Android 中的自定义适配器

Question

我正在尝试创建自定义 ListView，但是当我尝试创建 RegistrationInfo 对象时我的应用程序崩溃了。我收到 NULLPointerException。谁能告诉我为什么？

SecondActivity.java

public class SecondActivity extends Activity implements OnItemLongClickListener {

SharedPreferences pref;
CustomAdapter adapter;
ListView lv;
ArrayList<RegistrationInfo> registrationlist;

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

    lv = (ListView) findViewById(R.id.showdata);        

    File prefsdir = new File(getApplicationInfo().dataDir,"shared_prefs");
    String[] list = prefsdir.list();
    String[] preflist = new String[list.length];
    for(int i=0;i<list.length;i++)
    {
      String preffile = list[i].substring(0, list[i].length()-4);
      preflist[i]=preffile;
    }

    registrationlist = new ArrayList<RegistrationInfo>();
    for(int index=0;index<preflist.length;index++)
    {
          SharedPreferences spref = getSharedPreferences(preflist[index], MODE_PRIVATE);            
          String firstName = spref.getString("firstName", "");
          String lastName = spref.getString("lastName", "");
          String email = spref.getString("email", "");
          String contactno = spref.getString("contactno", "");
          Toast.makeText(this, firstName+" "+lastName+" "+email+" "+contactno, Toast.LENGTH_LONG).show();

//Crashing due to the statement below

          RegistrationInfo user = new RegistrationInfo(firstName, lastName, email, contactno);
          Toast.makeText(this, "abcd", Toast.LENGTH_LONG).show();
          registrationlist.add(user);
          Toast.makeText(this, "a", Toast.LENGTH_LONG).show();
          adapter = new CustomAdapter(registrationlist, this);
          Toast.makeText(this, "b", Toast.LENGTH_LONG).show();
          lv.setAdapter(adapter);         

    }
}

}

RegistrationInfo.java

public class RegistrationInfo 
{
    private String firstName,lastName,email,contactno;

public RegistrationInfo(String fName,String lName,String eid,String cno) 
{
    firstName = fName;
    lastName = lName;
    email = eid;                
    contactno = cno;
}   

public String getFirstName() {
    return firstName;
}

public void setFirstName(String firstName) {
    this.firstName = firstName;
}

public String getLastName() {
    return lastName;
}

public void setLastName(String lastName) {
    this.lastName = lastName;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getContactno() {
    return contactno;
}

public void setContactno(String contactno) {
    this.contactno = contactno;
}

}

CustomAdapter.java

public class CustomAdapter extends BaseAdapter 
{

Context context;
ArrayList<RegistrationInfo> registration;

public CustomAdapter(ArrayList list,Context c) 
{
    registration = list;
    context = c;
}

@Override
public int getCount() {
    return registration.size();
}

@Override
public RegistrationInfo getItem(int pos) {
    return registration.get(pos);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int pos, View convertView, ViewGroup parent) 
{
    View v = convertView;
    if(v==null)
    {
        LayoutInflater inflator = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        v = inflator.inflate(R.layout.customlistviewlayout, null);

        TextView name = (TextView) v.findViewById(R.id.name);
        TextView contactno = (TextView) v.findViewById(R.id.contactno);
        TextView email = (TextView) v.findViewById(R.id.email);

        RegistrationInfo user = registration.get(pos);

        //name.setText("Name : "+user.getFirstName()+" "+user.getLastName());
        //email.setText("Email id : "+user.getEmail());
        //contactno.setText("Contact No : "+user.getContactno());
        return v;
    }
    return null;
}

}

Logcat

02-01 20:20:30.984: E/AndroidRuntime(12355): FATAL EXCEPTION: main
02-01 20:20:30.984: E/AndroidRuntime(12355): java.lang.NullPointerException
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.AbsListView.obtainView(AbsListView.java:1581)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.ListView.measureHeightOfChildren(ListView.java:1251)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.ListView.onMeasure(ListView.java:1162)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.View.measure(View.java:8313)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3138)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1017)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.LinearLayout.measureVertical(LinearLayout.java:386)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.LinearLayout.onMeasure(LinearLayout.java:309)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.View.measure(View.java:8313)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3138)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.FrameLayout.onMeasure(FrameLayout.java:250)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.View.measure(View.java:8313)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.LinearLayout.measureVertical(LinearLayout.java:531)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.LinearLayout.onMeasure(LinearLayout.java:309)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.View.measure(View.java:8313)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3138)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.widget.FrameLayout.onMeasure(FrameLayout.java:250)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.View.measure(View.java:8313)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.ViewRoot.performTraversals(ViewRoot.java:845)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.view.ViewRoot.handleMessage(ViewRoot.java:1865)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.os.Handler.dispatchMessage(Handler.java:99)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.os.Looper.loop(Looper.java:130)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at android.app.ActivityThread.main(ActivityThread.java:3687)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at java.lang.reflect.Method.invokeNative(Native Method)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at java.lang.reflect.Method.invoke(Method.java:507)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
02-01 20:20:30.984: E/AndroidRuntime(12355):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

您的 getView 中不能 return null。改变

   return v;
    }
    return null;

到

}
return convertView;

您 return 在 getView 中的视图是您要在 GridView/[=14= 中显示的叶之一].系统必须对其进行测量和绘制。

Answer 2

在你的 getView() 内部，你可能 returning null。您总是需要 return 此处查看。

更改声明：

 return null;

到

 return v;

Answer 3

试试下面的代码：

public class CustomAdapter extends BaseAdapter  {
Context context;
ArrayList<RegistrationInfo> registration;

public CustomAdapter(ArrayList list,Context c) {
    registration = list;
    context = c;
}

@Override
public int getCount() {
    return registration.size();
}

@Override
public RegistrationInfo getItem(int pos) {
    return registration.get(pos);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int pos, View convertView, ViewGroup parent) 
{
    ViewHolder holder = null;
    if (convertView == null) {
        LayoutInflater inflator = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = inflator.inflate(R.layout.customlistviewlayout, null);
        holder = new ViewHolder();
        holder.name = (TextView) convertView.findViewById(R.id.name);
        holder.contactno = (TextView) convertView.findViewById(R.id.contactno);
        holder.email = (TextView) convertView.findViewById(R.id.email);

        convertView.setTag(holder);
    } else{
        holder = (ViewHolder) convertView.getTag();
    }

    //write your code
    return convertView;
}
private class ViewHolder{
    TextView name,contactno,email;
     }
}

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