1. 技术文章
  2. nullpointer exception raises when i click on the button

nullpointer exception raises when i click on the button

由 断了今生、忘了曾经 提交于 2019-12-28 04:37:25

问题


When I click on CheckData button on android, it is throwing a nullpointer exception.

SaveData.java

public class SaveData extends Activity implements OnClickListener {     
    static final int DIALOG_ID = 0;

    private Uri mImageCaptureUri;
    private ImageView mImageView;   
    public static class Certificates {
    private Bitmap bmp;

        public Certificates(Bitmap b) {
        bmp = b;
        }
        public Bitmap getBitmap() { return bmp; }
    }

    private static final int PICK_FROM_CAMERA = 1;
    private static final int PICK_FROM_FILE = 2;

    @Override
        protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.addname);

        View button1Click=findViewById(R.id.btn_choose);
        button1Click.setOnClickListener(this);
        View button2Click = findViewById(R.id.Button01add);
        button2Click.setOnClickListener(this);
        View button3Click = findViewById(R.id.Button01check);
        button3Click.setOnClickListener(this);  
    }

        public void onClick(View v){        
        switch(v.getId()){

        case R.id.Button01add:
            showDialog(DIALOG_ID);
            break;

        case R.id.Button01check:
            startActivity(new Intent (SaveData.this,CheckData.class));
            break;
        }

// picking an image from camera or gallery         
            final String [] items           = new String [] {"From Camera", "From SD Card"};                
            ArrayAdapter<String> adapter    = new ArrayAdapter<String> (this, android.R.layout.select_dialog_item,items);
            AlertDialog.Builder builder     = new AlertDialog.Builder(this);

            builder.setTitle("Select Image");
            builder.setAdapter( adapter, new DialogInterface.OnClickListener() {
                public void onClick( DialogInterface dialog, int item ) {
                    if (item == 0) {
                        Intent intent    = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
                        File file        = new File(Environment.getExternalStorageDirectory(), "tmp_avatar_" + String.valueOf(System.currentTimeMillis()) + ".jpg");
                        mImageCaptureUri = Uri.fromFile(file);

                        try {           
                            intent.putExtra(android.provider.MediaStore.EXTRA_OUTPUT, mImageCaptureUri);
                            intent.putExtra("return-data", true);                           
                            startActivityForResult(intent, PICK_FROM_CAMERA);
                    }   catch (Exception e) {
                            e.printStackTrace();
                        }           

                        dialog.cancel();
                    } else {
                        Intent intent = new Intent();

                        intent.setType("image/*");
                        intent.setAction(Intent.ACTION_GET_CONTENT);

                        startActivityForResult(Intent.createChooser(intent, "Complete action using"), PICK_FROM_FILE);
                    }
                }
            } );

            final AlertDialog dialog = builder.create();

            mImageView = (ImageView) findViewById(R.id.image1);

            ((Button) findViewById(R.id.btn_choose)).setOnClickListener(new View.OnClickListener() {            
                @Override
                public void onClick(View v) {
                    dialog.show();
                }
            });
        }

        @Override
        protected void onActivityResult(int requestCode, int resultCode, Intent data) {
            if (resultCode != RESULT_OK) return;

            Bitmap bitmap   = null;
            String path     = "";

            if (requestCode == PICK_FROM_FILE) {
                mImageCaptureUri = data.getData(); 
                path = getRealPathFromURI(mImageCaptureUri); //from Gallery 

                if (path == null)
                    path = mImageCaptureUri.getPath(); //from File Manager

                if (path != null) 
                    bitmap  = BitmapFactory.decodeFile(path);
            } else {
                path    = mImageCaptureUri.getPath();
                bitmap  = BitmapFactory.decodeFile(path);
            }

            mImageView.setImageBitmap(bitmap);      
        }

        public String getRealPathFromURI(Uri contentUri) {
            String [] proj      = {MediaStore.Images.Media.DATA};
            Cursor cursor       = managedQuery( contentUri, proj, null, null,null);

            if (cursor == null) return null;

            int column_index    = cursor.getColumnIndexOrThrow(MediaStore.Images.Media.DATA);

            cursor.moveToFirst();

            return cursor.getString(column_index);
        }

    protected final Dialog onCreateDialog(final int id) {
        Dialog dialog = null;
        switch(id) {
        case DIALOG_ID:
            AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setMessage("Information saved successfully ! Add Another Info?")
            .setCancelable(false)
            .setPositiveButton("No", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {

