Android 访问firebase数据库中的分层节点？

Helo，我想从firebase数据库层次结构中的

优惠券

节点（如下图所示）获取每个优惠券的优惠券详细信息。

我想从

优惠券

节点检索所有优惠券，并在列表视图中显示。。 我正在尝试用这段代码获取数据库引用，但每次它都返回null引用

 FirebaseDatabase database = FirebaseDatabase.getInstance();
    myRef = database.getReference("VendorDtl").child("vendor1").child("Coupons");

我也试过

FirebaseDatabase database = FirebaseDatabase.getInstance();
    myRef = database.getReference().child("VendorDtl").child("vendor1").child("Coupons");

但是每次我都找不到正确的数据库。。 因此，将显示listview，但其值为空

下面是用于获取值的代码

   protected void onStart() {
    super.onStart();
    //attaching value event listener
    myRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            //clearing the previous artist list
            cpnlist.clear();

            //iterating through all the nodes
            for (DataSnapshot ds : dataSnapshot.getChildren()) {
                Log.i("valueeventDb","tHIS IS CALLED");
                //getting artist
                CouponDetails coups= ds.getValue(CouponDetails.class);
                //adding artist to the list
                cpnlist.add(coups);           
            }

            //creating adapter
            CouponAdp cpnAdp = new CouponAdp(ShowCoupons.this, cpnlist);
            //attaching adapter to the listview
            couplist.setAdapter(cpnAdp);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(getApplicationContext(),"EROrrr Fetching value ... ",Toast.LENGTH_SHORT).show();
        }
    });
}

这是适配器代码

public class CouponAdp extends ArrayAdapter<CouponDetails> {
private Activity context;
List<CouponDetails> shpsv;

public CouponAdp(Activity context, List<CouponDetails> objects) {
    super(context,R.layout.item_list,objects);
    this.context=context;
    this.shpsv=objects;
}

@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {


    if (convertView == null) {
        convertView = ((Activity) getContext()).getLayoutInflater().inflate(R.layout.item_list, parent, false);
    }

    TextView coupNm = (TextView) convertView.findViewById(R.id.textView13);
    TextView coupDesc = (TextView) convertView.findViewById(R.id.textView15);
    TextView coupSts = (TextView) convertView.findViewById(R.id.textView17);
    TextView coupValidity = (TextView) convertView.findViewById(R.id.textView19);
    TextView coupValue = (TextView) convertView.findViewById(R.id.textView21);
    TextView coupCatg = (TextView) convertView.findViewById(R.id.textView23);

   CouponDetails shp=shpsv.get(position);

    coupNm.setText(shp.getScnm());
    coupDesc.setText(shp.getScdesc());
    coupSts.setText(shp.getScsts());
    coupValidity.setText(shp.getScval_from()+" to "+shp.getScval_to());
    coupValue.setText(shp.getSc_percentage());
    coupCatg.setText(shp.getSc_catg());

    return convertView;

}

公共类CouponAdp扩展了ArrayAdapter{
私人活动语境；
列出shpsv；
公共CouponAdp（活动上下文、列表对象）{
超级（上下文、右布局、项目列表、对象）；
this.context=context；
这个.shpsv=对象；
}
@非空
@凌驾
公共视图getView（int位置，@Nullable视图convertView，@NonNull视图组父级）{
if（convertView==null）{
convertView=（（活动）getContext（））.GetLayoutFlater（）.inflate（R.layout.item_列表，父项，false）；
}
TextView coupNm=（TextView）convertView.findViewById（R.id.textView13）；
TextView coupDesc=（TextView）convertView.findViewById（R.id.textView15）；
TextView coupSts=（TextView）convertView.findViewById（R.id.textView17）；
TextView coupValidity=（TextView）convertView.findViewById（R.id.textView19）；
TextView coupValue=（TextView）convertView.findViewById（R.id.textView21）；
TextView coupCatg=（TextView）convertView.findViewById（R.id.textView23）；
CouponDetails shp=shpsv.get（位置）；
coupNm.setText（shp.getScnm（））；
coupDesc.setText（shp.getScdesc（））；
setText（shp.getScsts（））；
coupValidity.setText（shp.getScval_from（）+”到“+shp.getScval_to（））；
coupValue.setText（shp.getSc_percentage（））；
coupCatg.setText（shp.getSc_catg（））；
返回视图；
}

