当我尝试将 listView 的最后一项中的文本设置为 MainActivity 中的 TextView 时遇到问题。它在“NullPointerException”运行时崩溃，因为我认为当我在 MainActivity 中调用它时，它没有完成在 ListView 上的下载，因此当 MainActivity 首次启动时，listView 没有完成他的工作和功能getCount()、getItem() 和我的函数 getLastElement() 仍然为空。 问题是我不太擅长处理线程(Wait()，notify()，..) 你能帮我解决这一切吗？
这是我的代码和 LogCat:
public class MainActivity extends Activity implements OnClickListener{
private static Context mContext;
public Button mExit, mHistory, mRating;
public TextView mSignal;
HistoryAdapt myAdapter;
HistoryItems m_myLastItem;
ArrayList<HistoryItems> m_myListItem;
Runnable m_run;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MainActivity.mContext=getApplicationContext();
mExit=(Button)findViewById(R.id.ExitButton);
mExit.setOnClickListener(this);
mHistory=(Button)findViewById(R.id.HistoryButton);
mHistory.setOnClickListener(this);
mRating=(Button)findViewById(R.id.RateButton);
mRating.setOnClickListener(this);
mSignal=(TextView)findViewById(R.id.SignalOfTheDayTV);
//### SET LAST ELEMENT INTO TEXTVIEW
m_myListItem = new ArrayList<HistoryItems>();
myAdapter= new HistoryAdapt(mContext, m_myListItem);
new Thread(
new Runnable(){
@Override
public void run(){
try{
///Need to wait until the data to be downloaded inside HistoryAdapt so it can show the last element from the ListView here
HistoryParser parser = new HistoryParser();
parser.parse(getInputStream(HistoryAct.RSS_LINK));
} catch (XmlPullParserException e) {
Log.w(e.getMessage(), e);
} catch (IOException e) {
Log.w(e.getMessage(), e);
} finally {
//notify that the data finished to download
MainActivity.this.runOnUiThread(
new Runnable(){
public void run(){
m_myLastItem = myAdapter.getLastElement();
//set last signal into TextView
mSignal.setText(m_myLastItem.getTitle());
}
}
);
}
}
}
).run();
//#RATER
// AppRater.app_launched(this);
// AppRater.showRateDialog(this, null);
//Get a Tracker (should auto-report)
((AppManager) getApplication()).getTracker(AppManager.TrackerName.APP_TRACKER);
}//oncreate
private InputStream getInputStream(String link) {
try {
URL url = new URL(link);
return url.openConnection().getInputStream();
} catch (IOException e) {
Log.w(Constants.DATA, "Exception while retrieving the input stream", e);
return null;
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
public static Context getAppContext(){
return MainActivity.mContext;
}
public void ExitState(){
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setTitle("You're about to quit Signals4Trading");
builder.setIcon(R.drawable.five);
//builder.setMessage("Your device has been registered successfully. You'll receive signals very soon.");
builder.setMessage("Are you sure you want to quit?");
builder.setCancelable(false);//can't click on the background of the activity
builder.setPositiveButton("Yes",new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
// TODO Auto-generated method stub
Toast.makeText(getApplicationContext(),"See you soon", Toast.LENGTH_LONG).show();
finish();
}//OnClickListener PositiveButton
});//anonymous class PositiveButton
builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
// TODO Auto-generated method stub
Toast.makeText(getApplicationContext(),"Enjoy your visit", Toast.LENGTH_LONG).show();
}
});
AlertDialog alertDialog = builder.create();
alertDialog.show();
}//ExitState
public void goToHistoryActivity(){
Intent intent = new Intent(MainActivity.this, HistoryAct.class );
startActivity(intent);
}
public void rateApp(){
Intent intent = new Intent(MainActivity.this, Rate.class);
startActivity(intent);
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
switch (v.getId()){
case R.id.ExitButton:
ExitState();
break;
case R.id.HistoryButton:
goToHistoryActivity();
break;
case R.id.RateButton:
rateApp();
break;
}
}
@Override
protected void onStart() {
// TODO Auto-generated method stub
super.onStart();
//Get an Analytics tracker to report app starts & uncaught exceptions etc.
GoogleAnalytics.getInstance(this).reportActivityStart(this);
}
@Override
protected void onStop() {
// TODO Auto-generated method stub
super.onStop();
//Stop the analytics tracking
GoogleAnalytics.getInstance(this).reportActivityStop(this);
}
}//MainActivity
package com.Signals4Trading.push.android;
import java.util.List;
import android.content.Context;
import android.content.Intent;
import android.os.StrictMode;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;
public class HistoryAdapt extends BaseAdapter {
private final List<HistoryItems>items;
private final Context context;
public HistoryAdapt(Context context,List<HistoryItems>items){
this.context=context;
this.items=items;
}//constructor
@Override
public int getCount() {
// TODO Auto-generated method stub
return items.size();
}
@Override
public Object getItem(int position) {
// TODO Auto-generated method stub
return items.get(position);
}
@Override
public long getItemId(int id) {
// TODO Auto-generated method stub
return id;
}
//###FUNCTION THAT RETURN LAST ELEMENT
public HistoryItems getLastElement(){
return items.get(items.size()-1);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
// TODO Auto-generated method stub
ViewHolder holder;
if (convertView == null) {
convertView = View.inflate(context, R.layout.historyitems, null);
holder = new ViewHolder();
holder.itemTitle = (TextView) convertView.findViewById(R.id.itemTitleTV);
holder.itemDate=(TextView) convertView.findViewById(R.id.itemDateTV);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
holder.itemTitle.setText(items.get(position).getTitle());
holder.itemDate.setText(items.get(position).getDate());
}
return convertView;
}
class ViewHolder {
TextView itemTitle;
TextView itemDate;
}
}
//HistoryAdapt
11-13 15:18:34.702: E/AndroidRuntime(7737): FATAL EXCEPTION: main
11-13 15:18:34.702: E/AndroidRuntime(7737): Process: com.Signals4Trading.push.android, PID: 7737
11-13 15:18:34.702: E/AndroidRuntime(7737): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.Signals4Trading.push.android/com.Signals4Trading.push.android.MainActivity}:
java.lang.ArrayIndexOutOfBoundsException: length=0; index=-1
11-13 15:18:34.702: E/AndroidRuntime(7737): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
最佳答案
Activity 中的 onCreate() 调用正在调用:
m_myLastItem= HistoryAdapt.getLastElement();
但是，HistoryAdapt 的构造函数尚未被调用，这就是您在其中初始化静态字段items 的地方。我建议重新考虑这个类及其用法，因为您以实例方式使用它，但将其部分编码为静态类。遵循的一个好的经验法则是不要使静态方法依赖于正在构造的类。那时它只是一个实例方法。
关于java - 线程问题 : NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26885693/