无法在片段内设置选项卡布局

Question

我正在尝试在片段内设置选项卡布局并通过导航抽屉内的选项调用它，但出现空对象引用错误。我正在 Main2Activity

中设置我的导航抽屉

Main2Activity


import android.content.Intent;
import android.os.Bundle;

import com.bumptech.glide.Glide;
import com.example.fireapp.model.Users;
import com.firebase.ui.auth.AuthUI;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.android.material.navigation.NavigationView;
import com.google.android.material.tabs.TabLayout;

import androidx.annotation.NonNull;
import androidx.appcompat.app.ActionBarDrawerToggle;
import androidx.core.view.GravityCompat;
import androidx.drawerlayout.widget.DrawerLayout;
import androidx.viewpager.widget.ViewPager;
import androidx.appcompat.app.AppCompatActivity;

import android.text.Layout;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;

import androidx.appcompat.widget.Toolbar;

import com.example.fireapp.ui.main.SectionsPagerAdapter;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
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 de.hdodenhof.circleimageview.CircleImageView;

public class Main2Activity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {

    private DrawerLayout drawerLayout;
    private NavigationView navigationView;
    private View headerView;
    private TextView usernameText;
    private TextView emailText;
    private CircleImageView userImage;
    private FirebaseUser firebaseUser;
    private DatabaseReference reference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);

        //toolbar
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("fireApp");

        //navigation drawer
        drawerLayout = findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawerLayout, toolbar,
                R.string.navigation_drawer_open,
                R.string.navigation_drawer_close);
        drawerLayout.addDrawerListener(toggle);
        toggle.syncState();


        //user details
        navigationView = findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
        headerView = navigationView.getHeaderView(0);
        usernameText = headerView.findViewById(R.id.usernameText);
        userImage = headerView.findViewById(R.id.userImage);
        emailText = headerView.findViewById(R.id.userEmail);
        setUserDetails();
//        if(savedInstanceState==null) {
//            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
//                    new chatFragment()).commit();
//            navigationView.setCheckedItem(R.id.nav_chat);
//        }
   }

    private void setUserDetails() {
        firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        reference = FirebaseDatabase.getInstance().getReference("User").child(firebaseUser.getUid());
        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                Users user = dataSnapshot.getValue(Users.class);
                usernameText.setText(user.getUsername());
                emailText.setText(firebaseUser.getEmail());
                if (user.getImageUrl().equals("default")) {
                    userImage.setImageResource(R.mipmap.ic_launcher);
                } else {
                    Glide.with(getApplicationContext()).load(user.getImageUrl()).into(userImage);
                }
            }

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

            }
        });
    }

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        switch(menuItem.getItemId()){
            case R.id.nav_chat:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                        new chatFragment()).commit();
                break;
            case R.id.nav_share:
                Toast.makeText(this, "Share!", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_feedback:
                Toast.makeText(this, "Feedback!", Toast.LENGTH_SHORT).show();
                break;
        }


        drawerLayout.closeDrawer(GravityCompat.START);
        return true;
    }

    @Override
    public void onBackPressed() {

        if (drawerLayout.isDrawerOpen(GravityCompat.START)) {
            drawerLayout.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }

    }

    //3 dots menu
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        super.onCreateOptionsMenu(menu);
        getMenuInflater().inflate(R.menu.main_menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        super.onOptionsItemSelected(item);
        if (item.getItemId() == R.id.logout) {
            AuthUI.getInstance()
                    .signOut(Main2Activity.this)
                    .addOnCompleteListener(new OnCompleteListener<Void>() {

                        @Override
                        public void onComplete(@NonNull Task<Void> task) {

                            startActivity(new Intent(Main2Activity.this, MainActivity.class));
                            finish();

                        }
                    });
            return true;
        } else
            return false;
    }

}

activity_main2.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout 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:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context=".Main2Activity"
    tools:openDrawer="end">

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical">

        <com.google.android.material.appbar.AppBarLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:theme="@style/ThemeOverlay.AppCompat.Dark">

            <androidx.appcompat.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="wrap_content">