                    SaveData.this.finish();
              }
            })
        .setNegativeButton("Yes", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                    dialog.cancel();
                }
            });
            AlertDialog alert = builder.create(); 
            dialog = alert;
            break;
        default:
        }
        return dialog;
    }
// menu option  
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.layout.mymenu, menu);        
        return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if(item.getItemId() == R.id.item1) {
            Log.d("Option", "Save option is clicked");           
        }
        if(item.getItemId() == R.id.item2) {
            Log.d("Option", "Delete option is clicked");
        }
        if(item.getItemId() == R.id.item3) {
            Log.d("Option", "Exit option is clicked");
        }       
        return super.onOptionsItemSelected(item);       
    }
}

DataManipulator.java:

public class DataManipulator {
public static final String KEY_IMG = "image";

 private DatabaseHelper mDbHelper;
    private SQLiteDatabase mDb;

    private static final String DATABASE_NAME = "DBtest";
    private static final int DATABASE_VERSION = 1;

    private static final String CERTIFICATES_TABLE = "certificates";

    private static final String CREATE_CERTIFICATES_TABLE = "create table "+CERTIFICATES_TABLE+" (" +KEY_IMG+" blob not null) ";

    private final Context mCtx;
    private static class DatabaseHelper extends SQLiteOpenHelper {
        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        public void onCreate(SQLiteDatabase db) {
            db.execSQL(CREATE_CERTIFICATES_TABLE);
        }

        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            db.execSQL("DROP TABLE IF EXISTS "+CERTIFICATES_TABLE);
            onCreate(db);
        }
    }
    public void Reset() { mDbHelper.onUpgrade(this.mDb, 1, 1); }

    public DataManipulator(Context ctx) {
        mCtx = ctx;
        mDbHelper = new DatabaseHelper(mCtx);
    }

    public DataManipulator open() throws SQLException {
        mDb = mDbHelper.getWritableDatabase();
        return this;
    }

    public void close() { mDbHelper.close(); }

    public void createCertificatesEntry(Certificates certificates) {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        certificates.getBitmap().compress(Bitmap.CompressFormat.PNG, 100, out);
        ContentValues cv = new ContentValues();
        cv.put(KEY_IMG, out.toByteArray());
        mDb.insert(CERTIFICATES_TABLE,  null, cv);
    }
    public Certificates getFirstCertificatesFromDB() throws SQLException {
        Cursor cur = mDb.query(true, CERTIFICATES_TABLE,  new String[] {KEY_IMG}, null, null, null, null, null, null);
        if(cur.moveToFirst()) {
            byte[] blob = cur.getBlob(cur.getColumnIndex(KEY_IMG));
            ByteArrayInputStream inputStream = new ByteArrayInputStream(blob);
            Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
            cur.close();
            return new Certificates(bitmap);
        }
        cur.close();
        return null;
    }    
}

DataManipulator.java:60 is certificates.getBitmap().compress(Bitmap.CompressFormat.PNG, 100, out);

CheckData.java:

    public class CheckData extends ListActivity  {     
    TextView selection;
    DataManipulator dm;
    private DataManipulator DataManipulator;

        protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.check);
        dm = new DataManipulator(this);

        LinearLayout layout = new LinearLayout(this);
        ImageView image = new ImageView(this);
        DataManipulator = new DataManipulator(this);

        Certificates testCertificates = new Certificates(BitmapFactory.decodeFile(Context.STORAGE_SERVICE));

        DataManipulator.open();
        DataManipulator.createCertificatesEntry( (Certificates) testCertificates);
        DataManipulator.close(); 

        testCertificates = null;

        DataManipulator.open();
        testCertificates = DataManipulator.getFirstCertificatesFromDB();
        DataManipulator.close();

        image.setImageBitmap(((Certificates) testCertificates).getBitmap());

        setContentView(layout);
    }
}

CheckData.java:29 is DataManipulator.createCertificatesEntry( (Certificates) testCertificates);

Logcat error:

java.lang.RuntimeException: Unable to start activity ComponentInfo{list.certificates/list.certificates.CheckData}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
at android.app.ActivityThread.access$600(ActivityThread.java:122)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4340)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at list.certificates.DataManipulator.createCertificatesEntry(DataManipulator.java:60)
at list.certificates.CheckData.onCreate(CheckData.java:29)
at android.app.Activity.performCreate(Activity.java:4465)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919)

回答1:


You're getting a null from certificates.getBitmap() because there's not a bitmap there. In your initial assignment, you're trying to decode a file that doesn't exist. Using Context.STORAGE_SERVICE doesn't work that way, it's just a string equaling storage.

