Question

从Firebase数据库检索数据时遇到问题。请原谅我缺乏技术演讲，因为这不是我的强项。我的应用程序是创建一个Vehicle列表，该列表发布到带有RecyclerView的页面上。通过“ NewVehicle.class”添加了新的载具，并在自动生成的节点下将其保存到Firebase。用户可以从“ VehicleList.class”中选择车辆，这会将车辆重定向到“ PasswordProtected.class”。用户需要输入密码才能查看与车辆有关的数据。车辆和“ PasswordProtected.class”都已链接，换句话说，代码知道在数据库中查看哪辆车辆。用户将被重定向到“ DataDisplay.class”，在这里他们可以从想要查看的图形中进行选择。我现在遇到的问题是从一辆车辆上检索特定的“ VehiclesData”并将其显示在图表中。我还添加了一张图像，显示我的数据库在Firebase中的样子。

NewVehicle.class

public class NewVehicle extends AppCompatActivity
{
//XML variables
private ImageView newVehicleImage;
private EditText vehicleMake, vehicleModel, vehicleReg, password, con_password, engineSize;
private ProgressDialog progress;

private Uri vehicleImage = null;

private StorageReference storageRef;
private DatabaseReference databaseRef;

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

    storageRef = FirebaseStorage.getInstance().getReference();
    databaseRef = FirebaseDatabase.getInstance().getReference().child("Vehicles");

    newVehicleImage = findViewById(R.id.new_vehicle_image);
    vehicleMake =  findViewById(R.id.new_vehicle_make);
    vehicleModel =  findViewById(R.id.new_vehicle_model);
    vehicleReg =  findViewById(R.id.new_vehicle_reg);
    engineSize =  findViewById(R.id.new_vehicle_engine);
    Button vehicleAdd = findViewById(R.id.save_btn);
    password = findViewById(R.id.password);
    con_password = findViewById(R.id.con_password);

    progress = new ProgressDialog(this);

    newVehicleImage.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            CropImage.activity()
                    .setGuidelines(CropImageView.Guidelines.ON)
                    .setMinCropResultSize(512, 512)
                    .setAspectRatio(1, 1)
                    .start(NewVehicle.this);
        }
    });

    //When user selects the 'Add' button
    vehicleAdd.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v)
        {
            StartPosting();
        }
    });

}//End onCreate()

private void StartPosting()
{
    progress.setMessage("Posting to Vehicle list");

    //The make, model and reg is saved to Strings
    final String make = vehicleMake.getText().toString().trim();
    final String model = vehicleModel.getText().toString().trim();
    final String reg = vehicleReg.getText().toString().trim();
    final String pass = password.getText().toString().trim();
    final String engine = engineSize.getText().toString().trim();
    final String con_pass = con_password.getText().toString().trim();

    //If the password length is less than six
    if(con_pass.length()<6)
    {
        con_password.setError(getText(R.string.mini_length));
        con_password.requestFocus();
    }

    if(pass.length()<6)
    {
        password.setError(getText(R.string.mini_length));
        password.requestFocus();
    }

    if(!TextUtils.isEmpty(make) &&  !TextUtils.isEmpty(model) && !TextUtils.isEmpty(reg) &&
            !TextUtils.isEmpty(engine) && !TextUtils.isEmpty(pass) &&
            !TextUtils.isEmpty(con_pass) && vehicleImage != null)
    {
        if (pass.equals(con_pass))
        {
            progress.show();

            StorageReference filePath = storageRef.child("vehicle_images").child(vehicleImage.getLastPathSegment());

            filePath.putFile(vehicleImage).addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>()
            {
                @Override
                public void onSuccess(UploadTask.TaskSnapshot taskSnapshot)
                {
                    Uri downloadUrl = taskSnapshot.getDownloadUrl();

