Question

登录时我的应用程序崩溃。问题源于加载功能。当它启动adapter.startListening（）;

时

我不确定它是否来自版本冲突或其他方式。请帮忙

我的logcat：

致命例外：主要处理：abcd.com.eatme，PID：10687 java.lang.RuntimeException：无法启动活动ComponentInfo {abcd.com.eatme / abcd.com.eatme.MainActivity}：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'void com.firebase.ui.database.FirebaseRecyclerAdapter.startListening（）' 在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:3319）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3415）在android.app.ActivityThread.access $ 1100（ActivityThread.java:229）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1821）在android.os.Handler.dispatchMessage（Handler.java:102）在android.os.Looper.loop（Looper.java:148）在android.app.ActivityThread.main（ActivityThread.java:7331） at java.lang.reflect.Method.invoke（Native Method）在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:1230）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:1120）引发者：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'void com.firebase.ui.database.FirebaseRecyclerAdapter.startListening（）' at abcd.com.eatme.MainActivity.onStart（MainActivity.java:55）在android.app.Instrumentation.callActivityOnStart（Instrumentation.java:1266）在android.app.Activity.performStart（Activity.java:6943）在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:3276）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3415）在android.app.ActivityThread.access $ 1100（ActivityThread.java:229）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1821）在android.os.Handler.dispatchMessage（Handler.java:102）在android.os.Looper.loop（Looper.java:148）在android.app.ActivityThread.main（ActivityThread.java:7331） at java.lang.reflect.Method.invoke（Native Method）在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:1230）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:1120）

我的家庭课程

public class Home extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    FirebaseDatabase database;
    DatabaseReference category;
    TextView textFullName;
    FirebaseRecyclerAdapter adapter;

    RecyclerView recycler_menu;
    RecyclerView.LayoutManager layoutManager;


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



        Toolbar toolbar = (Toolbar) findViewById ( R.id.toolbar );
        toolbar.setTitle ( "Menu" );
        setSupportActionBar ( toolbar );

        //initiate firebase

        database = FirebaseDatabase.getInstance ();
        category = database.getReference ("Category");


        FloatingActionButton fab = (FloatingActionButton) findViewById ( R.id.fab );
        fab.setOnClickListener ( new View.OnClickListener () {
            @Override
            public void onClick (View view) {
                Snackbar.make ( view, "Replace with your own action", Snackbar.LENGTH_LONG )
                        .setAction ( "Action", null ).show ();
            }
        } );

        DrawerLayout drawer = (DrawerLayout) findViewById ( R.id.drawer_layout );
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle (
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close );
        drawer.addDrawerListener ( toggle );
        toggle.syncState ();

        NavigationView navigationView = (NavigationView) findViewById ( R.id.nav_view );
        navigationView.setNavigationItemSelectedListener ( this );

        //set name for user
        View headerView = navigationView.getHeaderView ( 0 );
        textFullName = (TextView) headerView.findViewById ( R.id.textFullName );
        textFullName.setText ( Common.currentUser.getName () );


        //load menu

        recycler_menu =(RecyclerView) findViewById ( R.id.recycler_menu );
        recycler_menu.setHasFixedSize ( true );
        layoutManager = new LinearLayoutManager ( this );
        recycler_menu.setLayoutManager ( layoutManager );

        loadMenu();`enter code here`
    }


    private void loadMenu(){

        Query query = FirebaseDatabase
                .getInstance()
                .getReference()
                .child("Category");

        FirebaseRecyclerOptions<Category> options =
                new FirebaseRecyclerOptions.Builder<Category>()
                        .setQuery(query, Category.class)
                        .build();
        adapter = new FirebaseRecyclerAdapter<Category, MenuViewHolder>(options) {
            @Override
            public MenuViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
                View view = LayoutInflater.from(parent.getContext())
                        .inflate(R.layout.menu_item, parent, false);

                return new MenuViewHolder(view);
            }

            @Override
            protected void onBindViewHolder(@NonNull MenuViewHolder holder, int position, @NonNull Category model) {
                holder.textMenuName.setText(model.getName());
                Picasso.with(getBaseContext()).load(model.getImage())
                        .into(holder.imageView);
                final Category clickItem = model;
                holder.setItemClickListener(new ItemClickListener() {
                    @Override
                    public void onClick(View view, int position, boolean isLongClick) {
                        Toast.makeText(Home.this, ""+clickItem.getName(), Toast.LENGTH_SHORT).show();
                    }
                });
            }

        };

        recycler_menu.setAdapter(adapter);
    }


    @Override
    public void onBackPressed () {
        DrawerLayout drawer = (DrawerLayout) findViewById ( R.id.drawer_layout );
        if (drawer.isDrawerOpen ( GravityCompat.START )) {
            drawer.closeDrawer ( GravityCompat.START );
        } else {
            super.onBackPressed ();
        }
    }

    @Override
    public boolean onCreateOptionsMenu (Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater ().inflate ( R.menu.home, menu );
        return true;
    }