如何正确获取优惠券详细信息并在listview上显示详细信息

虽然我的应用程序没有直接显示任何错误！！ 日志显示

    01-19 21:02:58.502 27083-28252/com.example.vendorapp V/FA: Connection attempt already in progress
01-19 21:02:58.547 27083-27083/com.example.vendorapp W/View: requestLayout() improperly called by android.support.v7.widget.AppCompatTextView{522cae9 V.ED..... ......ID 103,94-256,143 #7f08002a app:id/bb_bottom_bar_title} during layout: running second layout pass
01-19 21:02:58.547 27083-27083/com.example.vendorapp W/View: requestLayout() improperly called by android.support.v7.widget.AppCompatTextView{2d9936e V.ED..... ......ID 40,94-319,143 #7f08002a app:id/bb_bottom_bar_title} during layout: running second layout pass
01-19 21:02:58.547 27083-27083/com.example.vendorapp W/View: requestLayout() improperly called by android.support.v7.widget.AppCompatTextView{6a37a0f V.ED..... ......ID 107,94-251,143 #7f08002a app:id/bb_bottom_bar_title} during layout: running second layout pass
01-19 21:02:58.551 27083-27092/com.example.vendorapp I/zygote64: Do partial code cache collection, code=121KB, data=108KB
01-19 21:02:58.552 27083-27092/com.example.vendorapp I/zygote64: After code cache collection, code=121KB, data=108KB
01-19 21:02:58.552 27083-27092/com.example.vendorapp I/zygote64: Increasing code cache capacity to 512KB
01-19 21:02:58.553 27083-27092/com.example.vendorapp I/zygote64: Compiler allocated 8MB to compile void android.widget.TextView.<init>(android.content.Context, android.util.AttributeSet, int, int)
01-19 21:02:58.567 27083-28252/com.example.vendorapp D/FA: Connected to remote service
01-19 21:02:58.567 27083-28252/com.example.vendorapp V/FA: Processing queued up service tasks: 4
01-19 21:03:03.050 27083-28252/com.example.vendorapp V/FA: Recording user engagement, ms: 4583
01-19 21:03:03.051 27083-28252/com.example.vendorapp V/FA: Activity paused, time: 264940263
01-19 21:03:03.056 27083-28252/com.example.vendorapp D/FA: Logging event (FE): user_engagement(_e), Bundle[{firebase_event_origin(_o)=auto, engagement_time_msec(_et)=4583, firebase_screen_class(_sc)=Promotion, firebase_screen_id(_si)=7031206903639617093}]
01-19 21:03:03.076 27083-27083/com.example.vendorapp V/FA: onActivityCreated
01-19 21:03:03.145 27083-27092/com.example.vendorapp I/zygote64: Compiler allocated 4MB to compile void android.view.View.<init>(android.content.Context, android.util.AttributeSet, int, int)
01-19 21:03:03.165 27083-28252/com.example.vendorapp V/FA: Activity resumed, time: 264940378
01-19 21:03:03.169 27083-28252/com.example.vendorapp D/FA: Logging event (FE): screen_view(_vs), Bundle[{firebase_event_origin(_o)=auto, firebase_previous_class(_pc)=Promotion, firebase_previous_id(_pi)=7031206903639617093, firebase_screen_class(_sc)=Coupons, firebase_screen_id(_si)=7031206903639617094}]
01-19 21:03:03.242 27083-27083/com.example.vendorapp W/View: requestLayout() improperly called by android.support.v7.widget.AppCompatTextView{b514d73 V.ED..... ......ID 103,94-256,143 #7f08002a app:id/bb_bottom_bar_title} during layout: running second layout pass
01-19 21:03:03.242 27083-27083/com.example.vendorapp W/View: requestLayout() improperly called by android.support.v7.widget.AppCompatTextView{1293430 V.ED..... ......ID 40,94-319,143 #7f08002a app:id/bb_bottom_bar_title} during layout: running second layout pass
01-19 21:03:03.243 27083-27083/com.example.vendorapp W/View: requestLayout() improperly called by android.support.v7.widget.AppCompatTextView{99208a9 V.ED..... ......ID 107,94-251,143 #7f08002a app:id/bb_bottom_bar_title} during layout: running second layout pass
01-19 21:03:03.258 27083-27092/com.example.vendorapp I/zygote64: Do full code cache collection, code=249KB, data=183KB
01-19 21:03:03.259 27083-27092/com.example.vendorapp I/zygote64: After code cache collection, code=240KB, data=145KB
01-19 21:03:03.334 27083-27220/com.example.vendorapp D/OpenGLRenderer: endAllActiveAnimators on 0x78bc848400 (RippleDrawable) with handle 0x78bdfc88a0
01-19 21:03:04.158 27083-28252/com.example.vendorapp V/FA: Screen exposed for less than 1000 ms. Event not sent. time: 994
01-19 21:03:04.159 27083-28252/com.example.vendorapp V/FA: Activity paused, time: 264941372
01-19 21:03:04.185 27083-27083/com.example.vendorapp V/FA: onActivityCreated
01-19 21:03:04.211 27083-27083/com.example.vendorapp E/ANDR-PERF-JNI: IPerf::tryGetService failed!
