigninActivity(this,0).execute("site_id","1","catlist","1","data_type","echo(/.../)");
}else{
Toast.makeText(getApplicationContext(), "لطفا جهت نوسازی اطلاعات،به اینترنت متصل شوید", Toast.LENGTH_LONG).show();
}
}
}
SigninActivity Class :
package com.example.mysqlex;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.lang.ref.WeakReference;
import java.lang.reflect.Array;
import java.net.URI;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLEncoder;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import android.app.ProgressDialog;
import android.content.Context;
import android.os.AsyncTask;
import android.widget.TextView;
import android.widget.Toast;
public class SigninActivity extends AsyncTask<String,Void,String>{
private ProgressDialog pdia;
String url="http://www.pbroilgas.com/index2.php";
private TextView roleField;
private Context context;
private int byGetOrPost = 0;
static public String results,classReferrer;
static public String[] resultArray;
//flag 0 means get and 1 means post.(By default it is get.)
public SigninActivity(Context context,int flag) {
this.context = context;
byGetOrPost = flag;
}
protected void onPreExecute(){
pdia = new ProgressDialog(context);
pdia.setMessage("در حال نوسازی اطلاعات...");
pdia.setProgressStyle(pdia.STYLE_SPINNER);
pdia.show();
super.onPreExecute();
}
@Override
protected String doInBackground(String... arg0) {
if(byGetOrPost == 0){ //means by Get Method
try{
String command1 = (String)arg0[0];
String value1 = (String)arg0[1];
StringBuilder link = new StringBuilder();
link.append(url+"?"+command1+"="+value1);
for(int i=2; i<arg0.length; i+=2){
link.append("&"+arg0[i]);
link.append("="+arg0[i+1]);
}
URL url = new URL(link.toString());
HttpClient client = new DefaultHttpClient();
HttpGet request = new HttpGet();
request.setURI(new URI(link.toString()));
HttpResponse response = client.execute(request);
BufferedReader in = new BufferedReader
(new InputStreamReader(response.getEntity().getContent()));
StringBuffer sb = new StringBuffer("");
String line;
while ((line = in.readLine()) != null) {
sb.append(line);
}
in.close();
return sb.toString();
}catch(Exception e){
return new String("Exception: " + e.getMessage());
}
}
else{
try{
String command = (String)arg0[0];
String value = (String)arg0[1];
String link=url;
String data = URLEncoder.encode(command, "UTF-8")
+ "=" + URLEncoder.encode(value, "UTF-8");
URL url = new URL(link);
URLConnection conn = url.openConnection();
conn.setDoOutput(true);
OutputStreamWriter wr = new OutputStreamWriter
(conn.getOutputStream());
wr.write( data );
wr.flush();
BufferedReader reader = new BufferedReader
(new InputStreamReader(conn.getInputStream()));
StringBuilder sb = new StringBuilder();
String line = null;
// Read Server Response
while((line = reader.readLine()) != null)
{
sb.append(line);
break;
}
return sb.toString();
}catch(Exception e){
return new String("Exception: " + e.getMessage());
}
}
}
@Override
protected void onPostExecute(String result){
pdia.dismiss();
results=result;
if(classReferrer=="Main"){
Main main= new Main();
Main.Totalresult=result.toString();
main.refresh("button");
}else if(classReferrer=="Content"){
Content content= new Content();
Content.Totalresult=result.toString();
content.refresh("button");
}
}
}
我的LogCat：
08-11 02:12:55.839: E/AndroidRuntime(8853): FATAL EXCEPTION: main
08-11 02:12:55.839: E/AndroidRuntime(8853): java.lang.NullPointerException
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.content.ContextWrapper.getPackageName(ContextWrapper.java:127)
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.content.ComponentName.<init>(ComponentName.java:75)
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.content.Intent.<init>(Intent.java:3350)
08-11 02:12:55.839: E/AndroidRuntime(8853): at com.example.mysqlex.Main$1.onClick(Main.java:99)
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.view.View.performClick(View.java:4147)
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.view.View$PerformClick.run(View.java:17161)
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.os.Handler.handleCallback(Handler.java:615)
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.os.Handler.dispatchMessage(Handler.java:92)
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.os.Looper.loop(Looper.java:213)
08-11 02:12:55.839: E/AndroidRuntime(8853): at android.app.ActivityThread.main(ActivityThread.java:4786)
08-11 02:12:55.839: E/AndroidRuntime(8853): at java.lang.reflect.Method.invokeNative(Native Method)
08-11 02:12:55.839: E/AndroidRuntime(8853): at java.lang.reflect.Method.invoke(Method.java:511)
08-11 02:12:55.839: E/AndroidRuntime(8853): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
08-11 02:12:55.839: E/AndroidRuntime(8853): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
08-11 02:12:55.839: E/AndroidRuntime(8853): at dalvik.system.NativeStart.main(Native Method)
我很感激你的帮助......
提前谢谢......
答案 0 :(得分：1)
错误表明它无法获取包名称。所以你的Main.this由于某种原因在那时是无效的。将其更改为：
public void onClick(View arg0) {
Intent goToContent = new Intent(arg0.getContext(), Content.class);
goToContent.putExtra("catid", id[1]);
goToContent.putExtra("pageTitle", values[1]);
startActivity(goToContent);
}
您的上下文引用无效，但调用该方法的视图永远不会无效。
答案 1 :(得分：0)
查看Intent(Context, Class<?>) ...
public Intent(Context packageContext, Class<?> cls) {
mComponent = new ComponentName(packageContext, cls);
}
...然后查看ComponentName(Context, Class<?>)构造函数的源代码......
public ComponentName(Context pkg, Class<?> cls) {
mPackage = pkg.getPackageName();
mClass = cls.getName();
}
...使用Intent(Context, Class<?>)投放NPE的唯一方法是mPackage = pkg.getPackageName(); - 换句话说，Main.this为空。
查看更新的代码，我可以看到原因。在您onPostExecute(...)的{{1}}的{{1}}中，您正在执行以下操作...
AsyncTask无法使用SigninActivity创建Main main= new Main();
的实例 - 它根本不起作用，这就是Activity为new的原因。
您将不得不重新考虑事情，因为您永远不应尝试从任何其他应用程序组件访问Main.this的任何变量或方法。
如果您需要从null访问Activity中的方法，最简单的方法是使Activity成为AsyncTask本身的内部类。