Question

我尝试按照本教程实现ListView适配器：

https://github.com/codepath/android_guides/wiki/Using-an-ArrayAdapter-with-ListView

然而，当我运行我的应用程序时，我收到以下错误：

无法启动活动ComponentInfo {com.john.test / com.john.test.MainActivity}：java.lang.NullPointerException：尝试调用虚方法＆＃39; void android.widget.ListView.setAdapter（android。 widget.ListAdapter）＆＃39;在空对象引用上

MainActivity.java

package com.john.test;

import android.content.Context;
import android.net.Uri;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;
import android.widget.ListView;
import android.widget.TextView;

import com.astuetz.PagerSlidingTabStrip;
import com.nostra13.universalimageloader.core.DisplayImageOptions;
import com.nostra13.universalimageloader.core.ImageLoader;
import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;
import com.nostra13.universalimageloader.core.display.RoundedBitmapDisplayer;
import com.john.test.API.ApiEndpointInterface;
import com.john.test.models.Post;

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

import retrofit.Callback;
import retrofit.RestAdapter;
import retrofit.RetrofitError;
import retrofit.client.Response;

public class MainActivity extends AppCompatActivity
    implements TrendingFragment.OnFragmentInteractionListener,
        HotFragment.OnFragmentInteractionListener,
        NewFragment.OnFragmentInteractionListener {

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        // Initialize the ViewPager and set an adapter
        ViewPager pager = (ViewPager) findViewById(R.id.pager);
        pager.setAdapter(new PagerAdapter(getSupportFragmentManager()));

        // Bind the tabs to the ViewPager
        PagerSlidingTabStrip tabs = (PagerSlidingTabStrip) findViewById(R.id.tabs);
        tabs.setViewPager(pager);

        // Construct the data source
        ArrayList<Post> arrayOfUsers = new ArrayList<Post>();
        // Create the adapter to convert the array to views
        PostAdapter adapter = new PostAdapter(this, arrayOfUsers);
        // Attach the adapter to a ListView
        ListView listView = (ListView) findViewById(R.id.listview_posts);
        listView.setAdapter(adapter);
    }

    @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);
    }

    public void onFragmentInteraction(Uri uri){
        //
    }

    class PagerAdapter extends FragmentPagerAdapter {

        private final String[] TITLES = {"Trending", "Hot", "New"};

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

        @Override
        public CharSequence getPageTitle(int position) {
            return TITLES[position];
        }

        @Override
        public int getCount() {
            return TITLES.length;
        }

        @Override
        public Fragment getItem(int position) {
            switch (position) {
                case 0:
                    return new TrendingFragment();
                case 1:
                    return new HotFragment();
                case 2:
                    return new NewFragment();
            }

            return null;
        }
    }
}

PostAdapter.java

package com.john.test;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;

import com.john.test.models.Post;

import java.util.ArrayList;

public class PostAdapter extends ArrayAdapter<Post> {

    public PostAdapter(Context context, ArrayList<Post> users) {
        super(context, 0, users);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // Get the data item for this position
        Post user = getItem(position);
        // Check if an existing view is being reused, otherwise inflate the view
        if (convertView == null) {
            convertView = LayoutInflater.from(getContext()).inflate(R.layout.post_layout, parent, false);
        }
        // Lookup view for data population
        TextView tvName = (TextView) convertView.findViewById(R.id.post_message);
        // Populate the data into the template view using the data object
        tvName.setText(user.message);
        // Return the completed view to render on screen
        return convertView;
    }

}

Post.java

package com.john.test.models;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Post {

    public String message;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

}

post_layout.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.support.v7.widget.CardView
        xmlns:card_view="http://schemas.android.com/apk/res-auto"
        android:id="@+id/card_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="5dp"
        card_view:cardCornerRadius="2dp"
        card_view:cardElevation="1dp"
        card_view:cardBackgroundColor="@color/white">

