java - 带有 Fragment 和 ViewPager 的 NullPointerException

Question

我的代码有一些问题。我在下面的代码中有一个 NullPointerException。 我正在尝试在 3 个不同的 fragment 之间创建滑动。

我不明白我的错误在哪里所以请帮助我:p

虚拟 fragment .java :

package com.example.testslide;

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

public class DummyFragment  extends Fragment {

    public static DummyFragment newInstance(String chaine) {
        DummyFragment fragment = new DummyFragment();
        Bundle args = new Bundle();
        args.putString("KEY_STRING", chaine);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.milieu, container);
        TextView textView = (TextView) v.findViewById(R.id.textView1);
        Bundle args = getArguments();
        textView.setText(args.getString("KEY_STRING"));
        return v;
    }

}

ViewPagerAdapter

package com.example.testslide;

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

public class ViewPagerAdapter extends FragmentPagerAdapter {
    public ViewPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int pos) {
        switch(pos) {
        case 1: return DummyFragment.newInstance("Je suis le premier écran !");
        case 2: return DummyFragment.newInstance("Je suis le second écran !");
        case 3: return DummyFragment.newInstance("Je suis le troisième écran !");
        }
        return null;
    }

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

ViewPagerActivity:

package com.example.testslide;

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


public class ViewPagerActivity extends FragmentActivity {
    private ViewPagerAdapter mSectionsPagerAdapter;
    private ViewPager mViewPager;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.viewpager);
        // Set up the adapter.
        mSectionsPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.viewPager);
        mViewPager.setAdapter(mSectionsPagerAdapter);
    }
}

viewpager.xml :

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

环境.xml

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


    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_margin="20dp"
        android:text="Page du milieu" />

</LinearLayout>

登录目录:

07-10 08:25:26.211: E/AndroidRuntime(607): FATAL EXCEPTION: main
07-10 08:25:26.211: E/AndroidRuntime(607): java.lang.NullPointerException
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:389)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.support.v4.view.ViewPager.populate(ViewPager.java:982)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.View.measure(View.java:10828)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4351)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:267)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.View.measure(View.java:10828)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4351)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1284)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.widget.LinearLayout.measureVertical(LinearLayout.java:613)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.widget.LinearLayout.onMeasure(LinearLayout.java:519)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.View.measure(View.java:10828)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4351)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:267)
07-10 08:25:26.211: E/AndroidRuntime(607):  at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:1889)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.View.measure(View.java:10828)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.ViewRoot.performTraversals(ViewRoot.java:938)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.view.ViewRoot.handleMessage(ViewRoot.java:2040)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.os.Handler.dispatchMessage(Handler.java:99)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.os.Looper.loop(Looper.java:132)
07-10 08:25:26.211: E/AndroidRuntime(607):  at android.app.ActivityThread.main(ActivityThread.java:4123)
07-10 08:25:26.211: E/AndroidRuntime(607):  at java.lang.reflect.Method.invokeNative(Native Method)
07-10 08:25:26.211: E/AndroidRuntime(607):  at java.lang.reflect.Method.invoke(Method.java:491)
07-10 08:25:26.211: E/AndroidRuntime(607):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
07-10 08:25:26.211: E/AndroidRuntime(607):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
07-10 08:25:26.211: E/AndroidRuntime(607):  at dalvik.system.NativeStart.main(Native Method)

Answer 1

我发现您的代码存在两个主要问题。

• View v = inflater.inflate(R.layout.milieu, container);

容器应该只用于布局参数，而不是用于向其中添加膨胀 View 。此外，您使用的 inflate 版本会返回容器本身。所以你在错误的地方寻找你的观点。改在

View v = inflater.inflate(R.layout.milieu, container, false);

• position 是 0-index 基础，所以如果你在 getCount 中返回 3，getItem 将被调用 position=0 ,1,2。这意味着您要为索引 0 返回 null