Android 字符串变量最终为空，我不知道为什么

我一直在努力创建一个活动，允许用户使用意图拍照并通过电子邮件发送此图像。我创建了文件名并将其存储在一个字符串中，但最终它的值被删除了。当它被重置为null时，我创建了另一个字符串变量来保存它的值，这样我就可以确保没有任何东西可以更改它的值，但是第二个字符串也会以null结束。我使用字符串的值并将其传递给intent，以便将刚刚拍摄的图像添加为附件。有人知道这个变量的值为什么会改变，以及如何改变吗

这是我的课堂，带着评论：

public class ImproveActivity extends Activity implements OnClickListener{
private static final String JPEG_FILE_PREFIX = "Improve_";
private static final String JPEG_FILE_SUFFIX = ".jpg";
private Bitmap mImageBitmap;
private ImageView mImageView;
private File storageDir;
private String mCurrentPhotoPath;
Button share, capture;
private String _path;
private File f;
//This variable holds file name
private String imageFileName;
//I am storing the file name variable here to keep its value
private String temp;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
       setContentView(R.layout.improve_layout);
       mImageView = (ImageView)findViewById(R.id.imgview);
       capture = (Button)findViewById(R.id.capture);
       share = (Button)findViewById(R.id.share);

       capture.setOnClickListener(this);
       share.setOnClickListener(this);

}

private void dispatchTakePictureIntent() {

    if(isIntentAvailable(getApplicationContext(), MediaStore.ACTION_IMAGE_CAPTURE)){

        Intent takePictureIntent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
        ;
        try {
            //f is the image
            f = createImageFile();

            takePictureIntent.putExtra(MediaStore.EXTRA_OUTPUT, Uri.fromFile(f));
            startActivityForResult(takePictureIntent, 1);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            Toast.makeText(getApplicationContext(), "error creating image", Toast.LENGTH_LONG).show();
        }

    }
    else{
        Toast.makeText(getApplicationContext(), "no camera available", Toast.LENGTH_LONG).show();
    }


}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent intent){
    Toast.makeText(getApplicationContext(), "returning", Toast.LENGTH_LONG).show();

    if (requestCode == 1) {

        mImageBitmap = BitmapFactory.decodeFile(mCurrentPhotoPath);
        if (mImageBitmap == null) {
            // bitmap still null
        } else {
            // set bitmap in imageview
            mImageView.setImageBitmap(mImageBitmap);
        }
    }

}

private File createImageFile() throws IOException {
    // Create an image file name
    String timeStamp = 
        new SimpleDateFormat("yyyyMMdd_HHmmss").format(new Date());
   imageFileName = JPEG_FILE_PREFIX + timeStamp + "_";
   //display the current image file name
    Toast.makeText(getApplicationContext(), imageFileName, Toast.LENGTH_LONG).show();

    File file = new File(getAlbumDir().getAbsolutePath());
    file.mkdirs();              
    Toast.makeText(getApplicationContext(), imageFileName, Toast.LENGTH_LONG).show();
    File image = File.createTempFile(
        imageFileName, 
        JPEG_FILE_SUFFIX  
    );


    mCurrentPhotoPath = image.getAbsolutePath();
    temp = imageFileName;
    return image;

}

private File getAlbumDir() {
    return storageDir = new File(
            Environment.getExternalStoragePublicDirectory(
                Environment.DIRECTORY_DCIM
            ), 
            "Camera"
        );
}

public static boolean isIntentAvailable(Context context, String action) {
    final PackageManager packageManager = context.getPackageManager();
    final Intent intent = new Intent(action);
    List<ResolveInfo> list =
            packageManager.queryIntentActivities(intent, PackageManager.MATCH_DEFAULT_ONLY);
    return list.size() > 0;
}


@Override
public void onClick(View v) {
    switch(v.getId()){
    case R.id.capture :
        dispatchTakePictureIntent();
        //displaying the value of the string imageFileName, it is what it should be, the file name
        Toast.makeText(getApplicationContext(), imageFileName, Toast.LENGTH_LONG).show();

        //displaying the file name in temp at this point it is still the file name I need
        Toast.makeText(getApplicationContext(), "temp is " +temp, Toast.LENGTH_LONG).show();
        break;
        //Once the image is saved, clicking share to open chooser
    case R.id.share:

        //displaying the temp string the file name is gone it is now null ????
        Toast.makeText(getApplicationContext(), "temp is " +temp, Toast.LENGTH_LONG).show();

        Intent picMessageIntent = new Intent(android.content.Intent.ACTION_SEND);  
        picMessageIntent.setType("image/jpg");  
        File downloadedPic =  new File(  
            Environment.getExternalStoragePublicDirectory(  
            Environment.DIRECTORY_DCIM),  
            "Camera/"+ temp+"jpg");  //I want to use the file name string to send the image via email
        picMessageIntent.putExtra(Intent.EXTRA_STREAM, Uri.fromFile(downloadedPic));  
        startActivity(Intent.createChooser(picMessageIntent, "Send your picture using:"));  
        break;
    }
}

