我正在尝试构建一个小型应用程序，该应用程序在外部存储中获取数据文件并通过电子邮件发送。我在 logcat 中不断收到“空指针异常”，然后我的应用程序就死了。我似乎找不到我的异常，我想知道是否有办法确定导致这种情况的代码行。我有 MainActivity 类，然后是一个名为 SendData 的类——代码如下。我是新来的，所以任何帮助将不胜感激 - 谢谢。
private static final String TAG = "MainActivity_ErrorLog";
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}
// Create the getData intent
Intent intentgetData = new Intent(MainActivity.this, SendData.class);
public void onStart()
{
{
try
{
File sd = Environment.getExternalStorageDirectory();
File data = Environment.getDataDirectory();
if (sd.canWrite())
{
// verify the paths
String currentDBPath = "TLC_COMMON/database.db";
String backupDBPath = "database.db";
File currentDB = new File(data, currentDBPath);
File backupDB = new File(sd, backupDBPath);
if (currentDB.exists())
{
FileChannel src = new FileInputStream(currentDB).getChannel();
FileChannel dst = new FileOutputStream(backupDB).getChannel();
dst.transferFrom(src, 0, src.size());
src.close();
dst.close();
}
}
}
catch (Exception e)
{
// change the V below to E when complete
Log.v(TAG,"ERROR: Database file not created");
}
startActivity(intentgetData);
}
}
}
——新班级
公共类发送数据扩展活动{私有静态最终字符串标签=“主活动”；
/* Checks if external storage is available to read */
public boolean isExternalStorageReadable()
{
String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state) ||
Environment.MEDIA_MOUNTED_READ_ONLY.equals(state))
{
return true;
}
return false;
}
/** Called when the user clicks the Send My Data button */
public SendData(View view)
{
// Send data by email
{
File root = Environment.getExternalStorageDirectory();
// verify it is saving as this file name; also path in previous class
String fileName = "database.db";
if (root.canWrite())
{
File attachment = new File(root, fileName);
Intent email = new Intent(Intent.ACTION_SENDTO);
email.putExtra(android.content.Intent.EXTRA_SUBJECT, "Exercise data");
email.putExtra(android.content.Intent.EXTRA_EMAIL, new String[]{"test@gmail.com"});
// is the Uri necessary?
email.putExtra(android.content.Intent.EXTRA_TEXT, Uri.fromFile(attachment));
// look at this VVV
startActivity(Intent.createChooser(email, "Send the data via Email"));}
else
{
// Change the V below to E when complete
Log.v(TAG, "Email send failed");
}
}
}
public void finish()
{
}
}
11-13 13:29:37.343: W/dalvikvm(3319): threadid=1: thread exiting with uncaught exception (group=0x418e3300)
11-13 13:29:37.343: E/AndroidRuntime(3319): FATAL EXCEPTION: main
11-13 13:29:37.343: E/AndroidRuntime(3319): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.va.datasender/com.example.va.datasender.MainActivity}: java.lang.NullPointerException
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.app.ActivityThread.access$600(ActivityThread.java:130)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.os.Handler.dispatchMessage(Handler.java:99)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.os.Looper.loop(Looper.java:137)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.app.ActivityThread.main(ActivityThread.java:4745)
11-13 13:29:37.343: E/AndroidRuntime(3319): at java.lang.reflect.Method.invokeNative(Native Method)
11-13 13:29:37.343: E/AndroidRuntime(3319): at java.lang.reflect.Method.invoke(Method.java:511)
11-13 13:29:37.343: E/AndroidRuntime(3319): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
11-13 13:29:37.343: E/AndroidRuntime(3319): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
11-13 13:29:37.343: E/AndroidRuntime(3319): at dalvik.system.NativeStart.main(Native Method)
11-13 13:29:37.343: E/AndroidRuntime(3319): Caused by: java.lang.NullPointerException
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.content.ContextWrapper.getPackageName(ContextWrapper.java:127)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.content.ComponentName.<init>(ComponentName.java:75)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.content.Intent.<init>(Intent.java:3301)
11-13 13:29:37.343: E/AndroidRuntime(3319): at com.example.va.datasender.MainActivity.<init>(MainActivity.java:36)
11-13 13:29:37.343: E/AndroidRuntime(3319): at java.lang.Class.newInstanceImpl(Native Method)
11-13 13:29:37.343: E/AndroidRuntime(3319): at java.lang.Class.newInstance(Class.java:1319)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
11-13 13:29:37.343: E/AndroidRuntime(3319): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
11-13 13:29:37.343: E/AndroidRuntime(3319): ... 11 more