                    DatabaseReference newpost = databaseRef.push();

                    newpost.child("Make").setValue(make);
                    newpost.child("Model").setValue(model);
                    newpost.child("Reg").setValue(reg);
                    newpost.child("Password").setValue(pass);
                    newpost.child("Engine").setValue(engine);
                    newpost.child("Confirmed_Password").setValue(con_pass);
                    newpost.child("Image").setValue(Objects.requireNonNull(downloadUrl).toString());

                    progress.dismiss();

                    startActivity(new Intent(NewVehicle.this, VehicleList.class));
                }
            });
        }

        if (!pass.equals(con_pass))
        {
            Toast.makeText(getApplicationContext(), "Passwords must match", Toast.LENGTH_LONG).show();
        }
    }
    else
    {
        Toast.makeText(getApplicationContext(), "Fields can not be left empty", Toast.LENGTH_LONG).show();
    }
}

protected void onActivityResult(int requestCode, int resultCode, Intent data)
{
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == CropImage.CROP_IMAGE_ACTIVITY_REQUEST_CODE)
    {
        CropImage.ActivityResult result = CropImage.getActivityResult(data);

        if (resultCode == RESULT_OK)
        {
            vehicleImage = result.getUri();
            newVehicleImage.setImageURI(vehicleImage);
        }
        //If the resultCode has an error
        else if (resultCode == CropImage.CROP_IMAGE_ACTIVITY_RESULT_ERROR_CODE)
        {
            Toast.makeText(NewVehicle.this, "Application Error", Toast.LENGTH_LONG).show();
        }
    }
}

}//End NewVehicle()

VehicleList.class

public class VehicleList extends AppCompatActivity
{
//Declaring RecyclerView
private RecyclerView vehicleList;
//Declaring DatabaseReference to FireBase
private DatabaseReference databaseRef;

@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    //Sets the layout according to the XML file
    setContentView(R.layout.activity_vehicle_list);

    //Linking to the FireBase Real-time Database
    databaseRef = FirebaseDatabase.getInstance().getReference().child("Vehicles");

    //XML variable
    vehicleList = findViewById(R.id.vehicle_list_view);
    vehicleList.setHasFixedSize(true);
    vehicleList.setLayoutManager(new LinearLayoutManager(this));
    FloatingActionButton addVehicleBtn = findViewById(R.id.add_vehicle_btn);

    //If the user taps the addPostBtn
    addVehicleBtn.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            //They will be redirected to the 'NewPost' class where they can add a new post
            Intent intent = new Intent(VehicleList.this, NewVehicle.class);
            startActivity(intent);
        }//End onClick()
    });//End addVehicleBtn()
}//End onCreate()

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

    //Recycler Adapter for Vehicles
    FirebaseRecyclerAdapter<VehicleLog, VehicleViewHolder> firebaseRecyclerAdapter
            = new FirebaseRecyclerAdapter<VehicleLog, VehicleViewHolder>(
            VehicleLog.class,
            R.layout.vehicle_itemlist,
            VehicleViewHolder.class,
            databaseRef
    ){
        @Override
        protected void populateViewHolder(VehicleViewHolder viewHolder, VehicleLog model, int position)
        {
            //Get the unique identifier for each record in the database
            final String vehicle_key = getRef(position).getKey();

            //Gathers the data
            viewHolder.setMakeText(model.getMake());
            viewHolder.setModelText(model.getModel());
            viewHolder.setRegText(model.getReg());
            viewHolder.setImage(getApplicationContext(), model.getImage());

            //If a record is tapped on
            viewHolder.mView.setOnClickListener(new View.OnClickListener()
            {
                @Override
                public void onClick(View v)
                {
                    //Users redirected to the next screen
                    Intent intent = new Intent(VehicleList.this, PasswordProtected.class);
                    intent.putExtra("Vehicle_id", vehicle_key);
                    startActivity(intent);
                }//End onClick()
            });//End OnClickListener()
        }//End populateViewHolder()
    };//End FirebaseRecyclerAdapter()

    vehicleList.setAdapter(firebaseRecyclerAdapter);

}//End onStart()

public static class VehicleViewHolder extends RecyclerView.ViewHolder
{
    View mView;

    public VehicleViewHolder(View itemView)
    {
        super(itemView);
        mView = itemView;
    }//End VehicleViewHolder()

