这是什么意思？我如何解决它？致命异常：主要

Question

我尝试在模拟器中打开我的android studio应用程序并发出错误：

12-06 00:50:30.993    1198-1198/com.coding.nero E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.coding.nero, PID: 1198
java.lang.NullPointerException
        at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394)
        at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:389)
        at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99)
        at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:982)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
        at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
        at android.view.View.measure(View.java:16497)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at android.view.View.measure(View.java:16497)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
        at android.view.View.measure(View.java:16497)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
        at android.view.View.measure(View.java:16497)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1916)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1113)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1295)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
        at android.view.Choreographer.doCallbacks(Choreographer.java:574)
        at android.view.Choreographer.doFrame(Choreographer.java:544)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5017)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)

我不知道这意味着什么或我如何解决它。如有必要，我可以提供更多代码。当我在模拟器中打开它时，它表示应用已崩溃并关闭。

这是我的主要活动XML文件

<android.support.v4.view.ViewPager
android:layout_height="match_parent"
android:layout_width="match_parent"
android:id="@+id/pager"
xmlns:android="http://schemas.android.com/apk/res/android">

这是我的MainActivity Java

    package com.coding.nero;

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

public class MainActivity extends FragmentActivity {

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


    /**
     * Created by John on 12/5/14.
     */

    //Links class to layout file fragment_a
    public static class FragmentA extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_a, container, false);
        }
    }

    /**
     * Created by John on 12/5/14.
     */

    //Links class to layout file fragment_a
    public static class FragmentB extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_a, container, false);
        }
    }

    /**
     * Created by John on 12/5/14.
     */

    //Links class to layout file fragment_a
    public static class FragmentC extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_a, container, false);
        }
    }

    /**
     * Created by John on 12/5/14.
     */

    //Links class to layout file fragment_a
    public static class FragmentD extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_d, container, false);
        }
    }
}

class MyAdapter extends FragmentPagerAdapter{

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

    @Override
    public Fragment getItem(int i) {
        Fragment fragment=null;
        if(i==0){
            fragment=new FragmentA_1();
        }
        if(i==1){
            fragment=new FragmentB_1();
        }
        if(i==2){
            fragment=new FragmentC_1();
        }
        if(i==3){
            fragment=new FragmentD_1();
        }
        return null;
    }

    @Override
    public int getCount() {
        return 4;
    }

适配器代码：

    package com.coding.nero;

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

public class MainActivity extends FragmentActivity {

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


    /**
     * Created by John on 12/5/14.
     */

    //Links class to layout file fragment_a
    public static class FragmentA extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_a, container, false);
        }
    }

    /**
     * Created by John on 12/5/14.
     */

    //Links class to layout file fragment_a
    public static class FragmentB extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_a, container, false);
        }
    }

    /**
     * Created by John on 12/5/14.
     */

    //Links class to layout file fragment_a
    public static class FragmentC extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_a, container, false);
        }
    }

    /**
     * Created by John on 12/5/14.
     */

    //Links class to layout file fragment_a
    public static class FragmentD extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_d, container, false);
        }
    }
}

class MyAdapter extends FragmentPagerAdapter{

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

    @Override
    public Fragment getItem(int i) {
        Fragment fragment=null;
        if(i==0){
            fragment=new FragmentA_1();
        }
        if(i==1){
            fragment=new FragmentB_1();
        }
        if(i==2){
            fragment=new FragmentC_1();
        }
        if(i==3){
            fragment=new FragmentD_1();
        }
        return null;
    }

    @Override
    public int getCount() {
        return (4);
    }
}

Answer 1

因为你的适配器“getItem（）”返回null。