        <TextView
            android:id="@+id/post_message"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Lorem ipsum"
            />

     </android.support.v7.widget.CardView>
</LinearLayout>

fragment_trending.xml

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" tools:context="com.john.test.TrendingFragment">

    <ListView
        android:id="@+id/listview_posts"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
    </ListView>

</LinearLayout>

activity_main.xml中

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

    <include layout="@layout/toolbar" />

    <com.astuetz.PagerSlidingTabStrip
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="48dp"
        android:layout_below="@+id/toolbar"
        android:background="@color/primary"
        android:textColorPrimary="@color/white"
        app:pstsDividerColor="@color/primary"
        app:pstsIndicatorColor="@color/white"
        app:pstsIndicatorHeight="2dp"
        app:pstsShouldExpand="true"
        app:pstsUnderlineHeight="0dp"
        app:pstsTabTextSize="12dp"
        />

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:layout_below="@+id/tabs"
        />
</LinearLayout>

toolbar.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/toolbar"
    app:theme="@style/ThemeOverlay.AppCompat.ActionBar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:minHeight="?attr/actionBarSize"
    android:background="@color/primary" />

TrendingFragment.java

package com.john.test;

import android.app.Activity;
import android.net.Uri;
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.ListView;

import com.john.test.models.Post;

import java.util.ArrayList;


/**
 * A simple {@link Fragment} subclass.
 * Activities that contain this fragment must implement the
 * {@link TrendingFragment.OnFragmentInteractionListener} interface
 * to handle interaction events.
 * Use the {@link TrendingFragment#newInstance} factory method to
 * create an instance of this fragment.
 */
public class TrendingFragment extends Fragment {
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    private OnFragmentInteractionListener mListener;

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment TrendingFragment.
     */
    // TODO: Rename and change types and number of parameters
    public static TrendingFragment newInstance(String param1, String param2) {
        TrendingFragment fragment = new TrendingFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

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

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment

        View view = inflater.inflate(R.layout.fragment_trending, container, false);
        ListView listView = (ListView) view.findViewById(R.id.listview_posts);

        // Construct the data source
        ArrayList<Post> arrayOfUsers = new ArrayList<Post>();

        Post post1 = new Post("Hey");
        arrayOfUsers.add(post1);

        // Create the adapter to convert the array to views
        PostAdapter adapter = new PostAdapter(getActivity(), arrayOfUsers);
        // Attach the adapter to a ListView
        listView.setAdapter(adapter);

        // return inflater.inflate(R.layout.fragment_trending, container, false);
    }

    // TODO: Rename method, update argument and hook method into UI event
    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnFragmentInteractionListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p/>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        public void onFragmentInteraction(Uri uri);
    }

}

导致错误的原因是什么？如何解决？我认为这与我的Fragment实施有关，但我不确定。

Answer 1

将ListView放在未充气的布局中。

<击> 更改此行

setContentView(R.layout.activity_main);

到

setContentView(R.layout.fragment_trending);

<击>

您应该将ListView移动到TrendingFragment并在片段内设置ListAdaper。

我不知道你是如何实现这个片段的，但它看起来应该是这样的。

<强> TrendingFragment

public class TrendingFragment extends Fragmnet { public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View view = inflater.inflate(R.layout.fragment_trending, container, false); ListView listView = (ListView) findViewById(R.id.listview_posts); // Construct the data source ArrayList<Post> arrayOfUsers = new ArrayList<Post>(); // Create the adapter to convert the array to views PostAdapter adapter = new PostAdapter(this, arrayOfUsers); // Attach the adapter to a ListView listView.setAdapter(adapter); return view; } }

Answer 2

无论如何，关键是，通过setContentView（）附加到MainActivity的布局不包含ListView。

ListView listView = (ListView) findViewById(R.id.listview_posts);

此findViewById方法返回null。

尝试通过片段访问ListView。

创建ListView适配器给出错误

2 个答案: