Java 无法在片段内设置选项卡布局_Java_Android_Android Fragments_Android Viewpager

Java 无法在片段内设置选项卡布局

java android android-fragments

Java 无法在片段内设置选项卡布局,java,android,android-fragments,android-viewpager,Java,Android,Android Fragments,Android Viewpager,我试图在片段中设置一个选项卡布局，并通过导航抽屉中的一个选项调用它，但我得到了一个空对象引用错误。我正在Main2活动中设置导航抽屉 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.goo

我试图在片段中设置一个选项卡布局，并通过导航抽屉中的一个选项调用它，但我得到了一个空对象引用错误。我正在Main2活动中设置导航抽屉


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;
    }

}

主要活动


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;
    }

}

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>

我得到以下错误

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$1.run(chatFragment.java:32)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:201)
        at android.app.ActivityThread.main(ActivityThread.java:6810)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)

我是android开发新手，请帮助我

java.lang.NullPointerException:尝试调用虚拟方法 'android.content.res.Resources android.content.Context.getResources（）' 关于空对象引用

NullPointerException在应用程序尝试使用具有null值的对象引用时引发

你可以试试这个

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

然后

-ViewPager可以调用此方法以获取描述指定页面的标题字符串

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$1.run(chatFragment.java:32)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:201)
        at android.app.ActivityThread.main(ActivityThread.java:6810)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)

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

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