For example code of how to use a storage manager properly, try this.

If you're just trying to decode a resource(drawable, raw, etc) use another form of decode:

InputStream is = context.getResources().openRawResource(resID);
Bitmap bitmap = BitmapFactory.decodeStream(is);


Update: Assuming I know what it is you're trying to do, the problem is that you're switching to the CheckData activity without passing the selected bitmap in any way. The activity has no way of knowing what bitmap to use, and Context.STORAGE_SERVICE is not a bitmap in any way, so you can't decode it.

For good answers on how to pass the bitmap to the new activity, look here.

In short, you could add an mBitmap variable to SaveData, and pass it along to CheckData.

For this, change the code in SaveData.onClick() to:

case R.id.Button01check:
    Intent intent = new Intent (SaveData.this,CheckData.class);
    intent.putExtra("bitmapData", mBitmap)
    startActivity(intent);
    break;

And, in onActivityResult(), add this at the end:

mBitmap = bitmap;

In CheckData.onCreate():

Bitmap bmp = getIntent().getExtras().get("bitmapData");
if(bmp != null) {
    Certificates testCertificates = new Certificates(bmp);
} else {
    // Back out gracefully //
}



回答2:


Problem root:

Your problem is in this line 

 Certificates testCertificates = 
             new Certificates(BitmapFactory.decodeFile(Context.STORAGE_SERVICE));

The parameter to decodeFile is invalid hence it returns null. The parameter should be the path to the image file, whereas you are passing a constant that is to be used for retrieving the storage service.

Fix:

You already have the bitmap in SaveData activity. You need to pass it to CheckData activity through intent. Add the bitmap to the intent :

   case R.id.Button01check:
        Intent intent = new Intent(this, CheckData.class);
        intent.putExtra("BitmapImage", bitmap);
        startActivity(intent);

Then in CheckData activity, retrieve the bitmap from intent and create the Certificates object.

Bitmap bitmap = (Bitmap) getIntent().getParcelableExtra("BitmapImage");
Certificates testCertificates = new Certificates(bitmap );

Reference: How can I pass a Bitmap object from one activity to another




回答3:


Either certificates is null, or certificates.getBitmap() is returning null. Add logging to find out which.




回答4:


check certificates is null or not before inserting data as:

public void createCertificatesEntry(Certificates certificates) {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
  if(certificates !=null)
{
        certificates.getBitmap().compress(Bitmap.CompressFormat.PNG, 100, out);
        ContentValues cv = new ContentValues();
        cv.put(KEY_IMG, out.toByteArray());
        mDb.insert(CERTIFICATES_TABLE,  null, cv);
}
else
{
}

    }



回答5:


generally , in order to handle exceptions, i would give you this tip:

when you get the exception on the logcat , double click (or press ENTER) each of the red lines of the logs in this chunk , till the text cursor goes to the correct line that you got the exception .

then , once you know on which of the variables you got the expcetion , put a breakpoint there ,on the event of clicking , and on the event of onCreate , to see why you got this exception .

however , your code has so many weird things , for example:

  1. why do you call setContentView twice in the same method ?

  2. why do you try to decode a bitmap file which its name is Context.STORAGE_SERVICE ?

  3. why do you have a class and and instance with the same name (DataManipulator) ? please use a more standard way to name fields and variables.



来源：https://stackoverflow.com/questions/11080994/nullpointer-exception-raises-when-i-click-on-the-button

标签
android
nullpointerexception
android-button
易学教程内所有资源均来自网络或用户发布的内容，如有违反法律规定的内容欢迎反馈！
该文章没有解决你所遇到的问题?点击提问,说说你的问题,让更多的人一起探讨吧!