android - 尝试调用虚方法'void android.widget.ListView.setAdapter

Question

我正在尝试创建一个 ListView ，其中包含带有选项卡的 viewpager 上的 fragment 中的图片和文本，因此我可以向左或向右滑动以调出另一个 fragment 。

我是 android 的新手，所以我所做的可能是完全错误的。我可以完美地构建项目，但是当模拟器运行时它崩溃了(日志显示在页面底部)。

我创建了一个单独的应用程序并成功创建了一个 ListView ，但是当我尝试像这样将它组合到一个 fragment 中时，它不起作用。这是我的以下代码

MainActivity.java

import java.util.ArrayList;
import java.util.List;
import java.util.Locale;

import android.app.ListFragment;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.app.FragmentPagerAdapter;
import android.os.Bundle;
import android.support.v4.view.ViewPager;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends ActionBarActivity implements ActionBar.TabListener {

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link FragmentPagerAdapter} derivative, which will keep every
     * loaded fragment in memory. If this becomes too memory intensive, it
     * may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    ViewPager mViewPager;
   private List<Posts> myPosts = new ArrayList<Posts>();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Set up the action bar.
        final ActionBar actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

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

        // When swiping between different sections, select the corresponding
        // tab. We can also use ActionBar.Tab#select() to do this if we have
        // a reference to the Tab.
        mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                actionBar.setSelectedNavigationItem(position);
            }
        });

        // For each of the sections in the app, add a tab to the action bar.
        for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
            // Create a tab with text corresponding to the page title defined by
            // the adapter. Also specify this Activity object, which implements
            // the TabListener interface, as the callback (listener) for when
            // this tab is selected.
            actionBar.addTab(
                    actionBar.newTab()
                            .setText(mSectionsPagerAdapter.getPageTitle(i))
                            .setTabListener(this));

        }

         Intent intent = new Intent(MainActivity.this, NotMainActivity.class);
        startActivity(intent);
        finish();
    }

    @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_main, 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);
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        // When the given tab is selected, switch to the corresponding page in
        // the ViewPager.
        mViewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    }
    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

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

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a PlaceholderFragment (defined as a static inner class below).
            switch (position){
                case 0:
                    return new TheWallFragment();
                case 1:
                    return new PeekFragment();
                case 2:
                    return new CameraFragment();
                default:
                    break;

            }

            return PlaceholderFragment.newInstance(position + 1);
        }




        @Override
        public int getCount() {
            // Show 3 total pages.
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();
            switch (position) {
                case 0:
                    return getString(R.string.title_section1).toUpperCase(l);
                case 1:
                    return getString(R.string.title_section2).toUpperCase(l);
                case 2:
                    return getString(R.string.title_section3).toUpperCase(l);
            }
            return null;
        }
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        /**
         * The fragment argument representing the section number for this
         * fragment.
         */
        private static final String ARG_SECTION_NUMBER = "section_number";

        /**
         * Returns a new instance of this fragment for the given section
         * number.
         */
        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.wall_layout, container, false);

            return rootView;
        }
    }

}

NotMainActivity.java

public class NotMainActivity extends FragmentActivity {

    private List<Posts> myPosts = new ArrayList<Posts>();

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

       registerClickCallBack();
        System.out.print("Main Activity Started");



    }


    private void populatePostList()     {
        myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
        myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
        myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
        myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
        myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
        myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
        myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
        myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));



    }

    private void populateListView() {
        ArrayAdapter<Posts> adapter = new MyListAdapter();
        ListView list = (ListView) findViewById(android.R.id.list);
        list.setAdapter(adapter);

        }


    //Handles clicks on the list items
    private void registerClickCallBack(){
        ListView list = (ListView) findViewById(android.R.id.list);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Posts clickedPost = myPosts.get(position);
                String message = "You clicked" + position
                        +"Location of post is" + clickedPost.getPostlocation();
                Toast.makeText(NotMainActivity.this, message, Toast.LENGTH_LONG).show();
            }
        });
    }


    private class MyListAdapter extends ArrayAdapter<Posts> {
        public MyListAdapter() {
            super(NotMainActivity.this, R.layout.item_view, myPosts);
        }


        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            // Make sure we have a view to work with {may have given null}
            View itemView = convertView;
            if (itemView != null) {
                itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);
            }
            // Find the post to work with.
            Posts currentPost = myPosts.get(position);


            //Fill the view
            ImageView imageView = (ImageView)itemView.findViewById(R.id.item_postImage);
            imageView.setImageResource(currentPost.getIconID());

            //Fill in Title

            TextView titleText = (TextView) itemView.findViewById(R.id.item_postText);
            titleText.setText(currentPost.getPosttitle());