01-19 21:03:04.306 27083-27083/com.example.vendorapp I/DynamiteModule: Considering local module com.google.android.gms.firebase_database:4 and remote module com.google.android.gms.firebase_database:6
01-19 21:03:04.306 27083-27083/com.example.vendorapp I/DynamiteModule: Selected remote version of com.google.android.gms.firebase_database, version >= 6
01-19 21:03:04.344 27083-27083/com.example.vendorapp W/zygote64: Skipping duplicate class check due to unrecognized classloader
01-19 21:03:04.387 27083-28252/com.example.vendorapp V/FA: Activity resumed, time: 264941600
01-19 21:03:04.392 27083-28252/com.example.vendorapp D/FA: Logging event (FE): screen_view(_vs), Bundle[{firebase_event_origin(_o)=auto, firebase_previous_class(_pc)=Coupons, firebase_previous_id(_pi)=7031206903639617094, firebase_screen_class(_sc)=ShowCoupons, firebase_screen_id(_si)=7031206903639617095}]
01-19 21:03:04.411 27083-28380/com.example.vendorapp D/NetworkSecurityConfig: No Network Security Config specified, using platform default
01-19 21:03:04.543 27083-27220/com.example.vendorapp D/OpenGLRenderer: endAllActiveAnimators on 0x78ba5afc00 (RippleDrawable) with handle 0x78d611cb40
01-19 21:03:05.840 27083-27092/com.example.vendorapp I/zygote64: Do partial code cache collection, code=249KB, data=169KB
01-19 21:03:05.841 27083-27092/com.example.vendorapp I/zygote64: After code cache collection, code=249KB, data=169KB
01-19 21:03:05.841 27083-27092/com.example.vendorapp I/zygote64: Increasing code cache capacity to 1024KB
01-19 21:03:06.508 27083-27083/com.example.vendorapp I/valueeventDb: tHIS IS CALLED
01-19 21:03:06.522 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponCategory found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.523 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponStatus found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.523 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponName found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.523 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponValue found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.523 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponDesc found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.523 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponvalidFrom found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.523 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponvalidTo found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.532 27083-27083/com.example.vendorapp I/valueeventDb: tHIS IS CALLED
01-19 21:03:06.532 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponCategory found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.533 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponStatus found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.533 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponName found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.533 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponValue found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.533 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponDesc found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.533 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponvalidFrom found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.533 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponvalidTo found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.541 27083-27083/com.example.vendorapp I/valueeventDb: tHIS IS CALLED
01-19 21:03:06.542 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponCategory found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.542 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponStatus found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.542 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponName found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.542 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponValue found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.542 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponDesc found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.542 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponvalidFrom found on class com.example.vendorapp.CouponDetails
01-19 21:03:06.542 27083-27083/com.example.vendorapp W/ClassMapper: No setter/field for CouponvalidTo found on class com.example.vendorapp.CouponDetails
01-19 21:03:09.477 27083-28252/com.example.vendorapp V/FA: Inactivity, disconnecting from the service

