Android：使用 viewPager 的 3 页闪屏出现 NullPointerException

Question

我正在尝试使用片段和 viewPager 创建一个 3 页的启动画面

我在运行时得到这个：

07-02 21:52:11.611  10400-10400/com.puxil.centre E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.puxil.centre, PID: 10400
java.lang.NullPointerException: Attempt to write to field 'android.support.v4.app.FragmentManagerImpl android.support.v4.app.Fragment.mFragmentManager' on a null object reference
        at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:416)
        at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:411)
        at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99)
        at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:837)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:987)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
        at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441)
        at android.view.View.measure(View.java:17525)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5478)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
        at android.view.View.measure(View.java:17525)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5478)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
        at android.view.View.measure(View.java:17525)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5478)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2629)
        at android.view.View.measure(View.java:17525)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2102)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1218)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1435)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1106)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6018)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:792)
        at android.view.Choreographer.doCallbacks(Choreographer.java:596)
        at android.view.Choreographer.doFrame(Choreographer.java:557)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:778)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:155)
        at android.app.ActivityThread.main(ActivityThread.java:5696)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)

这是我的主要内容activity:

    package com.puxil.centre;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;


public class splash extends FragmentActivity {

    ViewPager viewPager = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_splash);
        viewPager = (ViewPager) findViewById(R.id.pager);
        FragmentManager fragmentManager = getSupportFragmentManager();
        viewPager.setAdapter(new MyAdapter(fragmentManager));
    }






    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_splash, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

class MyAdapter extends FragmentPagerAdapter {

    public MyAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int i) {

        Fragment fragment = null;

        if(i==0) {
            fragment = new splashA();
        }
        if(i==1) {
            fragment = new splashB();
        }
        if(i==2) {
            fragment = new splashC();
        }

        return null;
    }

    @Override
    public int getCount() {
        return 3;
    }
}

这里是主要的布局 activity:

<android.support.v4.view.ViewPager
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:id="@+id/pager"
android:paddingBottom="@dimen/activity_vertical_margin"
>

这是我的一个片段的 java 文件，它与我的另外两个片段相同。

    package com.puxil.centre;


import android.os.Bundle;


import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;


/**
 * A simple {@link Fragment} subclass.
 */
public class splashB extends Fragment {


    public splashB() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_splash_b, container, false);
    }


}

这是片段的XML：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent">

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

Answer 1

在你的适配器中 getItem() 确保你 return 片段实例而不是 null

@Override
public Fragment getItem(int i) {
    Fragment fragment = null;

    if(i==0) {
        fragment = new splashA();
    }
    if(i==1) {
        fragment = new splashB();
    }
    if(i==2) {
        fragment = new splashC();
    }
    return fragment;
}

Android：使用 viewPager 的 3 页闪屏出现 NullPointerException

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