NullPointerException 与加载器 Android

Question

制作一个显示通过内容提供商通过其他应用插入的产品品牌的应用。我得到一个 NullPointerException。任何帮助将不胜感激！

package com.example.contentprovideruser;

import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.CursorLoader;
import android.support.v4.content.Loader;
import android.view.Menu;
import android.view.View;
import android.widget.TextView;

public class MainActivity extends FragmentActivity implements LoaderManager.LoaderCallbacks<Cursor> {
    TextView resultView=null;       
    CursorLoader cursorLoader;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        resultView= (TextView) findViewById(R.id.res);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }

    public void onClickDisplayNames(View view) {
        getSupportLoaderManager().initLoader(1, null, this);
    }

    @Override
    public Loader<Cursor> onCreateLoader(int arg0, Bundle arg1) {
        String[] column = {"name"};
        cursorLoader= new CursorLoader(this, Uri.parse("content://com.ah.wholesalers.MyProvider/brand"),column, null, null, null);
        return cursorLoader;
    }

    @Override
    public void onLoadFinished(Loader<Cursor> arg0, Cursor cursor) {
        cursor.moveToFirst();
        StringBuilder res=new StringBuilder();
        while (!cursor.isAfterLast()) {
            res.append("\n"+ cursor.getString(1));
            cursor.moveToNext();
        }
        resultView.setText(res);
    }

    @Override
    public void onLoaderReset(Loader<Cursor> arg0) {
        // TODO Auto-generated method stub
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
    }

}

布局activity_main.xml：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >
    <Button
        android:id="@+id/btnRetrieve"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:onClick="onClickDisplayNames"
        android:text="Display names" />
    <TextView
        android:id="@+id/res"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="20dp"
        android:clickable="false"
        android:ems="10" >
    </TextView>
</LinearLayout>

这是我的 logcat，除了所有的例外：

11-05 17:23:13.672: E/AndroidRuntime(2044): FATAL EXCEPTION: main
11-05 17:23:13.672: E/AndroidRuntime(2044): Process: com.example.contentprovideruser, PID: 2044
11-05 17:23:13.672: E/AndroidRuntime(2044): java.lang.NullPointerException
11-05 17:23:13.672: E/AndroidRuntime(2044): at com.example.contentprovideruser.MainActivity.onLoadFinished(MainActivity.java:46)
11-05 17:23:13.672: E/AndroidRuntime(2044): at com.example.contentprovideruser.MainActivity.onLoadFinished(MainActivity.java:1)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:427)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.app.LoaderManagerImpl$LoaderInfo.onLoadComplete(LoaderManager.java:395)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.content.Loader.deliverResult(Loader.java:103)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:81)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:35)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.content.AsyncTaskLoader.dispatchOnLoadComplete(AsyncTaskLoader.java:221)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.content.AsyncTaskLoader$LoadTask.onPostExecute(AsyncTaskLoader.java:61)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.content.ModernAsyncTask.finish(ModernAsyncTask.java:461)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.content.ModernAsyncTask.access$500(ModernAsyncTask.java:47)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.support.v4.content.ModernAsyncTask$InternalHandler.handleMessage(ModernAsyncTask.java:474)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.os.Handler.dispatchMessage(Handler.java:102)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.os.Looper.loop(Looper.java:136)
11-05 17:23:13.672: E/AndroidRuntime(2044): at android.app.ActivityThread.main(ActivityThread.java:5017)
11-05 17:23:13.672: E/AndroidRuntime(2044): at java.lang.reflect.Method.invokeNative(Native Method)
11-05 17:23:13.672: E/AndroidRuntime(2044): at java.lang.reflect.Method.invoke(Method.java:515)
11-05 17:23:13.672: E/AndroidRuntime(2044): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
11-05 17:23:13.672: E/AndroidRuntime(2044): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
11-05 17:23:13.672: E/AndroidRuntime(2044): at dalvik.system.NativeStart.main(Native Method)

Answer 1

问题可能出在您初始化 TextView 的位置。

resultView= (TextView) findViewById(R.id.res);

请检查 TextView 的资源 id，是否使用了错误的资源 id。 还要检查上下文，即您的 TextView R.id.res 存在于 R.layout.activity_main 布局中。