    //Sets the vehicles make on screen
    public void setMakeText(String make)
    {
        TextView vehicle_make = mView.findViewById(R.id.vehicle_make);
        vehicle_make.setText(make);
    }//End setMakeText()

    //Sets the vehicles model on screen
    public void setModelText(String model)
    {
        TextView vehicle_model = mView.findViewById(R.id.vehicle_model);
        vehicle_model.setText(model);
    }//End setModelText()

    //Sets the vehicles reg on screen
    public void setRegText(String reg)
    {
        TextView vehicle_reg = mView.findViewById(R.id.vehicle_reg);
        vehicle_reg.setText(reg);
    }//End setRegText()

    //Sets the vehicles image on screen using Glide
    public void setImage(Context ctx, String Image)
    {
        ImageView vehicle_image = mView.findViewById(R.id.post_image);
        Glide.with(ctx).load(Image).into(vehicle_image);
    }//End setImage()
}//End VehicleViewHolder()
}

PasswordProtected.class

public class PasswordProtected extends AppCompatActivity
{
//Declaring XML variables
private ImageView imageView;
private TextView vehicle_make, vehicle_model;
private EditText ent_pass;

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

    vehicle_model = findViewById(R.id.model);
    vehicle_make = findViewById(R.id.make);
    imageView = findViewById(R.id.image);

    Button cont = findViewById(R.id.cont);
    ent_pass = findViewById(R.id.editTextPassword);

    DatabaseReference databaseRef = FirebaseDatabase.getInstance().getReference().child("Vehicles");

    //Gathers the unique key of each record in the database
    String vehicle_key = 
Objects.requireNonNull(getIntent().getExtras()).getString("Vehicle_id");


 databaseRef.child(Objects.requireNonNull(vehicle_key))
.addValueEventListener(new ValueEventListener()
    {
        public void onDataChange(DataSnapshot dataSnapshot)
        {
            String model = (String) dataSnapshot.child("Model").getValue();
            String make = (String) dataSnapshot.child("Make").getValue();
            String image = (String) dataSnapshot.child("Image").getValue();

            vehicle_model.setText(model);
            vehicle_make.setText(make);
            Glide.with(PasswordProtected.this).load(image).into(imageView);
        }
        @Override
        public void onCancelled(DatabaseError databaseError)
        {
            Toast.makeText(PasswordProtected.this, "Database Error!", Toast.LENGTH_LONG).show();
        }
    });

    cont.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v)
        {
            final String enter_pass = ent_pass.getText().toString().trim();

            final DatabaseReference databaseRef = FirebaseDatabase.getInstance().getReference().child("Vehicles");

            String vehicle_key = Objects.requireNonNull(getIntent().getExtras()).getString("Vehicle_id");
            databaseRef.child(Objects.requireNonNull(vehicle_key)).addValueEventListener(new ValueEventListener() {
                public void onDataChange(DataSnapshot dataSnapshot)
                {
                    String pass = (String) dataSnapshot.child("Password").getValue();

                    Log.v("Firebase:", pass);

                    if(Objects.equals(pass, enter_pass))
                    {
                        Intent intent = new Intent(PasswordProtected.this, DataDisplay.class);
                        startActivity(intent);
                    }
                    else
                    {
                        Toast.makeText(getApplicationContext(),"PASSWORD DONT MATCH", Toast.LENGTH_LONG).show();
                    }//End else()
                }//End onDataChange()

                public void onCancelled(DatabaseError databaseError)
                {
                    Toast.makeText(PasswordProtected.this, "Database Error!", Toast.LENGTH_LONG).show();
                }
            });

            if(enter_pass.isEmpty())
            {
                ent_pass.setError(getText(R.string.pass_empt));
                ent_pass.requestFocus();
            }

            if(enter_pass.length()<6)
            {
                ent_pass.setError(getText(R.string.mini_length));
                ent_pass.requestFocus();
            }
        }
    });
}
}

DisplayData.class

public class DataDisplay extends AppCompatActivity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_data_display);

