我正在尝试使用自定义数组适配器在片段中填充ListView.但是我在自定义数组适配器的getView()方法中的方法findViewById()上得到NullPointerException.

这是CustomAdapter.java：

package graph.prathya.com.materialdesign;

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

/**
 * Created by Prathya on 5/19/2015.
 */
   public class CustomAdapter extends ArrayAdapter<Post>{
   Context context;
   int layoutid,singlerowimgid= R.id.imgIcon;
   Post data[]=null;
   public CustomAdapter(Context context, int layoutid, Post data[]){
    super(context, layoutid);
    this.layoutid = layoutid;
    this.data = data;
    this.context =context;
}

private class PostHolder{
    ImageView imgicon;
    TextView title;
}
@Override
public int getCount(){
    return data.length;
}
@Override
public View getView(int position, View convertView,ViewGroup parent){
    PostHolder holder;
    LayoutInflater li = (LayoutInflater) context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
    if(convertView == null){
        li.inflate(layoutid,parent,false);
        holder = new PostHolder();
        holder.imgicon = (ImageView)convertView.findViewById(singlerowimgid);
        holder.title =(TextView)convertView.findViewById(R.id.txtTitle);
    }
    else {
        holder = (PostHolder)convertView.getTag();
    }
    Post post = data[position];
    holder.title.setText(post.title);
    holder.imgicon.setImageResource(post.imgicon);

    return convertView;
}
}

这是我的XML文件,它定义了ListFragment中单行的外观
 singlerow.xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:padding="10dp">

<ImageView android:id="@+id/imgIcon"
    android:layout_width="wrap_content"
    android:layout_height="fill_parent"
    android:gravity="center_vertical"
    android:layout_alignParentTop="true"
    android:layout_alignParentBottom="true"
    android:layout_marginRight="15dp"
    android:layout_marginTop="5dp"
    android:layout_marginBottom="5dp" />

<TextView android:id="@+id/txtTitle"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:gravity="center_vertical"
    android:layout_alignParentTop="true"
    android:layout_alignParentBottom="true"
    android:textStyle="bold"
    android:textSize="22dp"
    android:textColor="#000000"
    android:layout_marginTop="5dp"
    android:layout_marginBottom="5dp" />

这是我的Fragment类：
  Fragment1.java：

package graph.prathya.com.materialdesign;

import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.ListFragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ListView;

/**
 * Created by Prathya on 5/19/2015.
 */
public class Fragment1 extends ListFragment{

  Post post[] = new Post[]{
        new Post(R.drawable.passion,"Passion"),
        new Post(R.drawable.event,"Event"),
        new Post(R.drawable.project,"Project"),
        new Post(R.drawable.groupstudy,"Group Study")};

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment1,container,false);

}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    setListAdapter(new CustomAdapter(getActivity(), R.layout.singlerow, post));

}
 }

这是fragment1.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"
android:background="#ffaa00">
    <ListView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@android:id/list"
    android:layout_gravity="center_horizontal" />

这是Post.java

package graph.prathya.com.materialdesign;

/**
 * Created by Prathya on 5/19/2015.
 */
public class Post {
public int imgicon;
public String title;
public Post(){
    super();
}
public Post(int imgicon, String title){
    this.imgicon = imgicon;
    this.title = title;
}
}

这是MainActivity.java

package graph.prathya.com.materialdesign;

import android.app.FragmentTransaction;
import android.support.v4.app.FragmentManager;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;


public class MainActivity extends ActionBarActivity {

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

public void addframent(){
    FragmentManager fragmentManager = getSupportFragmentManager();
    android.support.v4.app.FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    Fragment1 fm1 = new Fragment1();
    fragmentTransaction.add(R.id.sa,fm1);
    fragmentTransaction.commit();
}

}

这是错误信息

05-19 19:26:13.362 18306-18306/graph.prathya.com.materialdesign
E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: graph.prathya.com.materialdesign, PID: 18306
java.lang.NullPointerException: Attempt to invoke virtual method ‘android.view.View android.view.View.findViewById(int)’ on a null
object reference
at graph.prathya.com.materialdesign.CustomAdapter.getView(CustomAdapter.java:42)
at android.widget.AbsListView.obtainView(AbsListView.java:2347)
at android.widget.ListView.measureHeightOfChildren(ListView.java:1270)
at android.widget.ListView.onMeasure(ListView.java:1182)
at android.view.View.measure(View.java:17554)

..

解决方法:

在CustomAdapter.java中,您必须为convertView分配膨胀的布局：

convertView = li.inflate(layoutid,parent,false);

更新：
您必须将持有者设置为新增加的视图：

covnertView.setTag(holder);