我是编程的新手，这就是为什么我不理解What is a NullPointerException, and how do I fix it?的信息。
我写了一个应用程序，在那里你读了一本书的isbn，你将通过googlebooksAPI获得bookinfo，bookimage。
我设法将包含图像的数据存储在sqlite db中。我想在列表视图中显示存储的数据。 （我之前管理过，但没有存储图像 - 没有Nullpointer错误）。我得到了错误。
不幸的是我收到了这个错误： java.lang.NullPointerException：尝试获取null数组的长度
关注我的Logcat和使用过的活动：
logcat的：
Process: com.kasutwentyseven.gui4selfshelf, PID: 32327
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.kasutwentyseven.gui4selfshelf/com.kasutwentyseven.gui4selfshelf.Books.BookDataListActivity}: java.lang.NullPointerException: Attempt to get length of null array
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2357)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2419)
at android.app.ActivityThread.access$900(ActivityThread.java:154)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5291)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
Caused by: java.lang.NullPointerException: Attempt to get length of null array
at com.kasutwentyseven.gui4selfshelf.Books.BookDataListActivity.onCreate(BookDataListActivity.java:130)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2310)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2419)
at android.app.ActivityThread.access$900(ActivityThread.java:154)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5291)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
我认为错误发生在 BookDataListActivity
的onCreate方法中 @Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.book_data_list_layout);
Typeface myTypeface = Typeface.createFromAsset(getAssets(), "Lobster.ttf");
TextView myTextView = (TextView) findViewById(R.id.text_yourbooks);
myTextView.setTypeface(myTypeface);
booklistView = (ListView) findViewById(R.id.book_list_view);
inputSearch = (EditText) findViewById(R.id.search_bar);
bookListDataAdapter = new BookListDataAdapter(getApplicationContext(), R.layout.row_book_layout);
booklistView.setAdapter(bookListDataAdapter);
//onItemClickListener
booklistView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Intent intent = new Intent(getApplicationContext(), BookInfoActivity.class);
editTextBooktitle = (TextView) view.findViewById(R.id.text_book_title);
String book_title = editTextBooktitle.getText().toString();
intent.putExtra(EXTRA_MSG1, book_title);
editTextBookauthor = (TextView) view.findViewById(R.id.text_book_author);
String bookauthor = editTextBookauthor.getText().toString();
intent.putExtra(EXTRA_MSG2, bookauthor);
editTextBookdate = (TextView) view.findViewById(R.id.text_book_date);
String bookdate = editTextBookdate.getText().toString();
intent.putExtra(EXTRA_MSG3, bookdate);
editTextBookrating = (TextView) view.findViewById(R.id.text_book_rating);
String bookrating = editTextBookrating.getText().toString();
intent.putExtra(EXTRA_MSG4, bookrating);
editTextBookshelf = (TextView) view.findViewById(R.id.text_book_shelf);
String bookshelf = editTextBookshelf.getText().toString();
intent.putExtra(EXTRA_MSG5, bookshelf);
startActivity(intent);
}
});
inputSearch.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {
}
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
bookListDataAdapter.getFilter().filter(s);
}
@Override
public void afterTextChanged(Editable s) {
}
});
bookDBHelper = new BookDBHelper(getApplicationContext());
sqLiteDatabaseBooks = bookDBHelper.getReadableDatabase();
cursor2 = bookDBHelper.getInformations(sqLiteDatabaseBooks);
if (cursor2.moveToFirst()) {
do {
String booktitle, bookauthor, bookdate, bookrating, bookshelf;
Bitmap bookimage;
byte[] blob= cursor2.getBlob(0);
bookimage = BitmapFactory.decodeByteArray(blob,0,blob.length);
