java - 无法将数据从 firebase 添加到字符串数组

Question

因此，我尝试在其每个索引的字符串数组中传递来自 firebase 实时数据库的数据，但我的 Adapter 类的 get View 方法显示尝试读取空数组，但我在数组中添加了数据。

这是我启动数组 mCost 的 Activity

public class Department extends AppCompatActivity {

    ListView listView;
    String[] mItem = {"Vehicle Dynamics", "Chassis", "Power Train", "Brakes", "Electronics","Miscellaneous"};
    int[] mImage = {R.drawable.vehicledynamics, R.drawable.chassis, R.drawable.powertrain,R.drawable.brakes,
            R.drawable.electronics, R.drawable.miscellaneous};
    String[] mCost;
    int sumofdcost=0;
    String sumofdcostvalue;
    private DatabaseReference vdDbcost,cDbcost,ptDbcost,bDbcost,eDbcost,mDbcost, dDb,nDb;
    private FirebaseDatabase database;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_department);
        listView = findViewById(R.id.listView);
        Objects.requireNonNull(getSupportActionBar()).setDisplayHomeAsUpEnabled(true);
        database=FirebaseDatabase.getInstance();
        nDb=database.getReference().child("Navigation Activity").child("Cost");
        dDb=database.getReference().child("Department");

        vdDbcost=database.getReference().child("Department").child("Vehicle Dynamics").child("Cost");
        cDbcost=database.getReference().child("Department").child("Chassis").child("Cost");
        ptDbcost=database.getReference().child("Department").child("Brakes").child("Cost");
        bDbcost=database.getReference().child("Department").child("Power Train").child("Cost");
        eDbcost=database.getReference().child("Department").child("Electronics").child("Cost");
        mDbcost=database.getReference().child("Department").child("Miscellaneous").child("Cost");
        vdDbcost.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                Long cost=dataSnapshot.getValue(Long.class);
                mCost[0]=String.valueOf(cost);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { }
        });
        cDbcost.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                Long cost=dataSnapshot.getValue(Long.class);
                mCost[1]=String.valueOf(cost);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { }
        });
        ptDbcost.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                Long cost=dataSnapshot.getValue(Long.class);
                mCost[2]=String.valueOf(cost);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { }
        });
        bDbcost.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                Long cost=dataSnapshot.getValue(Long.class);
                mCost[3]=String.valueOf(cost);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { }
        });
        eDbcost.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                Long cost=dataSnapshot.getValue(Long.class);
                mCost[4]=String.valueOf(cost);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { }
        });
        mDbcost.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                Long cost=dataSnapshot.getValue(Long.class);
                mCost[5]=String.valueOf(cost);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { }
        });
        dDb.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                for(DataSnapshot ds: dataSnapshot.getChildren()){
                    Map<String,Object> map=(Map<String,Object>) ds.getValue();
                    Object totalcost=map.get("Cost");
                    int totalcostvalue=Integer.parseInt(String.valueOf(totalcost));
                    sumofdcost+=totalcostvalue;
                    sumofdcostvalue=String.valueOf(sumofdcost);
                }
                nDb.setValue(sumofdcostvalue);

            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });




        //Creating ADAPTER class
        MainAdapter adapter = new MainAdapter(this, mItem, mImage, mCost);
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                if (position == 0) {
                    //A//
                    //sub department, vehicle dynamics//
                    Intent intent = new Intent(Department.this, VehicleDynamicsActivity.class);
                    startActivity(intent);
                }
                if (position == 1) {
                    //B//
                    //Sub department,chassis//
                    Intent intent = new Intent(Department.this, ChassisItemActivity.class);
                    startActivity(intent);

                }
                if (position == 2) {
                    //C//
                    //Sub department,power train//
                    Intent intent = new Intent(Department.this, PowerTrainActivity.class);
                    startActivity(intent);

                }
                if (position == 3) {
                    //D//
                    //Sub department, brakes//
                    Intent intent = new Intent(Department.this, BrakesItemActivity.class);
                    startActivity(intent);

                }
                if (position == 4) {
                    //E//
                    //sub department,electronics//
                    Intent intent = new Intent(Department.this, ElectronicsItemActivity.class);
                    startActivity(intent);

                }
                if (position == 5) {
                    //F//
                    //sub department, miscellaneous//
                    Intent intent = new Intent(Department.this, MiscellaneousItemActivity.class);
                    startActivity(intent);

                }
            }
        });
    }

}

<小时/>

这就是适配器 getView 方法

   @Override
public View getView(int position, View convertView, ViewGroup parent) {
    if(inflater==null){
        inflater=(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }
    if(convertView==null){
        convertView = inflater.inflate(R.layout.row, null);
    }

    TextView itemName=convertView.findViewById(R.id.item_name);
    itemName.setText(mItem[position]);
    ImageView imageView = convertView.findViewById(R.id.d_image);
    imageView.setImageResource(mImage[position]);
    TextView cost1 = convertView.findViewById(R.id.cost_value);
    cost1.setText(mCost[position]);

    return convertView ;

<小时/>

这是原木猫

2020-03-12 15:13:49.032 29730-29730/com.example.android.zfr E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.zfr, PID: 29730
java.lang.NullPointerException: Attempt to read from null array
    at com.example.android.zfr.MainAdapter.getView(MainAdapter.java:56)
    at android.widget.AbsListView.obtainView(AbsListView.java:2397)
    at android.widget.ListView.makeAndAddView(ListView.java:2052)
    at android.widget.ListView.fillDown(ListView.java:786)
    at android.widget.ListView.fillFromTop(ListView.java:847)
    at android.widget.ListView.layoutChildren(ListView.java:1826)
    at android.widget.AbsListView.onLayout(AbsListView.java:2193)
    at android.view.View.layout(View.java:19693)
    at android.view.ViewGroup.layout(ViewGroup.java:6079)
    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
    at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
    at android.view.View.layout(View.java:19693)
    at android.view.ViewGroup.layout(ViewGroup.java:6079)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
    at android.view.View.layout(View.java:19693)
    at android.view.ViewGroup.layout(ViewGroup.java:6079)
    at androidx.appcompat.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:446)
    at android.view.View.layout(View.java:19693)
    at android.view.ViewGroup.layout(ViewGroup.java:6079)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
    at android.view.View.layout(View.java:19693)
    at android.view.ViewGroup.layout(ViewGroup.java:6079)
    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
    at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
    at android.view.View.layout(View.java:19693)
    at android.view.ViewGroup.layout(ViewGroup.java:6079)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
    at com.android.internal.policy.DecorView.onLayout(DecorView.java:764)
    at android.view.View.layout(View.java:19693)
    at android.view.ViewGroup.layout(ViewGroup.java:6079)
    at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2541)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2257)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1437)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6886)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1029)
    at android.view.Choreographer.doCallbacks(Choreographer.java:841)
    at android.view.Choreographer.doFrame(Choreographer.java:772)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1015)
    at android.os.Handler.handleCallback(Handler.java:794)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:176)
    at android.app.ActivityThread.main(ActivityThread.java:6651)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:824)

Answer 1

此错误是因为您的字符串数组为空，并且尚未初始化。

在您的 Department 类上尝试一下

String[] mCost = new String[5];

5是字符串数组的长度