公共类ImproveActivity扩展活动实现OnClickListener{
私有静态最终字符串JPEG\u文件\u PREFIX=“改进”；
私有静态最终字符串JPEG_FILE_SUFFIX=“.jpg”；
私有位图；
私有图像视图mImageView；
私有文件存储目录；
私有字符串mCurrentPhotoPath；
按钮共享、捕获；
私有字符串路径；
私有文件f；
//此变量保存文件名
私有字符串文件名；
//我将文件名变量存储在这里以保持其值
私有字符串温度；
@凌驾
创建时受保护的void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
setContentView（R.layout.改进_布局）；
mImageView=（ImageView）findViewById（R.id.imgview）；
capture=（按钮）findviewbyd（R.id.capture）；
share=（按钮）findViewById（R.id.share）；
capture.setOnClickListener（this）；
share.setOnClickListener（this）；
}
私有无效DispatchTakePictureContent（）{
如果（isIntentAvailable（getApplicationContext（），MediaStore.ACTION\u IMAGE\u CAPTURE））{
Intent takePictureIntent=新的意图（MediaStore.ACTION\u IMAGE\u CAPTURE）；
;
试一试{
//f是图像
f=createImageFile（）；
takePictureContent.putExtra（MediaStore.EXTRA_输出，Uri.fromFile（f））；
startActivityForResult（图片内容1）；
}捕获（IOE异常）{
//TODO自动生成的捕捉块
e、 printStackTrace（）；
Toast.makeText（getApplicationContext（），“创建图像时出错”，Toast.LENGTH_LONG.show（）；
}
}
否则{
Toast.makeText（getApplicationContext（），“没有可用的摄像头”，Toast.LENGTH_LONG.show（）；
}
}
@凌驾
ActivityResult上的公共void（int请求代码、int结果代码、意图）{
Toast.makeText（getApplicationContext（），“returning”，Toast.LENGTH_LONG.show（）；
if（requestCode==1）{
mImageBitmap=BitmapFactory.decodeFile（mCurrentPhotoPath）；
if（mImageBitmap==null）{
//位图仍然为空
}否则{
//在imageview中设置位图
setImageBitmap（mImageBitmap）；
}
}
}
私有文件createImageFile（）引发IOException{
//创建图像文件名
字符串时间戳=
新的SimpleDataFormat（“yyyyMMdd_HHmmss”）.格式（新日期（））；
imageFileName=JPEG\u文件\u前缀+时间戳+“\u”；
//显示当前图像文件名
Toast.makeText（getApplicationContext（），imageFileName，Toast.LENGTH_LONG）.show（）；
File File=新文件（getAlbumDir（）.getAbsolutePath（））；
mkdirs（）文件；
Toast.makeText（getApplicationContext（），imageFileName，Toast.LENGTH_LONG）.show（）；
File image=File.createTempFile(
imageFileName，
JPEG\u文件\u后缀
);
mCurrentPhotoPath=image.getAbsolutePath（）；
temp=imageFileName；
返回图像；
}
私有文件getAlbumDir（）{
return storageDir=新文件(
Environment.getExternalStoragePublicDirectory(
Environment.DIRECTORY\u DCIM
), 
“照相机”
);
}
公共静态布尔值isIntentAvailable（上下文、字符串操作）{
final-PackageManager-PackageManager=context.getPackageManager（）；
最终意图=新意图（行动）；
列表=
packageManager.QueryInputActivities（仅适用于意图、packageManager.MATCH_默认值）；
返回列表.size（）>0；
}
@凌驾
公共void onClick（视图v）{
开关（v.getId（））{
案例R.id.capture：
DispatchTakePictureContent（）；
//显示字符串imageFileName的值，它应该是什么，文件名
Toast.makeText（getApplicationContext（），imageFileName，Toast.LENGTH_LONG）.show（）；
//此时在temp中显示文件名仍然是我需要的文件名
Toast.makeText（getApplicationContext（），“temp is”+temp，Toast.LENGTH\u LONG.show（）；
打破
//保存图像后，单击“共享”打开选择器
案例R.id.share：
//显示临时字符串文件名消失，现在为空？？？？
Toast.makeText（getApplicationContext（），“temp is”+temp，Toast.LENGTH\u LONG.show（）；
Intent picMessageIntent=newintent（android.content.Intent.ACTION\u SEND）；
picMessageIntent.setType（“image/jpg”）；
文件下载PIC=新文件（
环境。getExternalStoragePublicDirectory（
环境。目录（DCIM），
“Camera/”+temp+“jpg”）；//我想使用文件名字符串通过电子邮件发送图像
picMessageIntent.putExtra（Intent.EXTRA_流，Uri.fromFile（downloadedPic））；
startActivity（Intent.createChooser（picMessageIntent，“使用以下方式发送图片”）；
打破
}
}