booktitle = cursor2.getString(1);
bookauthor = cursor2.getString(2);
bookdate = cursor2.getString(3);
bookrating = cursor2.getString(4);
bookshelf = cursor2.getString(5);
BookDataProvider bookDataProvider = new BookDataProvider(bookimage, booktitle, bookauthor, bookdate, bookrating, bookshelf);
bookListDataAdapter.add(bookDataProvider);
} while (cursor2.moveToNext());
}
}
}
我认为你需要为listview添加和获取Info数据的方法 - 它位于 BOOKDBHelper Activity
public class BookDBHelper extends SQLiteOpenHelper{
private static final String DATABASE_BOOKS_NAME = "BookINFO.DB";
private static final int DATABASE_BOOKS_VERS = 2;
private static final String CREATE_QUERY_BOOKS =
"CREATE TABLE "
+ BookContent.NewBookInfo.TABLE_NAME_BOOKS
+"("
+ BookContent.NewBookInfo.BOOK_IMAGE +" BLOB, "
+ BookContent.NewBookInfo.BOOK_TITLE+" TEXT, "
+ BookContent.NewBookInfo.BOOK_AUTHOR+" TEXT, "
+ BookContent.NewBookInfo.BOOK_DATE+" TEXT, "
+ BookContent.NewBookInfo.BOOK_RATING+" TEXT, "
+ BookContent.NewBookInfo.BOOK_SHELF+" TEXT);";
public BookDBHelper(Context context){
super(context, DATABASE_BOOKS_NAME, null, DATABASE_BOOKS_VERS);
Log.e("DATABASE OPERATIONS", " DATABASE CREATED");
}
@Override
public void onCreate(SQLiteDatabase bookdb) {
bookdb.execSQL(CREATE_QUERY_BOOKS);
Log.e("DATABASE OPERATIONS", " DATABASE CREATED");
}
public void addInformations(byte[] image, String booktitle, String bookauthor, String bookdate, String bookrating, String bookshelf, SQLiteDatabase bookdb)
{
ContentValues contentValues = new ContentValues();
contentValues.put(BookContent.NewBookInfo.BOOK_IMAGE, image);
contentValues.put(BookContent.NewBookInfo.BOOK_TITLE, booktitle);
contentValues.put(BookContent.NewBookInfo.BOOK_AUTHOR, bookauthor);
contentValues.put(BookContent.NewBookInfo.BOOK_DATE, bookdate);
contentValues.put(BookContent.NewBookInfo.BOOK_RATING, bookrating);
contentValues.put(BookContent.NewBookInfo.BOOK_SHELF, bookshelf);
bookdb.insert(BookContent.NewBookInfo.TABLE_NAME_BOOKS, null, contentValues);
Log.e("DATABASE OPERATIONS", "ON ROW INSERTED");
}
public Cursor getInformations(SQLiteDatabase bookdb){
Cursor cursor2;
String[] projections = {
BookContent.NewBookInfo.BOOK_IMAGE,
BookContent.NewBookInfo.BOOK_TITLE,
BookContent.NewBookInfo.BOOK_AUTHOR,
BookContent.NewBookInfo.BOOK_DATE,
BookContent.NewBookInfo.BOOK_RATING,
BookContent.NewBookInfo.BOOK_SHELF};
cursor2 = bookdb.query(BookContent.NewBookInfo.TABLE_NAME_BOOKS, projections,null, null, null, null, null);
return cursor2;
}
编辑：
public void saveBook(View view) { //Click on save Book saving it to SQL Database
String title = titleText.getText().toString();
String author = authorText.getText().toString();
String date = dateText.getText().toString();
String rating = ratingCountText.getText().toString();
String shelf = shelfText.getText().toString();
BitmapDrawable bitmapDrawable = (BitmapDrawable) thumbView.getDrawable();
Bitmap bitmap = bitmapDrawable.getBitmap();
ByteArrayOutputStream stream = new ByteArrayOutputStream();
bitmap.compress(Bitmap.CompressFormat.JPEG, 70, stream);
byte[] blob = stream.toByteArray();
bookDBHelper = new BookDBHelper(context1);
sqLiteDatabase1 = bookDBHelper.getWritableDatabase();
bookDBHelper.addInformations(blob,title, author, date, rating, shelf, sqLiteDatabase1);
Toast.makeText(getBaseContext(), "Data Saved", Toast.LENGTH_LONG).show();
bookDBHelper.close();
}
我没有发现我的错误，这就是我写信给你的原因。也许你的专家可以帮我学习新东西。
抱歉这个愚蠢的问题......我刚刚开始编程
EDITED
好的，我有一个Nullpointer 'bookimage = BitmapFactory.decodeByteArray（blob，0，blob.length）;`
我不知道为什么，因为我保存了一张图片，而且blob不应该是空的。
有解决方法吗？如何添加If子句或其他任何内容？