    //XML variables
    Button mass_air = findViewById(R.id.mass_air);
    Button engine_load = findViewById(R.id.engine_throttle);
    Button engine_RPM = findViewById(R.id.RPM);
    Button engine_temps = findViewById(R.id.engine_temps);

    mass_air.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            Intent intent = new Intent(DataDisplay.this, GraphEngineAirflow.class);
            startActivity(intent);
        }
    });

    engine_temps.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            Intent intent = new Intent(DataDisplay.this, GraphTempSpecs.class);
            startActivity(intent);
        }
    });

    engine_load.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            Intent intent = new Intent(DataDisplay.this, GraphEngineSpecs.class);
            startActivity(intent);
        }
    });

    engine_RPM.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(DataDisplay.this, GraphEngineRPM.class);
            startActivity(intent);
        }
    });
}
}

GraphEngineAirflow.class

public class GraphEngineAirflow extends Activity implements
    OnChartGestureListener, OnChartValueSelectedListener
{
private static final String TAG = "GraphEngineAirflow";

private LineChart chart;

//Array to hold Mass Airflow data from Firebase
ArrayList<Entry> engineAirflow = new ArrayList<>();

//Variables
LineDataSet set1;
LineData data;

protected void onCreate(Bundle savedInstanceState) {

    /*This creates an Alert dialog on this screen, it also sets it so the user can cancel the message
      for the Mass Airflow rate information*/
    AlertDialog.Builder builder = new AlertDialog.Builder(GraphEngineAirflow.this);
    builder.setCancelable(true);

    //Setting the title and message from the string.xml
    builder.setTitle(GraphEngineAirflow.this.getString(R.string.engine_airflow_title));
    builder.setMessage(GraphEngineAirflow.this.getString(R.string.engine_airflow_def));

    //When the user selects the Cancel button the page will redirect back to the VehicleSpec page
    builder.setNegativeButton(GraphEngineAirflow.this.getString(R.string.cancel), new DialogInterface.OnClickListener()
    {
        public void onClick(DialogInterface dialog, int whichButton) {
            dialog.cancel();
            Intent intent = new Intent(GraphEngineAirflow.this, DataDisplay.class);
            startActivity(intent);
        }//End onClick()
    });//End setNegativeButton()

    builder.setPositiveButton(GraphEngineAirflow.this.getString(R.string.Ok), new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which)
        {

        }//End onClick()
    });//End setPositiveButton()

    builder.show();

    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    //Sets the layout according to the XML file
    setContentView(R.layout.activity_graph_engine_airflow);

    //XML reference
    chart = findViewById(R.id.linechart);

    //Listens for on chart taps
    chart.setOnChartGestureListener(GraphEngineAirflow.this);
    chart.setOnChartValueSelectedListener(GraphEngineAirflow.this);

    //Enable touch gestures
    chart.setTouchEnabled(true);

    //Enable scaling and dragging
    chart.setDragEnabled(true);
    chart.setScaleEnabled(false);
    chart.setDrawGridBackground(false);

    //Enable pinch zoom
    chart.setPinchZoom(true);

    //Background color
    chart.setBackgroundColor(Color.LTGRAY);

    //Setting YAxis
    YAxis left = chart.getAxisLeft();
    left.setAxisMinimum(0f);
    left.setAxisMaximum(50f);
    left.setTextSize(13f);
    left.enableGridDashedLine(10f, 10f, 0f);

    YAxis left2 = chart.getAxisRight();
    left2.setEnabled(false);

    chart.getAxisRight().setEnabled(false);

    //Value string
    String[] vals = new String[] {"0s", "1s", "2s", "3s", "4s", "5s", "6s", "7s", "8s", "9s", "10s", "11s"};

    //Legend object
    Legend i = chart.getLegend();
    //Customise legend
    i.setTextSize(15f);
    i.setForm(Legend.LegendForm.CIRCLE);
    i.setTextColor(Color.BLACK);

    //Setting XAis
    XAxis x = chart.getXAxis();
    x.setTextSize(13f);
    x.setValueFormatter(new MyXAxisValueFormatter(vals));
    x.setGranularity(1);
    x.setPosition(XAxis.XAxisPosition.BOTTOM);

