android - 如何避免 Android Studio 中的 Fragment 崩溃？

Question

我正在尝试创建一个显示从网站检索到的字符串列表的片段。我使用 Android Studio 的命令“new -> Fragment (List)”创建了一些片段。片段是指主导航抽屉活动。当我尝试使用模拟器执行应用程序时，单击片段名称时应用程序崩溃。我能做些什么来解决这个问题？

MainActivity.java

package com.example.is2_app;

import android.os.Bundle;

import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;

import com.example.is2_app.ui.dummy.DummyContent;
import com.example.is2_app.ui.municipio.municipioFragment;
import com.google.android.material.navigation.NavigationView;

import androidx.drawerlayout.widget.DrawerLayout;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;

public class MainActivity extends AppCompatActivity implements municipioFragment.OnListFragmentInteractionListener {

    private AppBarConfiguration mAppBarConfiguration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        NavigationView navigationView = findViewById(R.id.nav_view);
        // 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_municipio, R.id.nav_home, R.id.nav_gallery, R.id.nav_slideshow,
                R.id.nav_tools, R.id.nav_share, R.id.nav_send)
                .setDrawerLayout(drawer)
                .build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
        NavigationUI.setupWithNavController(navigationView, navController);
    }

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

    @Override
    public void onListFragmentInteraction(DummyContent.DummyItem item) {

    }
}

activity_main_drawer.xml

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

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/nav_gallery"
            android:icon="@drawable/ic_menu_gallery"
            android:title="@string/menu_comune" />

        <item
            android:id="@+id/nav_municipio"
            android:icon="@drawable/ic_menu_gallery"
            android:title="@string/menu_municipio" />

        <item
            android:id="@+id/nav_slideshow"
            android:icon="@drawable/ic_menu_slideshow"
            android:title="@string/menu_slideshow" />
        <item
            android:id="@+id/nav_tools"
            android:icon="@drawable/ic_menu_manage"
            android:title="@string/menu_tools" />
    </group>

    <item android:title="Communicate">
        <menu>
            <item
                android:id="@+id/nav_share"
                android:icon="@drawable/ic_menu_share"
                android:title="@string/menu_share" />
            <item
                android:id="@+id/nav_send"
                android:icon="@drawable/ic_menu_send"
                android:title="@string/menu_send" />
        </menu>
    </item>

</menu>

ComuneFragment.java

package com.example.is2_app.ui.comune;

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

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

import com.example.is2_app.R;

import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;

import java.io.IOException;

public class ComuneFragment extends Fragment {

    private ComuneViewModel comuneViewModel;
    private Button getBtn;
    private TextView result;

    public ComuneFragment() {
    }


    public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        comuneViewModel =
        ViewModelProviders.of(this).get(ComuneViewModel.class);
        View root = inflater.inflate(R.layout.fragment_comune, container, false);


        result = result.findViewById(R.id.result);
        getBtn = getBtn.findViewById(R.id.getBtn);
        getBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getWebsite();
            }
        });
        return root;
    }

    private void getWebsite() {

        new Thread(new Runnable() {
            @Override
            public void run() {
                final StringBuilder builder = new StringBuilder();
                try {
                    Document doc = Jsoup.connect("http://www.ssaurel.com/blog").get();
                    String title = doc.title();
                    Elements links = doc.select("a[href]");
                    builder.append(title).append("\n");

                    for (Element link : links) {
                        builder.append("\n").append("Link: ").append(link.attr("href"))
                                .append("\n").append("Text: ").append(link.text());

                    }

                } catch (IOException e) {
                    builder.append("Error: ").append(e.getMessage()).append("\n");
                }

                if (getActivity() != null) {
                    getActivity().runOnUiThread(new Runnable() {

                        @Override
                        public void run() {
                            result.setText(builder.toString());
                        }

                    });
                }
            }
        }).start();
    }
}

ComuneViewModel.java

package com.example.is2_app.ui.comune;

import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

public class ComuneViewModel extends ViewModel {

    private MutableLiveData<String> mText;

    public ComuneViewModel() {
        mText = new MutableLiveData<>();
        mText.setValue("This is gallery fragment");
    }

    public LiveData<String> getText() {
        return mText;
    }
}

fragment_comune.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">


    <Button
        android:id="@+id/getBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/result"
        android:layout_marginStart="148dp"
        android:layout_marginLeft="148dp"
        android:layout_marginTop="132dp"
        android:text="@string/get_website"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.006"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.495" />

    <TextView
        android:id="@+id/result"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/getBtn"
        android:layout_marginStart="8dp"
        android:layout_marginTop="488dp"
        android:layout_marginEnd="8dp"
        android:text="Result..."
        android:textAlignment="center"
        android:textSize="20sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


</androidx.constraintlayout.widget.ConstraintLayout>

日志猫

2020-01-21 16:05:54.043 25410-25410/com.example.is2_app E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.is2_app, PID: 25410
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.TextView.findViewById(int)' on a null object reference
        at com.example.is2_app.ui.comune.ComuneFragment.onCreateView(ComuneFragment.java:39)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2600)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:881)
        at androidx.fragment.app.FragmentManagerImpl.addAddedFragments(FragmentManagerImpl.java:2100)
        at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1874)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1830)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
        at androidx.fragment.app.FragmentManagerImpl$2.run(FragmentManagerImpl.java:150)
        at android.os.Handler.handleCallback(Handler.java:790)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

标签： androidlistviewandroid-fragmentsfragment