android - 在 android 中运行启动器 Activity 时出现空指针异常

Question

我试图制作一个 android 视频播放器来播放 sd 卡中的一些视频文件。为此，我编写了两个类 Main.java 和 VideoSelector.java，但在启动我的应用程序时它显示空指针异常。请帮我解决这个错误

主.java

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.VideoView;

public class Main extends Activity {

    private VideoView m_vvPlayer;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        m_vvPlayer = (VideoView) findViewById(R.id.vvMain);
        Button btnSelectFile = (Button) findViewById(R.id.btnSelectFile);
        btnSelectFile.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                Intent i = new Intent();
                i.setClass(v.getContext(), VideoSelector.class);
                startActivityForResult(i, 0);
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (RESULT_OK == resultCode) {
            Bundle bd = data.getExtras();
            String path = bd.getString(VideoSelector.FILE_PATH);
            m_vvPlayer.setVideoPath(path);
            m_vvPlayer.start();
        }
    }
}

VideoSelector.java

import android.app.ListActivity;
import android.content.ContentResolver;
import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.provider.MediaStore;
import android.view.View;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;

public class VideoSelector extends ListActivity {

    public static final String FILE_PATH = "FILE_PATH"; 
    public void onCreate(Bundle icicle)
    {
        super.onCreate(icicle);
        init();
    }

    public void init() {
        setContentView(R.layout.video_selector);

        MakeCursor();

        // Map Cursor columns to views defined in media_list_item.xml
        SimpleCursorAdapter adapter = new SimpleCursorAdapter(
                this,
                android.R.layout.simple_list_item_1,
                mCursor,
                new String[] { MediaStore.Video.Media.TITLE},
                new int[] { android.R.id.text1 });

        setListAdapter(adapter);
    }

    @Override
    protected void onListItemClick(ListView l, View v, int position, long id)
    {
        String filePath = mCursor.getString(mCursor.getColumnIndex(MediaStore.Video.Media.DATA));
        mCursor.moveToPosition(position);
        Intent result = new Intent();
        result.putExtra(FILE_PATH, filePath);
        setResult(RESULT_OK, result);
        finish();
    }

    private void MakeCursor() {
        String[] cols = new String[] {
                MediaStore.Video.Media._ID,
                MediaStore.Video.Media.TITLE,
                MediaStore.Video.Media.DATA,
                MediaStore.Video.Media.MIME_TYPE,
                MediaStore.Video.Media.ARTIST
        };
        ContentResolver resolver = getContentResolver();
        if (resolver == null) {
            System.out.println("resolver = null");
        } else {
            mSortOrder = MediaStore.Video.Media.TITLE + " COLLATE UNICODE";
            mWhereClause = MediaStore.Video.Media.TITLE + " != ''";
            mCursor = resolver.query(MediaStore.Video.Media.EXTERNAL_CONTENT_URI,
                cols, mWhereClause , null, mSortOrder);
        }
    }

    private Cursor mCursor;
    private String mWhereClause;
    private String mSortOrder;
}

逻辑猫

08-04 11:28:18.862: I/ApplicationPackageManager(12180): cscCountry is not German : INS
08-04 11:28:18.933: W/dalvikvm(12180): threadid=1: thread exiting with uncaught exception (group=0x40018578)
08-04 11:28:18.933: E/AndroidRuntime(12180): FATAL EXCEPTION: main
08-04 11:28:18.933: E/AndroidRuntime(12180): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.Main}: java.lang.NullPointerException
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.os.Handler.dispatchMessage(Handler.java:99)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.os.Looper.loop(Looper.java:130)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.app.ActivityThread.main(ActivityThread.java:3687)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at java.lang.reflect.Method.invokeNative(Native Method)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at java.lang.reflect.Method.invoke(Method.java:507)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at dalvik.system.NativeStart.main(Native Method)
08-04 11:28:18.933: E/AndroidRuntime(12180): Caused by: java.lang.NullPointerException
08-04 11:28:18.933: E/AndroidRuntime(12180):    at com.example.test.Main.onCreate(Main.java:22)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
08-04 11:28:18.933: E/AndroidRuntime(12180):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
08-04 11:28:18.933: E/AndroidRuntime(12180):    ... 11 more

Answer 1

在activity_main.xml中没有找到类似R.id.btnSelectFile的资源