            //Set Vote Number
            TextView voteText = (TextView) itemView.findViewById(R.id.item_postVoteText);
            voteText.setText(currentPost.getPostvote());



            return itemView;
        }

    }

}

Posts.java

public class Posts {
    private String postlocation;
    private String postvote;
    private String posttitle;
    private int iconID;

    public Posts(String postlocation, String postvote, String posttitle, int iconID) {
        this.postlocation = postlocation;
        this.postvote = postvote;
        this.posttitle = posttitle;
        this.iconID = iconID;
    }







    public String getPostlocation() {
        return postlocation;
    }

    public String getPostvote() {
        return postvote;
    }

    public String getPosttitle() {
        return posttitle;
    }
    public int getIconID() {
        return iconID;
    }

TheWallFragment.java

public class TheWallFragment extends android.support.v4.app.ListFragment {
    List<Posts> myPosts = new ArrayList<Posts>();
    private String[] strListView;
    private ListView myListView;
    private int number;
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.wall_layout, container, false);
        return rootView;
    }


}

activity_main.xml

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

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.gnumbu.errolgreen.testing" >

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
<activity
    android:name=".NotMainActivity">
    </activity>
    </application>

</manifest>

item_view.xml

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

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/item_postImage"
        android:src="@drawable/dog"
        android:maxWidth="80dp"
        android:minHeight="80dp"
        android:adjustViewBounds="true"
        android:layout_marginLeft="14dp"
        android:layout_marginStart="14dp"
        android:layout_alignParentTop="true"
        android:layout_toRightOf="@+id/item_upvoteArrow"
        android:layout_toEndOf="@+id/item_upvoteArrow"
        android:layout_alignBottom="@+id/item_downvotearrow" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="This is my dog! My Dog is amazing, give it a lick!"
        android:id="@+id/item_postText"
        android:paddingStart="5dp"
        android:layout_above="@+id/item_downvotearrow"
        android:layout_toRightOf="@+id/item_postImage"
        android:layout_toEndOf="@+id/item_postImage" />

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/item_upvoteArrow"
        android:src="@drawable/upvotearrow"
        android:maxHeight="24dp"
        android:maxWidth="24dp"
        android:adjustViewBounds="true"
        android:layout_alignParentTop="true"
        android:layout_alignLeft="@+id/item_downvotearrow"
        android:layout_alignStart="@+id/item_downvotearrow"
        android:clickable="true" />

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/item_downvotearrow"
        android:src="@drawable/downvotearrow"
        android:maxWidth="24dp"
        android:maxHeight="24dp"
        android:adjustViewBounds="true"
        android:layout_marginLeft="9dp"
        android:layout_marginStart="9dp"
        android:layout_below="@+id/item_postVoteText"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:longClickable="false"
        android:clickable="true" />

    <TextView
        android:layout_height="wrap_content"
        android:text="22"
        android:id="@+id/item_postVoteText"
        android:maxHeight="20dp"
        android:focusableInTouchMode="false"
        android:visibility="visible"
        android:singleLine="true"
        android:gravity="center"
        android:enabled="true"
        android:layout_below="@+id/item_upvoteArrow"
        android:layout_alignLeft="@+id/item_upvoteArrow"
        android:layout_alignStart="@+id/item_upvoteArrow"
        android:nestedScrollingEnabled="false"
        android:layout_width="wrap_content"
        android:layout_toStartOf="@+id/item_postImage" />

</RelativeLayout>

wall_layout.xml

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

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="New Text"
        android:id="@+id/textView"
        android:layout_gravity="center_horizontal" />

    <ListView
        android:id="@android:id/list"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
            />
</LinearLayout>

Error Log

 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5223)
            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:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
            at com.gnumbu.errolgreen.testing.NotMainActivity.populateListView(NotMainActivity.java:70)
            at com.gnumbu.errolgreen.testing.NotMainActivity.onCreate(NotMainActivity.java:43)
            at android.app.Activity.performCreate(Activity.java:5937)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5223)
            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:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

New Error

 Process: com.gnumbu.errolgreen.testing, PID: 5304
    java.lang.NullPointerException: Attempt to invoke virtual method 'int android.view.View.getImportantForAccessibility()' on a null object reference
            at android.widget.AbsListView.obtainView(AbsListView.java:2360)
            at android.widget.ListView.measureHeightOfChildren(ListView.java:1270)
            at android.widget.ListView.onMeasure(ListView.java:1182)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            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:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            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:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1166)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
            at android.view.Choreographer.doCallbacks(Choreographer.java:580)
            at android.view.Choreographer.doFrame(Choreographer.java:550)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            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:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Answer 1

NotMainActivity电话 setContentView与 R.layout.activity_main作为包含 ViewPager 的参数而不是 ListView .由于您没有在该布局中声明 ListView，findViewById(android.R.id.list)返回 null，当您尝试访问它时(调用 setAdapter ) NullPointerException被抛出。

在你的MyAdapter , 改变自

if (itemView != null) {
      itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);
}

到

if (itemView == null) {
      itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);
}