Android java.lang.NullPointerException on Fragment Page

Question

Я знаю, что существует много потоков для NullPointerException, и я также знаю, что это вызвано нулевым значением где-то, но я новичок в разработке Android (а также Java) и i Я пытаюсь запустить первый пример Tabs, поэтому я не очень практичен.

На примере это одна: https://github.com/codepath/android_guides/wiki/Google-Play-Style-Tabs-using-TabLayout

Когда я запустить приложение это происходит сбой и возвращает следующие ошибки:

9892-9892/com.mycompany.myfirstapp E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mycompany.myfirstapp/com.mycompany.myfirstapp.TabActivity}: java.lang.NullPointerException 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 
      at android.app.ActivityThread.access$600(ActivityThread.java:141) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 
      at android.os.Handler.dispatchMessage(Handler.java:99) 
      at android.os.Looper.loop(Looper.java:137) 
      at android.app.ActivityThread.main(ActivityThread.java:5041) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:511) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
      at dalvik.system.NativeStart.main(Native Method) 
    Caused by: java.lang.NullPointerException 
      at com.mycompany.myfirstapp.TabActivity.onCreate(TabActivity.java:20) 
      at android.app.Activity.performCreate(Activity.java:5104) 
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 
            at android.app.ActivityThread.access$600(ActivityThread.java:141) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 
            at android.os.Handler.dispatchMessage(Handler.java:99) 
            at android.os.Looper.loop(Looper.java:137) 
            at android.app.ActivityThread.main(ActivityThread.java:5041) 
            at java.lang.reflect.Method.invokeNative(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:511) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
            at dalvik.system.NativeStart.main(Native Method) 

Это /res/layout/activity_tab_test.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:app="http://schemas.android.com/apk/res-auto" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:orientation="vertical"> 

<android.support.design.widget.TabLayout 
    android:id="@+id/sliding_tabs" 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    app:tabMode="scrollable" /> 

<android.support.v4.view.ViewPager 
    android:id="@+id/viewpager" 
    android:layout_width="match_parent" 
    android:layout_height="0px" 
    android:layout_weight="1" 
    android:background="@android:color/white" /> 

</LinearLayout> 

Это /res/layout/fragment_page.xml

<?xml version="1.0" encoding="utf-8"?> 
<TextView xmlns:android="http://schemas.android.com/apk/res/android" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:gravity="center" /> 

PageFragment.java

package com.mycompany.myfirstapp; 

import android.support.v4.app.Fragment; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.TextView; 

public class PageFragment extends Fragment { 
public static final String ARG_PAGE = "ARG_PAGE"; 

private int mPage; 

public static PageFragment newInstance(int page) { 
    Bundle args = new Bundle(); 
    args.putInt(ARG_PAGE, page); 
    PageFragment fragment = new PageFragment(); 
    fragment.setArguments(args); 
    return fragment; 
} 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    mPage = getArguments().getInt(ARG_PAGE); 
} 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, 
         Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.fragment_page, container, false); 
    TextView textView = (TextView) view; 
    textView.setText("Fragment #" + mPage); 
    return view; 
} 
} 

SampleFragmentPagerAdapter.java

package com.mycompany.myfirstapp; 

import android.support.v4.app.Fragment; 
import android.support.v4.app.FragmentManager; 
import android.content.Context; 
import android.support.v4.app.FragmentPagerAdapter; 

public class SampleFragmentPagerAdapter extends FragmentPagerAdapter { 
final int PAGE_COUNT = 3; 
private String tabTitles[] = new String[] { "Tab1", "Tab2", "Tab3" }; 
private Context context; 

public SampleFragmentPagerAdapter(FragmentManager fm, Context context) { 
    super(fm); 
    this.context = context; 
} 

@Override 
public int getCount() { 
    return PAGE_COUNT; 
} 

@Override 
public Fragment getItem(int position) { 
    return PageFragment.newInstance(position + 1); 
} 

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

TabActivity.java (The MainActivity примера)

package com.mycompany.myfirstapp; 

import android.os.Bundle; 
import android.support.design.widget.TabLayout; 
import android.support.v4.app.FragmentActivity; 
import android.support.v4.view.ViewPager; 

public class TabActivity extends FragmentActivity { 

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

    // Get the ViewPager and set it's PagerAdapter so that it can display items 
    ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager); 
    viewPager.setAdapter(new SampleFragmentPagerAdapter(getSupportFragmentManager(), 
      TabActivity.this)); 

    // Give the TabLayout the ViewPager 
    TabLayout tabLayout = (TabLayout) findViewById(R.id.sliding_tabs); 
    tabLayout.setupWithViewPager(viewPager); 
} 
} 

Где может быть нулевое значение?

Answer 1

В соответствии с тем, что вы разместили, ViewPager принадлежит activity_tab_test.xml, а не fragment_page.xml, который состоит только из одного TextView. Изменение

setContentView(R.layout.fragment_page); 

с

setContentView(R.layout.activity_tab_test);