Android：片段查找失败（返回 null）

Question

我在 Android Studio 中开始了一个新项目（我是 Android Studio 的新手），它使用了谷歌地图。我希望在我点击导航抽屉中的map 时打开地图。这导致一个片段称为

MapFragment

.

但是当我按下 map 时它抛出这个错误，然后它关闭了应用程序：

这是抽屉导航的样子：

另外我使用的是 8.0 版（奥利奥）。

我已经使用这些代码来实现谷歌地图：

主要：活动：

package com.example.policetracker;

import android.os.Bundle;
import android.view.View;
import android.view.Menu;

import com.example.policetracker.ui.map.MapFragment;
import com.google.android.material.snackbar.Snackbar;
import com.google.android.material.navigation.NavigationView;

import androidx.fragment.app.Fragment;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;
import androidx.drawerlayout.widget.DrawerLayout;
import androidx.appcompat.app.AppCompatActivity;

import com.example.policetracker.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

    private AppBarConfiguration mAppBarConfiguration;
    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

        setSupportActionBar(binding.appBarMain.toolbar);
        binding.appBarMain.fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
        DrawerLayout drawer = binding.drawerLayout;
        NavigationView navigationView = binding.navView;
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        mAppBarConfiguration = new AppBarConfiguration.Builder(
                R.id.nav_home, R.id.nav_map)
                .setOpenableLayout(drawer)
                .build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_content_main);
        NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
        NavigationUI.setupWithNavController(navigationView, navController);

       // Fragment fragment = new MapFragment();
        //getSupportFragmentManager().beginTransaction().replace(R.id.frame_layout,fragment).commit();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onSupportNavigateUp() {
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_content_main);
        return NavigationUI.navigateUp(navController, mAppBarConfiguration)
                || super.onSupportNavigateUp();
    }
}

MapFragment.java:

package com.example.policetracker.ui.map;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.ViewModelProvider;

import com.example.policetracker.R;
import com.example.policetracker.databinding.FragmentMapBinding;
import com.example.policetracker.ui.map.MapViewModel;
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.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;

public class MapFragment extends Fragment {

    private MapViewModel mapViewModel;

    public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {

        SupportMapFragment supportMapFragment = (SupportMapFragment)
            getChildFragmentManager().findFragmentById(R.id.google_map);

        supportMapFragment.getMapAsync(new OnMapReadyCallback() {
            @Override
            public void onMapReady(@NonNull GoogleMap googleMap) {
                googleMap.setOnMapClickListener(new GoogleMap.OnMapClickListener() {
                    @Override
                    public void onMapClick(@NonNull LatLng latLng) {
                        MarkerOptions markerOptions = new MarkerOptions();
                        markerOptions.position(latLng);
                        markerOptions.title(latLng.latitude + " : " + latLng.longitude);
                        googleMap.clear();
                        googleMap.animateCamera(CameraUpdateFactory.newLatLngZoom(
                                latLng,10
                        ));
                        googleMap.addMarker(markerOptions);
                    }
                });
            }
        });

        View view = inflater.inflate(R.layout.fragment_map, container, false);
        return view;
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
    }

}

fregment_map.xml：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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=".ui.map.MapFragment">

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

    <fragment
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/google_map"
        android:name="com.google.android.gms.maps.SupportMapFragment"/>
</androidx.constraintlayout.widget.ConstraintLayout>

AndroidManifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
    <uses-permission android:name="android.permission.INTERNET" />

    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/Theme.PoliceTracker"
        tools:targetApi="31">
        <activity
            android:name=".MapsActivity"
            android:exported="false" />
        <activity
            android:name=".MainActivity"
            android:exported="true"
            android:label="@string/app_name"
            android:theme="@style/Theme.PoliceTracker.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <meta-data
            android:name="com.google.android.geo.API_KEY"
            android:value="//API KEY"/>
    </application>

</manifest>

我把钥匙藏起来了