Java 线程问题：NullPointerException

当我试图将listView最后一项中的文本设置为MainActivity中的文本视图时，出现了一个问题。它在运行“NullPointerException”时崩溃，因为我认为在MainActivity中调用它时，它没有完成在ListView上的下载，所以当MainActivity首次启动时，ListView没有完成他的工作，函数getCount（）、getItem（）和我的函数getLastElement（）仍然为null。 问题是我不太擅长处理线程（Wait（），notify（），…） 你能帮我做这一切吗

这是我的代码和日志：

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)

public类MainActivity扩展活动实现OnClickListener{
私有静态上下文mContext；
公共按钮mExit、mHistory、mRating；
公共文本视图mSignal；
历史适应；
历史项目m_myLastItem；
ArrayList m_Mylistem；
可运行的m_运行；
@凌驾
创建时受保护的void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
setContentView（R.layout.activity_main）；
MainActivity.mContext=getApplicationContext（）；
mExit=（按钮）findViewById（R.id.ExitButton）；
mExit.setOnClickListener（this）；
mHistory=（按钮）findViewById（R.id.HistoryButton）；
mHistory.setOnClickListener（this）；
mRating=（按钮）findviewbyd（R.id.RateButton）；
mRating.setOnClickListener（此）；
mSignal=（TextView）findViewById（R.id.signalOfDayTV）；
//###将最后一个元素设置为TEXTVIEW
m_myListItem=新的ArrayList（）；
myAdapter=new HistoryAdapt（mContext，m_myListItem）；
新线程(
新的Runnable（）{
@凌驾
公开募捐{
试一试{
///需要等待HistoryAdapt中要下载的数据，以便在此处显示ListView中的最后一个元素
HistoryParser=新的HistoryParser（）；
parser.parse（getInputStream（HistoryAct.RSS_链接））；
}catch（XMLPullParseRexE）{
Log.w（e.getMessage（），e）；
}捕获（IOE异常）{
Log.w（e.getMessage（），e）；
}最后{
//通知数据已完成下载
main activity.this.runOnUiThread(
新的Runnable（）{
公开募捐{
m_myLastItem=myAdapter.getLastElement（）；
//将最后一个信号设置为文本视图
mSignal.setText（m_myLastItem.getTitle（））；
}
}
);
}
}
}
).run（）；
//#评分员
//apper.app_启动（本次）；
//AppRater.showRateDialog（此为空）；
//获取跟踪程序（应自动报告）
（（AppManager）getApplication（））.getTracker（AppManager.TrackerName.APP_TRACKER）；
}//一次创建
私有InputStream getInputStream（字符串链接）{
试一试{
URL=新的URL（链接）；
返回url.openConnection（）.getInputStream（）；
}捕获（IOE异常）{
Log.w（Constants.DATA，“检索输入流时异常”，e）；
返回null；
}
}
@凌驾
公共布尔onCreateOptions菜单（菜单）{
//为菜单充气；这会将项目添加到操作栏（如果存在）。
getMenuInflater（）.充气（R.menu.main，menu）；
返回true；
}
@凌驾
公共布尔值onOptionsItemSelected（菜单项项）{
//处理操作栏项目单击此处。操作栏将
//自动处理Home/Up按钮上的点击，只要
//在AndroidManifest.xml中指定父活动时。
int id=item.getItemId（）；
if（id==R.id.action\u设置）{
返回true；
}
返回super.onOptionsItemSelected（项目）；
}
公共静态上下文getAppContext（）{
返回MainActivity.mContext；
}
公共无效退出状态（）{
AlertDialog.Builder=新建AlertDialog.Builder（此）；
setTitle（“您将要退出Signals4Trading”）；
builder.setIcon（R.drawable.5）；
//setMessage（“您的设备已成功注册。您将很快收到信号。”）；
setMessage（“您确定要退出吗？”）；
builder.setCancelable（false）；//无法单击活动的背景
builder.setPositiveButton（“是”，新建DialogInterface.OnClickListener（）{
@凌驾
public void onClick（DialogInterface dialog，int which）{
//TODO自动生成的方法存根
Toast.makeText（getApplicationContext（），“再见”，Toast.LENGTH_LONG.show（）；
完成（）；
}//onclick监听器正按钮
})；//匿名类正按钮
setNegativeButton（“否”，新的DialogInterface.OnClickListener（）{
@凌驾
public void onClick（DialogInterface dialog，int which）{
//TODO自动生成的方法存根
Toast.makeText（getApplicationContext（），“祝您访问愉快”，Toast.LENGTH_LONG.show（）；
}
});
AlertDialog AlertDialog=builder.create（）；
alertDialog.show（）；
}//退出状态
公共无效goToHistoryActivity（）{
意向意向=新意向（MainActivity.this、HistoryAct.class）；
星触觉（意向）；
}
公共应用程序（）
Thread run = new Runnable() {

            @Override
            public void run() {
                // TODO Auto-generated method stub

            }

    run =  new Thread( new Runnable(){
                public void run(){

            if(!HistoryAdapt.hasBeenInitiated){

                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 
                }

            }
                }
            });

        run.start();
        run.join();

m_myLastItem= HistoryAdapt.getLastElement();

new Thread(
    new Runnable()
    {
        @Override
        public void Run()
        {
            //your code here
        }
    }
).Run();

synchronized(this){
    this = newvalue;   
}

MainActivity.this.runOnUiThread(new Runnable() {
    public void run() {
        m_myLastItem = HistoryAdapt.getLastElement();
        //set last signal into TextView
        mSignal.setText(m_myLastItem.getTitle());
    }
});

new Thread(
    new Runnable()
    {
        @Override
        public void run()
        {
            if(!HistoryAdapt.hasBeenInitiated)
            {
                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()
                            {
                                if (!HistoryAdapt.hasBeenInitiated)
                                {
                                    m_myLastItem = HistoryAdapt.getLastElement();
                                    //set last signal into TextView
                                    mSignal.setText(m_myLastItem.getTitle());
                                }
                            }
                        }
                    );
                }
            }
        }
    }
).run();