    @Override
    public boolean onOptionsItemSelected (MenuItem item) {


        return super.onOptionsItemSelected ( item );
    }

    @SuppressWarnings ("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected (MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId ();

        if (id == R.id.nav_menu) {
            // Handle the camera action
        } else if (id == R.id.nav_cart) {

        } else if (id == R.id.nav_orders) {

        } else if (id == R.id.nav_logout) {

        }

        DrawerLayout drawer = (DrawerLayout) findViewById ( R.id.drawer_layout );
        drawer.closeDrawer ( GravityCompat.START );
        return true;
    }

    }

我的MainActivity类

public class MainActivity extends AppCompatActivity {

    Button btnSignUp, btnSignIn;
    TextView textSlogan;
    FirebaseRecyclerAdapter adapter;

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

        btnSignIn = (Button) findViewById ( R.id.btnSignIn );
        btnSignUp = (Button) findViewById ( R.id.btnSignUp );

        textSlogan = (TextView) findViewById ( R.id.textSlogan );
        Typeface face = Typeface.createFromAsset ( getAssets (),"font/NABILA.TTF" );
        textSlogan.setTypeface ( face);



        btnSignUp.setOnClickListener ( new View.OnClickListener () {
            @Override
            public void onClick (View view) {
                Intent signUp = new Intent ( MainActivity.this, SignUp.class );
                startActivity ( signUp );

            }
        } );
        btnSignIn.setOnClickListener ( new View.OnClickListener () {
            @Override
            public void onClick (View v) {
                Intent signIn = new Intent ( MainActivity.this, SignIn.class );
                startActivity ( signIn );
            }
        } );

    }

    @Override
    protected void onStart() {
        super.onStart();
        adapter.startListening();


    }


    @Override
    protected void onStop() {
        super.onStop();
        adapter.stopListening();
    }
}

我的傻瓜：

apply plugin: 'com.android.application'

android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "abcd.com.eatme"
        minSdkVersion 17
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation 'com.android.support:appcompat-v7:27.0.2'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    implementation 'com.android.support:design:27.0.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
    implementation 'com.google.firebase:firebase-core:11.8.0'
    implementation 'com.google.android.gms:play-services-auth:11.8.0'
    implementation 'com.google.firebase:firebase-core:11.8.0'
    implementation 'com.google.firebase:firebase-database:11.8.0'
    implementation 'com.github.jd-alexander:android-flat-button:v1.1'
    implementation 'com.rengwuxian.materialedittext:library:2.1.4'
    implementation 'com.android.support:cardview-v7:27.0.2'
    implementation 'com.android.support:recyclerview-v7:27.0.2'
    implementation 'com.firebaseui:firebase-ui-database:3.2.1'
    implementation 'com.squareup.picasso:picasso:2.5.2'
}

apply plugin: 'com.google.gms.google-services'

我的firebase数据库：

androideatit-63d7daddclose

 Category
    01
     Image: 
     Name: 
   02
     Image: 
     Name:
   03
     Image: 
     Name:
   04
     Image: 
     Name:  


  users
    Roger
     name: Mac
     password: 12345

ViewHolder

viewholder是公共类MenuViewHolder扩展RecyclerView.ViewHolder实现了View.OnClickListener {

public TextView textMenuName;
public ImageView imageView;

private ItemClickListener itemClickListener;

public MenuViewHolder (View itemView) {
    super ( itemView );


    textMenuName = (TextView) itemView.findViewById ( R.id.menu_name );
    imageView = (ImageView)itemView.findViewById ( R.id.menu_image);

    itemView.setOnClickListener ( this );

}

public void setItemClickListener (ItemClickListener itemClickListener) {
    this.itemClickListener = itemClickListener;
}

@Override
public void onClick (View view) {
    itemClickListener.onClick ( view, getAdapterPosition (), false );


}

}

Answer 1

您得到的错误是：

尝试在abcd.com.eatme.MainActivity.onStart（MainActivity.java:55）的空对象引用上调用虚方法'void com.firebase.ui.database.FirebaseRecyclerAdapter.startListening（）'

如果您转到错误提及的MainActivity.java第55行，则它位于onStart()：

@Override
protected void onStart() {
    super.onStart();
    adapter.startListening();
}

具体而言，adapter.startListening();正在倾听，因为adapter还没有值。

实际上，您共享的其余代码从未创建FirebaseRecyclerAdapter，因此您无法开始监听它。我建议您查看documentation for FirebaseUI，其中显示了如何创建FirebaseRecyclerAdapter：

 FirebaseRecyclerOptions<Chat> options =
            new FirebaseRecyclerOptions.Builder<Chat>()
                    .setQuery(query, Chat.class)
                    .build();
FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Chat, ChatHolder>(options) {
  ...

Answer 2

我设法解决了它

而不是：

@Override
protected void onStart() {
    super.onStart();
    adapter.startListening();
}

MainActivity中的

。转移此行adapter.startListening（）;在设置适配器之前在loadMenu（）内部：

adapter.startListening();
recycler_menu.setAdapter(adapter);

Firebase用户界面

2 个答案: