NavigationView 引發空對象引用異常

Question

我在 Android Studio 中做了一個 DrawerLayout，其中包含一個谷歌地圖片段、一個 NavigationView 和一個 FrameLayout。

我的抽屜布局xml：

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:map="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

        <com.google.android.material.navigation.NavigationView
            android:id="@+id/navigation"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:layout_gravity="start"
            map:menu="@menu/nav_items"/>

        <FrameLayout
            android:id="@+id/frame"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">


        <fragment
            android:id="@+id/map"
            android:name="com.google.android.gms.maps.SupportMapFragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            tools:context=".app" />

        <Button
            android:id="@+id/pop_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:layout_margin="10dp"
            android:background="@drawable/button_back"
            android:text="Pozovi"
            map:layout_constraintBottom_toBottomOf="parent"
            map:layout_constraintEnd_toEndOf="parent"
            map:layout_constraintStart_toStartOf="parent"
            map:layout_constraintTop_toTopOf="@+id/map"
            map:layout_constraintVertical_bias="0.9" />

        <ImageButton
            android:onClick="open_drawer"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:src="@drawable/ic_menu"
            map:layout_constraintBottom_toBottomOf="parent"
            map:layout_constraintEnd_toEndOf="parent"
            map:layout_constraintStart_toStartOf="parent"
            map:layout_constraintTop_toTopOf="parent"
            map:layout_constraintVertical_bias="0.01"
            map:layout_constraintHorizontal_bias="0.01"/>

    </androidx.constraintlayout.widget.ConstraintLayout>
</androidx.drawerlayout.widget.DrawerLayout>

在我的 java 活動代碼中，我添加了一個 onNavigationItemSelected 偵聽器：

dLayout = findViewById(R.id.drawer_layout);
    navView = (NavigationView) findViewById(R.id.navigation);
    navView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            Toast.makeText(getApplicationContext(),"Hello",Toast.LENGTH_SHORT).show();
            return true;
        }
    });

問題是這會引發以下錯誤：

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.navigation.NavigationView.setNavigationItemSelectedListener(com.google.android.material.navigation.NavigationView$OnNavigationItemSelectedListener)' on a null object reference

這是我的完整 Java 類代碼：

package com.example.soferapp;
import androidx.annotation.ColorInt;
import androidx.annotation.NonNull;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import androidx.core.view.GravityCompat;
import androidx.drawerlayout.widget.DrawerLayout;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentActivity;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;

import android.Manifest;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Context;
import android.content.pm.PackageManager;
import android.graphics.Color;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.text.Editable;
import android.util.Log;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.Toast;

import com.example.soferapp.databinding.ActivityAppBinding;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.BitmapDescriptor;
import com.google.android.gms.maps.model.BitmapDescriptorFactory;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.Marker;
import com.google.android.gms.maps.model.MarkerOptions;
import com.google.android.material.bottomsheet.BottomSheetDialog;
import com.google.android.material.navigation.NavigationView;
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 app extends FragmentActivity implements OnMapReadyCallback{
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference driver_data = database.getReference("Data").child("Drivers");
    DatabaseReference user_data = database.getReference("Data").child("Users");
    private GoogleMap mMap;
    private ActivityAppBinding binding;
    LocationManager locationManager;
    LocationListener locationListener;
    int permission_id = 1;
    Button pop_button;
    int num = 0;
    private DrawerLayout mDrawerLayout;
    List<String> list = new ArrayList<>();
    EditText phone_number_input;
    DrawerLayout dLayout;
    LatLng bih_location = new LatLng(43.86073594535343, 17.771002261242077);
    NavigationView navView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        dLayout = findViewById(R.id.drawer_layout);
        navView = (NavigationView) findViewById(R.id.navigation);
        navView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                Toast.makeText(getApplicationContext(),"Hello",Toast.LENGTH_SHORT).show();
                return true;
            }
        });

        binding = ActivityAppBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);
        pop_button = findViewById(R.id.pop_button);
        pop_button.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            BottomSheetDialog bottomSheetDialog = new BottomSheetDialog(app.this,
                    R.style.BottomSheetDialogTheme);
            View bottomSheetView = LayoutInflater.from(getApplicationContext()).inflate(R.layout.bottom_window, (LinearLayout) findViewById(R.id.bottom_containter));
            phone_number_input = bottomSheetView.findViewById(R.id.phone_number);
            bottomSheetView.findViewById(R.id.call).setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    String phone_number = phone_number_input.getText().toString();
                    String key = user_data.push().getKey();
                    user_data.child(key).child("Phone_number").setValue(phone_number);
                    user_data.child(key).child("Location").setValue("My coordinates");
                    user_data.child(key).child("Key").setValue(key);
                    Toast.makeText(app.this, "Poslat je zahtjev za prevoz", Toast.LENGTH_SHORT).show();
                    bottomSheetDialog.dismiss();
                }
            });

            bottomSheetDialog.setContentView(bottomSheetView);
            bottomSheetDialog.show();
        }

    });
}

    @Override
    public void onMapReady(GoogleMap googleMap) {
        googleMap.animateCamera(CameraUpdateFactory.newLatLngZoom(bih_location, 7));
        driver_data.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                googleMap.clear();
                list.clear();
                for(DataSnapshot ds : dataSnapshot.getChildren()) {
                    String driver_coordinates = ds.getValue(String.class);
                    list.add(driver_coordinates);
                }

                for(int i = 0; i < list.size(); i++){
                    String driver_location = list.get(i);
                    Double lat = Double.parseDouble(driver_location.split(",")[0]);
                    Double lon = Double.parseDouble(driver_location.split(",")[1]);
                    mMap.addMarker(new MarkerOptions().title("Vozač").position(new LatLng(lat, lon)).icon(BitmapDescriptorFactory.fromResource(R.drawable.car_icon)));
                }
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {}
        });
        mMap = googleMap;
        enableMyLocation();

    }
    private void enableMyLocation() {
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION)
                == PackageManager.PERMISSION_GRANTED) {
            if (mMap != null) {
                mMap.setMyLocationEnabled(true);
                //View locationButton = ((View) findViewById(Integer.parseInt("1")).getParent()).findViewById(Integer.parseInt("2"));
                //locationButton.setBackgroundColor(Color.parseColor("#20262b"));
            }
        } else {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION, Manifest.permission.ACCESS_COARSE_LOCATION}, permission_id);
            enableMyLocation();
        }
    }
    public void open_drawer(View view){
        //dLayout.openDrawer(Gravity.LEFT);
    }
}

錯誤在第 78 行出現。我嘗試做很多事情來解決這個問題，但沒有任何效果。

Answer 1

在找到任何視圖之前，您應該先 setContentView。 所以像這樣的順序：

binding = ActivityAppBinding.inflate(getLayoutInflater());
setContentView(binding.getRoot());
dLayout = findViewById(R.id.drawer_layout);
navView = (NavigationView) findViewById(R.id.navigation);
navView.setNavigationItemSelectedListener(