我的logcat中出现以下错误
2020-03-21 16:58:54.611 20322-20322/com.example.bookitup E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.bookitup, PID: 20322
java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference
at com.example.bookitup.RecyclerView_Config.setConfig(RecyclerView_Config.java:22)
at com.example.bookitup.MainActivity$2.DataIsLoaded(MainActivity.java:90)
at com.example.bookitup.BookDatabaseEdit$1.onDataChange(BookDatabaseEdit.java:40)
at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@19.2.1:75)
at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@19.2.1:63)
at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@19.2.1:55)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
2020-03-21 16：58：54.622 2004-2019 /？ W / ActivityTaskManager：强制完成活动com.example.bookitup / .MainActivity 2020-03-21 16：58：54.622 2004-20389 /？ I / DropBoxManagerService：添加标签= data_app_crash isTagEnabled = true标志= 0x2
这是导致此NPE的RecyclerView_Config的代码。
public class RecyclerView_Config {
private Context mContext;
private BooksAdapter mBooksAdapter;
public void setConfig(RecyclerView recyclerView, Context context, List<BookActivity> books, List<String> keys){
mContext = context;
mBooksAdapter = new BooksAdapter(books,keys);
recyclerView.setLayoutManager(new LinearLayoutManager(context));/*here!!!!!!!!!!!!!!!!!!!!!!!!!*/
recyclerView.setAdapter(mBooksAdapter);
}
class BookItemView extends RecyclerView.ViewHolder {
private TextView mTitle;
private TextView mAuthor;
private TextView mISBN;
private TextView mCategory;
private String key;
public BookItemView(ViewGroup parent){
super(LayoutInflater.from(mContext).inflate(R.layout.book_list_item,parent,false));
mTitle = (TextView) itemView.findViewById(R.id.title_txtView);
mAuthor = (TextView) itemView.findViewById(R.id.author_txtView);
mCategory = (TextView) itemView.findViewById(R.id.category_txtView);
mISBN = (TextView) itemView.findViewById(R.id.isbn_txtView);
}
public void bind(BookActivity book, String key){
mTitle.setText(book.getXbook());
mAuthor.setText(book.getXauthor());
mCategory.setText(book.getXdescription());
mISBN.setText(book.getXisbn());
this.key = key;
}
}
class BooksAdapter extends RecyclerView.Adapter<BookItemView>{
private List<BookActivity> mBookList;
private List<String> mKeys;
public BooksAdapter(List<BookActivity> mBookList, List<String> mKeys) {
this.mBookList = mBookList;
this.mKeys = mKeys;
}
@NonNull
@Override
public BookItemView onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
return new BookItemView((parent));
}
@Override
public void onBindViewHolder(@NonNull BookItemView holder, int position) {
holder.bind(mBookList.get(position), mKeys.get(position));
}
@Override
public int getItemCount() {
return mBookList.size();
}
}
}
这是我活动中的代码
public class MainActivity extends AppCompatActivity {
private AppBarConfiguration mAppBarConfiguration;
private RecyclerView mRecyclerView;
//private ProgressBar progressBar;
@Override
protected void onCreate(Bundle savedInstanceState) {
setTheme(R.style.AppTheme_NoActionBar);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
FirebaseUser user= FirebaseAuth.getInstance().getCurrentUser();
if (user != null) {
//setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = findViewById(R.id.fab);
Toast.makeText(getApplicationContext(), "on the main page", Toast.LENGTH_LONG).show();
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//Snackbar.make(view, "Adding new book, please wait!", Snackbar.LENGTH_LONG).setAction("Action", null).show();
Toast.makeText(getApplicationContext(), "Adding new book, please wait ", Toast.LENGTH_LONG).show();
//progressBar.setVisibility(View.GONE);
Intent intent = new Intent(MainActivity.this, AddBookActivity.class);
startActivity(intent);
}
});
DrawerLayout drawer = findViewById(R.id.drawer_layout);
NavigationView navigationView = findViewById(R.id.nav_view);
// Passing each menu ID as a set of Ids because each
// menu should be considered as top level destinations.
mAppBarConfiguration = new AppBarConfiguration.Builder(
R.id.nav_home, R.id.nav_gallery, R.id.nav_slideshow,
R.id.nav_tools, R.id.nav_share, R.id.nav_send, R.id.nav_profile_edit)
.setDrawerLayout(drawer)
.build();
NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
NavigationUI.setupWithNavController(navigationView, navController);
//piling all books on the main page
mRecyclerView = (RecyclerView) findViewById(R.id.recyclerview_books);
new BookDatabaseEdit().readBooks(new BookDatabaseEdit.DataStatus() {
@Override
public void DataIsLoaded(List<BookActivity> books, List<String> keys) {
new RecyclerView_Config().setConfig(mRecyclerView,MainActivity.this,books,keys);
}
@Override
public void DataIsInserted() {
}
@Override
public void DataIsUpdated() {
}
@Override
public void DataIsDeleted() {
}
});
}
else {
Intent intent = new Intent(MainActivity.this, RegistrationActivity.class);
startActivity(intent);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.action_settings:
// User chose the "Settings" item, show the app settings UI...
return true;
case R.id.action_logout:
FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
FirebaseAuth.getInstance().signOut();
Intent intent = new Intent(MainActivity.this, LoginActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
startActivity(new Intent(intent));
return true;
default:
// If we got here, the user's action was not recognized.
// Invoke the superclass to handle it.
return super.onOptionsItemSelected(item);
}
}
@Override
public boolean onSupportNavigateUp() {
NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
return NavigationUI.navigateUp(navController, mAppBarConfiguration)
|| super.onSupportNavigateUp();
}
}
我的BookDatabaseEdit.java
public class BookDatabaseEdit {
private FirebaseDatabase mDatabase;
private DatabaseReference mReferenceBooks;
private List<BookActivity> books = new ArrayList<>();
public interface DataStatus{
void DataIsLoaded(List<BookActivity> books, List<String> keys);
void DataIsInserted();
void DataIsUpdated();
void DataIsDeleted();
}
public BookDatabaseEdit() {
mDatabase = FirebaseDatabase.getInstance();
mReferenceBooks = mDatabase.getReference("Booklist");
}
public void readBooks(final DataStatus dataStatus){
mReferenceBooks.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange( DataSnapshot dataSnapshot) {
books.clear();
List<String> keys = new ArrayList<>();
for(DataSnapshot keyNode : dataSnapshot.getChildren()){
keys.add(keyNode.getKey());
BookActivity book = keyNode.getValue(BookActivity.class);
books.add(book);
}
dataStatus.DataIsLoaded(books,keys);
System.out.println(keys);
System.out.println(books);
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}
答案 0 :(得分：0)
我认为您应该为Context实现一个getter和setter，并使用getter通过null检查获取上下文。
答案 1 :(得分：0)
因此，我已修复此错误，并且我的代码现在可以正常工作。问题出在RecyclerView_Config.java上。问题是，基本上，我的程序试图在创建应用之前将Firebase中的数据填充到应用中。
它是固定的，谢谢大家。