如果有人能告诉我这是怎么发生的，我将不胜感激

这是logcat，我打开活动，拍照，保存，然后它返回到活动视图，我按share，toast在选择器打开时显示temp的值

11-25 17:56:34.573: D/dalvikvm(12037): GC_EXTERNAL_ALLOC freed 1176 objects / 87312 bytes in 53ms
11-25 17:56:39.313: W/IInputConnectionWrapper(12037): showStatusIcon on inactive InputConnection
11-25 17:56:49.803: W/System.err(12037): java.lang.NullPointerException: Argument must not be null
11-25 17:56:49.808: W/System.err(12037):    at java.io.FileInputStream.<init>(FileInputStream.java:78)
11-25 17:56:49.808: W/System.err(12037):    at java.io.FileInputStream.<init>(FileInputStream.java:134)
11-25 17:56:49.808: W/System.err(12037):    at android.graphics.BitmapFactory.decodeFile(BitmapFactory.java:349)
11-25 17:56:49.808: W/System.err(12037):    at android.graphics.BitmapFactory.decodeFile(BitmapFactory.java:399)
11-25 17:56:49.808: W/System.err(12037):    at com.seweb.app.ImproveActivity.onActivityResult(ImproveActivity.java:94)
11-25 17:56:49.808: W/System.err(12037):    at android.app.Activity.dispatchActivityResult(Activity.java:3890)
11-25 17:56:49.808: W/System.err(12037):    at android.app.ActivityThread.deliverResults(ActivityThread.java:3511)
11-25 17:56:49.808: W/System.err(12037):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3115)
11-25 17:56:49.808: W/System.err(12037):    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3143)
11-25 17:56:49.813: W/System.err(12037):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2684)
11-25 17:56:49.813: W/System.err(12037):    at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3815)
11-25 17:56:49.813: W/System.err(12037):    at android.app.ActivityThread.access$2400(ActivityThread.java:125)
11-25 17:56:49.813: W/System.err(12037):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2037)
11-25 17:56:49.818: W/System.err(12037):    at android.os.Handler.dispatchMessage(Handler.java:99)
11-25 17:56:49.818: W/System.err(12037):    at android.os.Looper.loop(Looper.java:123)
11-25 17:56:49.818: W/System.err(12037):    at android.app.ActivityThread.main(ActivityThread.java:4627)
11-25 17:56:49.818: W/System.err(12037):    at java.lang.reflect.Method.invokeNative(Native Method)
11-25 17:56:49.818: W/System.err(12037):    at java.lang.reflect.Method.invoke(Method.java:521)
11-25 17:56:49.823: W/System.err(12037):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
11-25 17:56:49.823: W/System.err(12037):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
11-25 17:56:49.823: W/System.err(12037):    at dalvik.system.NativeStart.main(Native Method)
11-25 17:56:50.273: D/dalvikvm(12037): GC_EXTERNAL_ALLOC freed 1920 objects / 108152 bytes in 136ms

11-2517:56:34.573:D/dalvikvm（12037）：GC_EXTERNAL_ALLOC在53ms内释放了1176个对象/87312个字节
11-25 17:56:39.313:W/IIInputConnectionWrapper（12037）：在非活动输入连接上显示状态图标
11-2517:56:49.803:W/System.e
String temp = "";

java.lang.NullPointerException: Argument must not be null
    ...
    at com.seweb.app.ImproveActivity.onActivityResult(ImproveActivity.java:94)

mImageBitmap = BitmapFactory.decodeFile(mCurrentPhotoPath);

private void savePreferences(){
    // We need an Editor object to make preference changes.
    SharedPreferences.Editor editor = getPreferences(MODE_PRIVATE).edit();
    editor.putString("imageFileName", imageFileName);
    editor.putString("mCurrentPhotoPath", mCurrentPhotoPath);

    // Commit the edits!
    editor.commit();
}

private void restorePreferences() {
    SharedPreferences settings = getPreferences(MODE_PRIVATE);
    imageFileName = settings.getString("imageFileName", "");
    mCurrentPhotoPath = settings.getString("mCurrentPhotoPath", "");
}

try {
    savePreferences();
    ...
    startActivityForResult(takePictureIntent, 1);
} 

if (requestCode == 1) {
    if(mCurrentPhotoPath == null)
        restorePreferences();

    mImageBitmap = BitmapFactory.decodeFile(mCurrentPhotoPath);
    ...

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    savedInstanceState.putAll(getIntent().getExtras());
    savedInstanceState.putString("currentPhotoPath", mCurrentPhotoPath);
    super.onSaveInstanceState(savedInstanceState);
}

        if (savedInstanceState != null) {
mCurrentPhotoPath = savedInstanceState.getString("currentPhotoPath", mCurrentPhotoPath);
    }