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我正在製作一個帶有多個片段的選項卡式活動，這些片段使用RecyclerView填充自己，並且將從Firebase實時數據庫和存儲中檢索數據作爲後端。Android-Firebase- recyclerView中的java.lang.NullPointerException
錯誤：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.visan.peds, PID: 1522
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference
at com.example.visan.peds.Landing_mother.getData(Landing_mother.java:76)
at com.example.visan.peds.Landing_mother.onCreateView(Landing_mother.java:43)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:2087)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1113)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1295)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:801)
at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:1643)
at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:679)
at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:143)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1272)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1120)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1646)
at android.view.View.measure(View.java:18991)
at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:716)
at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:462)
at android.view.View.measure(View.java:18991)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5952)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
at android.view.View.measure(View.java:18991)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5952)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18991)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5952)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.view.View.measure(View.java:18991)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5952)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18991)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5952)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2645)
at android.view.View.measure(View.java:18991)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2403)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1342)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1591)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1233)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6552)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:919)
at android.view.Choreographer.doCallbacks(Choreographer.java:710)
at android.view.Choreographer.doFrame(Choreographer.java:645)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:905)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5582)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
我的主要活動，Landing.java
package com.example.visan.peds;
import android.os.Bundle;
import android.support.design.widget.TabLayout;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
public class Landing extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_landing);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
TabLayout tabLayout=(TabLayout)findViewById(R.id.tab_layout);
tabLayout.addTab(tabLayout.newTab().setText("CHILD"));
tabLayout.addTab(tabLayout.newTab().setText("MOTHER"));
tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);
final ViewPager viewPager=(ViewPager) findViewById(R.id.pager);
final Landing_frag_manager frag_manager=new Landing_frag_manager(getSupportFragmentManager(),tabLayout.getTabCount(),Landing.this);
viewPager.setAdapter(frag_manager);
viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener(){
@Override
public void onTabReselected(TabLayout.Tab tab) {
viewPager.setCurrentItem(tab.getPosition());
}
@Override
public void onTabSelected(TabLayout.Tab tab) {
}
@Override
public void onTabUnselected(TabLayout.Tab tab) {
}
});
}
}
它的佈局文件，activity_landing.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
android:id="@+id/main_layout"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:background="?attr/colorPrimary"
android:elevation="6dp"
android:minHeight="?attr/actionBarSize"
android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/>
<android.support.design.widget.TabLayout
android:id="@+id/tab_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/toolbar"
android:background="?attr/colorPrimary"
android:elevation="6dp"
android:minHeight="?attr/actionBarSize"
android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"/>
<android.support.v4.view.ViewPager
android:id="@+id/pager"
android:layout_width="match_parent"
android:layout_height="fill_parent"
android:layout_below="@id/tab_layout"/>
</RelativeLayout>
片段適配器，Landing_f rag_manager.java
package com.example.visan.peds;
import android.content.Context;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
public class Landing_frag_manager extends FragmentPagerAdapter {
private int num_of_tabs;
Context context;
public Landing_frag_manager(FragmentManager fm, int numOfTabs, Context context){
super(fm);
this.num_of_tabs=numOfTabs;
this.context=context;
}
@Override
public Fragment getItem(int position) {
switch (position){
case 0:
Landing_child child=new Landing_child();
return child;
case 1:
Landing_mother mother=new Landing_mother();
return mother;
default:
return null;
}
}
@Override
public int getCount() {
return num_of_tabs;
}
}
代碼片段，Landing_mother.java
package com.example.visan.peds;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import java.util.ArrayList;
import java.util.List;
public class Landing_mother extends Fragment {
private List<Ailment> ailments;
private RecyclerView recyclerView;
LinearLayoutManager layoutManager;
private RecyclerView.Adapter adapter;
public Landing_mother(){}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final View view=inflater.inflate(R.layout.fragment_landing_mother,container,false);
ailments=new ArrayList<>();
getData();
recyclerView =(RecyclerView)view.findViewById(R.id.recyclerView1);
recyclerView.setHasFixedSize(true);
layoutManager=new LinearLayoutManager(getActivity());
layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
recyclerView.setLayoutManager(layoutManager);
return view;
}
private void getData(){
FirebaseDatabase db=FirebaseDatabase.getInstance();
DatabaseReference ref=db.getReference().child("Mother").child("0");
ref.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (dataSnapshot.exists()){
Log.v("SNAPSHOT::",dataSnapshot.toString());
for (DataSnapshot snapshot:dataSnapshot.getChildren()){
Ailment ailment=snapshot.getValue(Ailment.class);
ailments.add(ailment);
}
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
adapter=new LandingAdapter(ailments,getActivity());
recyclerView.setAdapter(adapter);
}
}
它的佈局文件，fragment_landing_mother.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:orientation="vertical"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">
<android.support.v7.widget.RecyclerView
android:id="@+id/recyclerView1"
android:layout_width="match_parent"
android:layout_height="wrap_content">
</android.support.v7.widget.RecyclerView>
</LinearLayout>
病痛的對象類，Ailment.java
package com.example.visan.peds;
public class Ailment {
private String name, image_url;
public void setName(String name){
this.name=name;
}
public String getName(){
return name;
}
public void setImage_url(String image_url){
this.image_url=image_url;
}
public String getImage_url(){
return image_url;
}
}
Adap terfor RecyclerView，LandingAdapter.java
爲recyclerView單列package com.example.visan.peds;
import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;
import com.bumptech.glide.Glide;
import com.firebase.ui.storage.images.FirebaseImageLoader;
import com.google.firebase.storage.FirebaseStorage;
import com.google.firebase.storage.StorageReference;
import java.util.List;
public class LandingAdapter extends RecyclerView.Adapter<LandingAdapter.ViewHolder>{
private List<Ailment> list;
private Context context;
FirebaseStorage storage;
LayoutInflater inflater;
public LandingAdapter(List<Ailment> list, Context context){
super();
this.list=list;
this.context=context;
inflater=LayoutInflater.from(context);
}
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.landing_list_row,parent,false);
ViewHolder viewHolder=new ViewHolder(itemView);
return viewHolder;
}
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
Ailment ailment=list.get(position);
StorageReference storageReference=storage.getReferenceFromUrl(ailment.getImage_url());
Glide.with(context).using(new FirebaseImageLoader()).load(storageReference).into(holder.imageView);
holder.textView.setText(ailment.getName());
}
@Override
public int getItemCount() {
return list.size();
}
class ViewHolder extends RecyclerView.ViewHolder{
public ImageView imageView;
public TextView textView;
public ViewHolder(View view){
super(view);
imageView=(ImageView)view.findViewById(R.id.row_image);
textView=(TextView)view.findViewById(R.id.row_name);
}
}
}
佈局文件，landing_list_row.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/row_image"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/row_name"
android:layout_below="@+id/row_image"/>
</RelativeLayout>