    //Adding value to array as system will crash without
    engineAirflow.add(new Entry(0, 0));
    engineAirflow.add(new Entry(1, 0));

    //Setting the line
    set1 = new LineDataSet(engineAirflow, "Engine AirFlow ");
    set1.setFillAlpha(110);
    set1.setColor(Color.RED);
    set1.setLineWidth(3f);
    set1.setValueTextSize(10f);
    set1.setValueTextColor(Color.BLACK);

    data = new LineData(set1);

    chart.setData(data);

    //Calls the downloadDatt()
    downloadData();
    //Change the chart when a change occurs
    chart.notifyDataSetChanged();

    //XML button
    Button checkD = findViewById(R.id.checkdata);

    //If the user taps the button
    checkD.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            /*This creates an Alert dialog on this screen, it also sets it so the user can cancel the message
            for the Mass Airflow rate information retrieved from the database*/
            AlertDialog.Builder builder2 = new AlertDialog.Builder(GraphEngineAirflow.this);
            builder2.setCancelable(true);

            //Setting the title and message from the string.xml
            builder2.setTitle(GraphEngineAirflow.this.getString(R.string.IMPORTANT));
            builder2.setMessage(GraphEngineAirflow.this.getString(R.string.airflow_info));

            //When the user selects the Cancel button the page will redirect back to the VehicleSpec page
            builder2.setNegativeButton(GraphEngineAirflow.this.getString(R.string.cancel), new DialogInterface.OnClickListener()
            {
                @Override
                public void onClick(DialogInterface dialog, int whichButton)
                {
                    dialog.cancel();
                    Intent intent = new Intent(GraphEngineAirflow.this, DataDisplay.class);
                    startActivity(intent); }//End onClick()
            });//End setNegativeButton()

            //If the user taps Ok
            builder2.setPositiveButton(GraphEngineAirflow.this.getString(R.string.Ok), new DialogInterface.OnClickListener()
            {
                @Override
                public void onClick(DialogInterface dialog, int which)
                {

                }//End onClick()
            });//End setPositiveButton()

            //Show the Dialogs on screen
            builder2.show();
        }//End onClick()
    });//End OnClickListener()
}//End onCreate

//Downloads Data from FireBase
private void downloadData()
{
    //ArrayAdapter
    final ArrayAdapter<String> arrayAdapter = new ArrayAdapter<>(this,R.layout.activity_graph_engine_airflow);

    //Connecting into table "VehicleData" on the FireBase database
    DatabaseReference database = FirebaseDatabase.getInstance().getReference("Vehicles");

    //ChildEventListener allows child events to be listened for
    database.addChildEventListener(new ChildEventListener()
    {
        public void onChildAdded(DataSnapshot dataSnapshot, String prevChildKey)
        {
            //Holds the DataSnapshot value of the database as type String
            VehicleData vehicleData = dataSnapshot.getValue(VehicleData.class);

            //Prints values to console to prove the download is working
            System.out.println("getmassAirflowRate: " + Objects.requireNonNull(vehicleData).getMassAirflowRate());
            System.out.println("prevChildKey: " + prevChildKey);
            System.out.println("data.key" + dataSnapshot.getKey());

            //Converting value to integer
            setData(Integer.parseInt(dataSnapshot.getKey()), vehicleData);

            //Will refresh app when the data changes in the database
            arrayAdapter.notifyDataSetChanged();
        }//End onChildAdded()

        //Will run when data within the database is changed/edited
        public void onChildChanged(DataSnapshot dataSnapshot, String s)
        {

        }//End onChildChanged()

        //Will run when data within the database is removed
        public void onChildRemoved(DataSnapshot dataSnapshot)
        {

        }//End onChildRemoved()

        //Will run when data within the database is moved to different location
        public void onChildMoved(DataSnapshot dataSnapshot, String s)
        {

        }//End onChildMoved()

        //Will run when any sort of error occurs
        public void onCancelled(DatabaseError databaseError)
        {

        }//End onCancelled()
    });//End addChildEventListener()
}//End DownloadData()