            </androidx.appcompat.widget.Toolbar>

        </com.google.android.material.appbar.AppBarLayout>

        <FrameLayout
            android:id="@+id/fragment_container"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </LinearLayout>
    <com.google.android.material.navigation.NavigationView
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:id="@+id/nav_view"
        app:headerLayout="@layout/drawer_header"
        app:menu="@menu/drawer_menu"></com.google.android.material.navigation.NavigationView>
</androidx.drawerlayout.widget.DrawerLayout>

聊天片段

package com.example.fireapp;


import android.os.Bundle;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.viewpager.widget.ViewPager;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.example.fireapp.ui.main.SectionsPagerAdapter;
import com.google.android.material.tabs.TabLayout;

public class chatFragment extends Fragment {
    public  static TabLayout tabLayout;
    public  static ViewPager viewPager;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        final View view = inflater.inflate(R.layout.fragment_chat,container,false);
        tabLayout=view.findViewById(R.id.tabs);
        viewPager=view.findViewById(R.id.view_pager);
        viewPager.setAdapter(new SectionsPagerAdapter(getChildFragmentManager()));
        tabLayout.post(new Runnable() {
            @Override
            public void run() {
                tabLayout.setupWithViewPager(viewPager);
            }
        });
        return view;
    }
}

fragment_chat.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context=".chatFragment">

    <!-- TODO: Update blank fragment layout -->
    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="@string/hello_blank_fragment" />

        <com.google.android.material.tabs.TabLayout
            android:id="@+id/tabs"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="?attr/colorPrimary" />


    <androidx.viewpager.widget.ViewPager
        android:id="@+id/view_pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior" />

</FrameLayout>

SectionPagerAdapter

package com.example.fireapp.ui.main;

import android.content.Context;

import androidx.annotation.Nullable;
import androidx.annotation.StringRes;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentActivity;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentPagerAdapter;

import com.example.fireapp.R;
import com.example.fireapp.tab1;
import com.example.fireapp.tab2;
import com.example.fireapp.tab3;

import java.util.List;

/**
 * A [FragmentPagerAdapter] that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class SectionsPagerAdapter extends FragmentPagerAdapter {

    @StringRes
    private static final int[] TAB_TITLES = new int[]{R.string.tab_text_1, R.string.tab_text_2,R.string.tab_text_3};
    private Context mContext ;



    public SectionsPagerAdapter(Context context, FragmentManager fm) {
        super(fm);
        mContext = context;
    }

    public SectionsPagerAdapter(FragmentManager childFragmentManager) {
        super(childFragmentManager);
    }


    @Override
    public Fragment getItem(int position) {
       switch (position){
           case 0 :
               tab1 tab1 = new tab1();
               return tab1;
           case 1:
               tab2 tab2 = new tab2();
               return tab2;
           case  2:
               tab3 tab3 = new tab3();
               return tab3;

       }
       return null;
    }

    @Nullable
    @Override
    public CharSequence getPageTitle(int position) {
        return mContext.getResources().getString(TAB_TITLES[position]);
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }
}

我收到以下错误

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.fireapp, PID: 6785
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
        at com.example.fireapp.ui.main.SectionsPagerAdapter.getPageTitle(SectionsPagerAdapter.java:61)
        at com.google.android.material.tabs.TabLayout.populateFromPagerAdapter(TabLayout.java:1323)
        at com.google.android.material.tabs.TabLayout.setPagerAdapter(TabLayout.java:1314)
        at com.google.android.material.tabs.TabLayout.setupWithViewPager(TabLayout.java:1227)
        at com.google.android.material.tabs.TabLayout.setupWithViewPager(TabLayout.java:1188)
        at com.google.android.material.tabs.TabLayout.setupWithViewPager(TabLayout.java:1168)
        at com.example.fireapp.chatFragment.run(chatFragment.java:32)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:201)
        at android.app.ActivityThread.main(ActivityThread.java:6810)
        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:873)

我是 android 开发的新手，请帮忙。

Answer 1

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference

NullPointerException 当应用程序尝试使用具有空值的 object 引用时抛出。

你可以试试这个

private String TAB_TITLES[] = new String[]{"Tab1", "Tab2", "Tab3"};

然后

@Override
public CharSequence getPageTitle(int position) {
    // Generate title based on item position
    return TAB_TITLES[position];
}

getPageTitle() - ViewPager 可以调用此方法来获取描述指定页面的标题字符串。

无法在片段内设置选项卡布局

cannot set tab layout inside fragment

java

android

android-fragments

android-viewpager