01-19 21:02:58.502 27083-28252/com.example.vendorapp V/FA:连接尝试已在进行中
01-19 21:02:58.547 27083-27083/com.example.vendorapp W/View:requestLayout（）被android.support.v7.widget.AppCompatTextView调用不正确{522cae9 V.ED.…..…ID 103,94-256143#7f08002a app:ID/bb_bottom_bar_title}在布局期间运行第二个布局传递
01-19 21:02:58.547 27083-27083/com.example.vendorapp W/View:requestLayout（）被android.support.v7.widget.AppCompatTextView调用不正确{2d9936e V.ED.…..…ID 40,94-319143#7f08002a app:ID/bb_bottom_bar_title}在布局期间运行第二次布局传递
01-19 21:02:58.547 27083-27083/com.example.vendorapp W/View:requestLayout（）被android.support.v7.widget.AppCompatTextView调用不正确{6a37a0f V.ED.…..…ID 107,94-251143#7f08002a app:ID/bb_bottom_bar_title}在布局期间运行第二个布局传递
01-19 21:02:58.551 27083-27092/com.example.vendorapp I/zygote64:Do部分代码缓存收集，代码=121KB，数据=108KB
01-19 21:02:58.552 27083-27092/com.example.vendorapp I/zygote64:在代码缓存收集之后，代码=121KB，数据=108KB
01-19 21:02:58.552 27083-27092/com.example.vendorapp I/zygote64:将代码缓存容量增加到512KB
01-19 21:02:58.553 27083-27092/com.example.vendorapp I/zygote64:编译器分配了8MB来编译void android.widget.TextView。（android.content.Context，android.util.AttributeSet，int，int）
01-19 21:02:58.567 27083-28252/com.example.vendorapp D/FA:连接到远程服务
01-19 21:02:58.567 27083-28252/com.example.vendorapp V/FA:处理排队服务任务：4
01-19 21:03:03.050 27083-28252/com.example.vendorapp V/FA：记录用户参与度，ms:4583
01-19 21:03:03.051 27083-28252/com.example.vendorapp V/FA:活动暂停，时间：264940263
01-19 21:03:03.056 27083-28252/com.example.vendorapp D/FA：日志记录事件（FE）：用户参与（_e），Bundle[{firebase\u event\u origin（_o）=auto，参与时间_msec（_et）=4583，firebase\u screen\u class（_sc）=促销，firebase\u screen\u id（_si）=703120693; 7093}]
01-19 21:03:03.076 27083-27083/com.example.vendorapp V/FA:on活动已创建
01-19 21:03:03.145 27083-27092/com.example.vendorapp I/zygote64:编译器分配了4MB来编译void android.view.view。（android.content.Context，android.util.AttributeSet，int，int）
01-19 21:03:03.165 27083-28252/com.example.vendorapp V/FA:活动已恢复，时间：264940378
01-19 21:03:03.169 27083-28252/com.example.vendorapp D/FA：日志事件（FE）：屏幕视图（_vs），捆绑[{firebase_event_event_origin（_o）=auto，firebase_previous_class（_pc）=促销，firebase_previous_id（_pi）=703120690; 7039617093，firebase_screen_class（_sc）=优惠券，firebase_screen_id（(si）=703120690; 7039617094]
01-19 21:03:03.242 27083-27083/com.example.vendorapp W/View:requestLayout（）被android.support.v7.widget.AppCompatTextView调用不正确{b514d73 V.ED.…..…ID 103,94-256143#7f08002a app:ID/bb_bottom_bar_title}在布局期间运行第二个布局传递
01-19 21:03:03.242 27083-27083/com.example.vendorapp W/View:requestLayout（）被android.support.v7.widget.AppCompatTextView调用不正确{1293430 V.ED.…..…ID 40,94-319143#7f08002a应用程序：ID/bb_bottom_bar_title}在布局期间运行第二个布局传递
01-19 21:03:03.243 27083-27083/com.example.vendorapp W/View:requestLayout（）被android.support.v7.widget.AppCompatTextView调用不正确{99208a9 V.ED.…..…ID 107,94-251143#7f08002a应用程序：ID/bb_bottom_bar_title}在布局期间运行第二个布局传递
01-19 21:03:03.258 27083-27092/com.example.vendorapp I/zygote64:执行完整代码缓存收集，代码=249KB，数据=183KB
01-19 21:03:03.259 27083-27092/com.example.vendorapp I/zygote64:在代码缓存收集之后，代码=240KB，数据=145KB
01-19 21:03:03.334 27083-27220/com.example.vendorapp D/OpenGLRenderer:0x7上的EndAllActivateAnimators
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference couponsRef = rootRef.child("VendorDtl").child("vendor1").child("Coupons");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<CouponDetails> cpnlist = new ArrayList<>();
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            CouponDetails coups = ds.getValue(CouponDetails.class);
            cpnlist.add(coups);
            Log.d("TAG", coups.getCouponName());
        }
        couplist.setAdapter(cpnAdp);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
couponsRef.addListenerForSingleValueEvent(eventListener);