//Function sets the data on the graph
private void setData(int key, VehicleData vehicleData)
{
    //Prints to console first
    System.out.println("Using key: " + key);
    System.out.println("Setting Mass Intake Airflow Rate: " + vehicleData.getMassAirflowRate());

    //Adds new entries to the arrayList and converts the string into a float
    engineAirflow.add(new Entry(key + 2, Float.parseFloat(vehicleData.getMassAirflowRate())));

    //Change the chart when changes occurs
    set1.notifyDataSetChanged();
    data.notifyDataChanged();
    this.chart.notifyDataSetChanged();

    //Redisplay chart
    chart.invalidate();
}//End setData()

//Using the String to change the values of the XAis
public class MyXAxisValueFormatter implements IAxisValueFormatter
{
    private String[] mVals;
    private MyXAxisValueFormatter(String[] vals)
    {
        this.mVals = vals;
    }//End MyXAxisValueFormatter()

    @Override
    public String getFormattedValue(float value, AxisBase axis)
    {
        return mVals[(int)value];
    }//End getFormattedValue()

}//End MyXAxisValueFormatter()

@Override
//Sends log message if action is performed
public void onChartGestureStart(MotionEvent me, ChartTouchListener.ChartGesture lastPerformedGesture)
{
    Log.i(TAG, "onChartGestureStart: X:" + me.getX() + "Y:" + me.getY());
}//End()

@Override
//Sends log message if action is performed
public void onChartGestureEnd(MotionEvent me, ChartTouchListener.ChartGesture lastPerformedGesture)
{
    Log.i(TAG, "onChartGestureEnd: " + lastPerformedGesture);
}//End()

@Override
//Sends log message if action is performed
public void onChartLongPressed(MotionEvent me)
{
    Log.i(TAG, "onChartLongPressed: ");
}//End()

@Override
//Sends log message if action is performed
public void onChartDoubleTapped(MotionEvent me)
{
    Log.i(TAG, "onChartDoubleTapped: ");
}//End()

@Override
//Sends log message if action is performed
public void onChartSingleTapped(MotionEvent me)
{
    Log.i(TAG, "onChartSingleTapped: ");
}//End()

@Override
//Sends log message if action is performed
public void onChartFling(MotionEvent me1, MotionEvent me2, float velocityX, float velocityY)
{
    Log.i(TAG, "onChartFling: veloX: " + velocityX + "veloY" + velocityY);
}//End()

@Override
//Sends log message if action is performed
public void onChartScale(MotionEvent me, float scaleX, float scaleY)
{
    Log.i(TAG, "onChartScale: ScaleX: " + scaleX + "ScaleY: " + scaleY);
}//End()

@Override
//Sends log message if action is performed
public void onChartTranslate(MotionEvent me, float dX, float dY)
{
    Log.i(TAG, "onChartTranslate: dX" + dX + "dY" + dY);
}//End()

@Override
//Sends log message if action is performed
public void onValueSelected(Entry e, Highlight h) {
    Log.i(TAG, "onValueSelected: " + e.toString());
    Toast.makeText(this, "onValueSelected: " + e.toString(), Toast.LENGTH_SHORT).show();
}//End()

@Override
//Sends log message if action is performed
public void onNothingSelected() {
    Log.i(TAG, "onNothingSelected: ");
}//End()

}//End GraphEngineAirflow()

Answer 1

更改：

 DatabaseReference database = FirebaseDatabase.getInstance().getReference("Vehicles").child(vehicle_key).child("Vehicle data");

到

 DatabaseReference database = FirebaseDatabase.getInstance().getReference("Vehicles").child(vehicle_key).child("VehiclesData");

您的代码应该没问题。

Answer 2

在第一种情况下，请使用像这样的参考（不在根目录中）

 DatabaseReference database = FirebaseDatabase.getInstance().getReference().child("VehicleData").child(your id here mean 0 1 2 how you set that simillarly get as it (check if is not null first) );

您未引用0 1 2 3 id，这意味着在Vehicle Data的根目录下，您会获得完整的子代作为值，这不是VehicleData类的有效类。因此，您需要移至0 1 2 3，您将拥有完整的车辆数据类

Firebase数据检索问题

2 个答案: