问题描述
我创建了一个ViewPager,但是我一直收到错误消息说它为null。我尝试更改ViewPager,检查了R ID,即XML中指定的ID。我怀疑这与我构造XML文件的方式有关。谢谢。
这是我得到的错误:
E/in.mycampuscha: [qarth_debug:] get PatchStore::createdisableExceptionQarthFile method fail.
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.devin.mycampuschat,PID: 2034
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.devin.mycampuschat/com.devin.MyCampusChat.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference
at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3430)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3614)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:86)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2199)
at android.os.Handler.dispatchMessage(Handler.java:112)
at android.os.Looper.loop(Looper.java:216)
at android.app.ActivityThread.main(ActivityThread.java:7625)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:524)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:987)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference
at com.devin.MyCampusChat.MainActivity.onCreate(MainActivity.java:63)
at android.app.Activity.performCreate(Activity.java:7458)
at android.app.Activity.performCreate(Activity.java:7448)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1286)
at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3409)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3614)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:86)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2199)
at android.os.Handler.dispatchMessage(Handler.java:112)
at android.os.Looper.loop(Looper.java:216)
at android.app.ActivityThread.main(ActivityThread.java:7625)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:524)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:987)
这是XML:
<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout
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=".MainActivity">
<com.google.android.material.appbar.AppBarLayout
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<include layout="@layout/app_bar_layout" android:id="@+id/toolbar" />
<com.google.android.material.tabs.TabLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/tab_layout">
<com.google.android.material.tabs.TabItem
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:icon="@drawable/ic_chats"
android:text="Chats" />
<com.google.android.material.tabs.TabItem
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:icon="@drawable/ic_friends"
android:text="Friends" />
<com.google.android.material.tabs.TabItem
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:icon="@drawable/ic_requests"
android:text="Requests" />
</com.google.android.material.tabs.TabLayout>
</com.google.android.material.appbar.AppBarLayout>
<androidx.viewpager.widget.ViewPager
android:layout_below="@+id/view_pager"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_toEndOf="@+id/toolbar"
app:layout_behavior="com.google.android.material.appbar.AppBarLayout$ScrollingViewBehavior">
</androidx.viewpager.widget.ViewPager>
</androidx.coordinatorlayout.widget.CoordinatorLayout>
这是活动:
package com.devin.MyCampusChat;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentPagerAdapter;
import androidx.viewpager.widget.ViewPager;
import android.app.DownloadManager;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import com.devin.MyCampusChat.Fragments.ChatsFragment;
import com.devin.MyCampusChat.Fragments.FriendsFragment;
import com.devin.MyCampusChat.Fragments.RequestsFragment;
import com.google.android.material.tabs.TabLayout;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends AppCompatActivity {
private FirebaseAuth fAuth;
private FirebaseUser fUser;
private Toolbar mToolbar;
private TabLayout tabLayout;
private ViewPager viewPager;
//Fragments.
private ChatsFragment chatsFragment;
private FriendsFragment friendsFragment;
private RequestsFragment requestsFragment;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tabLayout = findViewById(R.id.tab_layout);
viewPager = findViewById(R.id.view_pager);
fAuth = FirebaseAuth.getInstance();
mToolbar = findViewById(R.id.toolbar);
//fragments
chatsFragment = new ChatsFragment();
friendsFragment = new FriendsFragment();
requestsFragment = new RequestsFragment();
setSupportActionBar(mToolbar);
getSupportActionBar().setTitle("MyCampus Chats");
tabLayout.setupWithViewPager(viewPager);
ViewPagerAdapter viewPagerAdapter = new MainActivity.ViewPagerAdapter(getSupportFragmentManager(),0);
viewPager.setAdapter(viewPagerAdapter);
viewPagerAdapter.addFragment(chatsFragment,"Chats");
viewPagerAdapter.addFragment(friendsFragment,"Friends");
viewPagerAdapter.addFragment(requestsFragment,"Requests");
}
@Override
public void onStart() {
super.onStart();
fUser = fAuth.getCurrentUser();
//Check to see if the current user is null or not,if so,they are not logged in.
if(fUser == null){
Intent startIntent = new Intent(MainActivity.this,StartActivity.class);
startActivity(startIntent);
finish();
}
}
/** Creates the options menu **/
@Override
public boolean onCreateOptionsMenu(Menu menu) {
super.onCreateOptionsMenu(menu);
getMenuInflater().inflate(R.menu.main_menu,menu);
return true;
}
/** Gets the options selected from the menu resource **/
@Override
public boolean onoptionsItemSelected(@NonNull MenuItem item) {
super.onoptionsItemSelected(item);
switch(item.getItemId()) {
case R.id.log_out_btn:
fAuth.signOut();
sendToStart();
break;
case R.id.all_users_btn:
case R.id.settings_btn:
}
return true;
}
/** Simply sends the user to the startActivity (state not logged in) **/
private void sendToStart(){
Intent intent = new Intent(MainActivity.this,StartActivity.class);
startActivity(intent);
finish();
}
public class ViewPagerAdapter extends FragmentPagerAdapter {
private List<Fragment> mFragments = new ArrayList<>();
private List<String> fragmentTitles = new ArrayList<>();
public ViewPagerAdapter(@NonNull FragmentManager fm,int behavior) {
super(fm,behavior);
}
public void addFragment(Fragment fragment,String title){
mFragments.add(fragment);
fragmentTitles.add(title);
}
@NonNull
@Override
public Fragment getItem(int position) {
return mFragments.get(position);
}
@Override
public int getCount() {
return mFragments.size();
}
@Nullable
@Override
public CharSequence getPageTitle(int position) {
return fragmentTitles.get(position);
}
}
}
解决方法
您的代码中没有问题。布局xml出现问题。
似乎未提供ViewPager的ID。
您将id设置为错误的属性android:layout_below
必须将其设置为android:id
<androidx.viewpager.widget.ViewPager
android:id="@+id/view_pager"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_toEndOf="@+id/toolbar"
app:layout_behavior="com.google.android.material.appbar.AppBarLayout$ScrollingViewBehavior">
</androidx.viewpager.widget.ViewPager>