Android ViewPager为空
我创建了一个ViewPager,但是我一直收到一个错误,说它是空的。我尝试过更改ViewPager,检查了R Id,XML中指定的Id。我怀疑这与我构建XML文件的方式有关。谢谢 这就是我得到的错误:Android ViewPager为空,android,xml,android-viewpager,android-tablayout,Android,Xml,Android Viewpager,Android Tablayout,我创建了一个ViewPager,但是我一直收到一个错误,说它是空的。我尝试过更改ViewPager,检查了R Id,XML中指定的Id。我怀疑这与我构建XML文件的方式有关。谢谢 这就是我得到的错误: E/in.mycampuscha: [qarth_debug:] get PatchStore::createDisableExceptionQarthFile method fail. E/AndroidRuntime: FATAL EXCEPTION: main Proces
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>
这是一项活动:
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);
}
}
}
package com.devin.MyCampusChat;
导入androidx.annotation.NonNull;
导入androidx.annotation.Nullable;
导入androidx.appcompat.app.appcompat活动;
导入androidx.appcompat.widget.Toolbar;
导入androidx.fragment.app.fragment;
导入androidx.fragment.app.FragmentManager;
导入androidx.fragment.app.FragmentPagerAdapter;
导入androidx.viewpager.widget.viewpager;
导入android.app.DownloadManager;
导入android.content.Intent;
导入android.os.Bundle;
导入android.view.Menu;
导入android.view.MenuItem;
导入com.devin.MyCampusChat.Fragments.ChatsFragment;
导入com.devin.MyCampusChat.Fragments.FriendsFragment;
导入com.devin.MyCampusChat.Fragments.RequestsFragment;
导入com.google.android.material.tabs.TabLayout;
导入com.google.firebase.auth.FirebaseAuth;
导入com.google.firebase.auth.FirebaseUser;
导入java.util.ArrayList;
导入java.util.List;
公共类MainActivity扩展了AppCompatActivity{
私人消防队;
专用FirebaseUser fUser;
私有工具栏mToolbar;
私人摊位摊位;
私人视页机视页机;
//碎片。
私人聊天室聊天室聊天室;
私人朋友,朋友,朋友;
私有请求片段请求片段;
@凌驾
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tabLayout=findviewbyd(R.id.tab\u布局);
viewPager=findViewById(R.id.view\u pager);
fAuth=FirebaseAuth.getInstance();
mToolbar=findviewbyd(R.id.toolbar);
//碎片
chatsFragment=新chatsFragment();
friendsFragment=新的friendsFragment();
requestsFragment=新的requestsFragment();
设置支持操作栏(mToolbar);
getSupportActionBar().setTitle(“MyCampus聊天”);
tabLayout.setupWithViewPager(viewPager);
ViewPagerAdapter ViewPagerAdapter=新的MainActivity.ViewPagerAdapter(getSupportFragmentManager(),0);
viewPager.setAdapter(viewPagerAdapter);
viewPagerAdapter.addFragment(chatsFragment,“Chats”);
viewPagerAdapter.addFragment(friendsFragment,“Friends”);
addFragment(requestsFragment,“Requests”);
}
@凌驾
public void onStart(){
super.onStart();
fUser=fAuth.getCurrentUser();
//检查当前用户是否为空,如果为空,则表示他们未登录。
if(fUser==null){
Intent startIntent=新的Intent(MainActivity.this、StartActivity.class);
startActivity(startinent);
完成();
}
}
/**创建选项菜单**/
@凌驾
公共布尔onCreateOptions菜单(菜单){
super.onCreateOptions菜单(菜单);
getMenuInflater().充气(右菜单.主菜单,菜单);
返回true;
}
/**获取从菜单资源中选择的选项**/
@凌驾
公共布尔值OnOptionItemSelected(@NonNull MenuItem item){
super.onOptionsItemSelected(项目);
开关(item.getItemId()){
案例R.id.log\u out\u btn:
fAuth.signOut();
sendToStart();
打破
案例R.id.所有用户\u btn:
案例R.id.settings\u btn:
}
返回true;
}
/**只需将用户发送到startActivity(未登录状态)**/
私有void sendToStart(){
意向意向=新意向(MainActivity.this、StartActivity.class);
星触觉(意向);
完成();
}
公共类ViewPagerAdapter扩展了FragmentPagerAdapter{
私有列表MFFragments=newArrayList();
private List fragmentTitles=new ArrayList();
public ViewPagerAdapter(@NonNull FragmentManager fm,int-behavior){
超级(fm,行为);
}
public void addFragment(片段片段,字符串标题){
添加(片段);
碎片标题。添加(标题);
}
@非空
@凌驾
公共片段getItem(int位置){
返回mFragments.get(位置);
}
@凌驾
public int getCount(){
返回mffragments.size();
}
@可空
@凌驾
公共字符序列getPageTitle(int位置){
返回碎片标题。获取(位置);
}
}
}
您的代码中没有问题。布局xml存在问题
似乎没有提供ViewPager的id
您将id设置为错误的属性android:layout\u
必须将其设置为android:id
我怎么没发现?谢谢