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference couponsRef = rootRef.child("VendorDtl").child("vendor1").child("Coupons");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<CouponDetails> cpnlist = new ArrayList<>();
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String couponCategory = ds.child("CouponCategory").getValue(String.class);
            String couponDesc = ds.child("CouponDesc").getValue(String.class);
            String couponName= ds.child("CouponName").getValue(String.class);
            String couponStatus = ds.child("CouponStatus").getValue(String.class);
            String couponValue = ds.child("CouponValue").getValue(String.class);
            String couponValidFrom = ds.child("CouponvalidFrom").getValue(String.class);
            String couponValidTo = ds.child("CouponValidTo").getValue(String.class);

            CouponDetails shp = new CouponDetails(couponCategory, couponDesc, couponName, couponStatus, couponValue, couponValidFrom, couponValidTo);
            cpnlist.add(shp);
            Log.d("TAG", couponName);
        }
        couplist.setAdapter(cpnAdp);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
couponsRef.addListenerForSingleValueEvent(eventListener);

public class ActivityClassName extends AppCompatActivity {
    ArrayList<CouponDetails> couponDetailsArrayList;
    FirebseDatabase database;
    ListView couponListView;
    CouponAdp couponAdp;

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

        couponDetailsArrayList = new ArrayList<>();
        database = FirebaseDatabase.getInstance();
        couponListView = findViewbyId(R.id.your_list_view_id);
        couponAdp = new CouponAdp(ActivityClassName.this,couponDetailsArrayList);

        getCoupons();

    }

    private void getCoupons(){
        couponDetailsArrayList.clear();
        DatabaseReference vendorGroup = database.getReference("VendorDtl");
        vendorGroup.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                // for each vendor get inside the block(node)
                for (DataSnapshot snapshot : dataSnapshot.getChildren())
                    // inside each vendor get all the coupons
                    for(DataSnapshot coupon : snapshot.child("Coupons").getChildren());
                        couponDetailsArrayList.add(coupon.getValue(CouponDetails.class));
                couponAdp.notifyDataSetChanged();
                couponListView.setAdapter(couponAdp);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.i("CouponFetchError: ",databaseError.getMessage();
            }
        });
    }
}

vendorGroup.addValueEventListener(new ValueEventListener() {

vendorGroup.addListenerForSingleValueEvent(new ValueEventListener() {

getCoupons();