java - 带 BaseAdapter 的 ListView

Question

我正在学习BaseAdapter。我有自定义 ListView 。我想从数据库中填充 ListView 中的数据。我收到空指针错误。这是我的代码。

DbTools.java

public class DbTools extends SQLiteOpenHelper {

    public DbTools(Context context) {
         super(context, "local.db", null, 1);

}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("create table ashu(Name varchar, address varchar)");
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("Drop Table if Exists ashu");
    onCreate(db);
}

public void InsertDb(String Name,String address) {

    SQLiteDatabase db = getWritableDatabase();
    ContentValues cv = new ContentValues();
    cv.put("Name", Name);
    cv.put("address", address);
    db.insert("ashu", null, cv);
    db.close();
}

public ArrayList<NameAddress> getAllData(){
    ArrayList<NameAddress> dataFromDb = new ArrayList<NameAddress>();
    String SQL = "Select Name, address from ashu";
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(SQL, null);
    if (cursor.moveToFirst()){
        do{
            NameAddress na = new NameAddress();
            na.NamesDb = cursor.getString(0).toString();
            na.AddressDb = cursor.getString(1).toString();
            dataFromDb.add(na);             
        } while(cursor.moveToNext());
    }
    return dataFromDb;

   }
}

这是我的 Adapter 类，我在其中扩展了 BaseAdapter

适配器.java

public class Adapter extends BaseAdapter{

 public ArrayList<NameAddress> result;
ListView lv;
private Context context;
DbTools dbTools = new DbTools(context);
private LayoutInflater inflater = null;


public Adapter(Context context,ArrayList<NameAddress> rslt){
    rslt=result;
    this.context=context;
    inflater =  (LayoutInflater)(this.context).getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}


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

@Override
public Object getItem(int position) {       
    return position;
}

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

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    final NameAddress tmpData = result.get(position);
    Holder h = new Holder();
    DbTools dbTools = new DbTools(context);

    View rootView;
    rootView = inflater.inflate(R.layout.row, null);
    h.textView1 = (TextView) rootView.findViewById(R.id.mainText);
    h.textView2 = (TextView) rootView.findViewById(R.id.subText);
    h.textView1.setText(tmpData.NamesDb);
    h.textView2.setText(tmpData.AddressDb);

    return rootView;
}

class Holder{   
    public TextView textView1, textView2;
}
}

MainActivity.java

public class MainActivity extends Activity {

ListView mainList;
Button save;
EditText name,address;
DbTools dbTools = new DbTools(this);
ArrayList<NameAddress> addList;
private Adapter adpter;

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

    mainList = (ListView)findViewById(R.id.mainList);
    save = (Button)findViewById(R.id.button1);
    name = (EditText)findViewById(R.id.nameEditText);
    address = (EditText)findViewById(R.id.addressEditText);

    save.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {

            String tempName = name.getText().toString();
            String tempAdd = address.getText().toString();
            dbTools.InsertDb(tempName, tempAdd);

            Toast.makeText(getApplicationContext(), "Added Successfully", Toast.LENGTH_SHORT).show();
            name.setText("");
            address.setText("");

        }
    });

    addList = new ArrayList<NameAddress>();
    addList = dbTools.getAllData();
    adpter = new Adapter(this, addList);
    adpter.notifyDataSetChanged();
    mainList.setAdapter(adpter);

}
}

我有模型类

NameAddress.java

public class NameAddress {
  String NamesDb;
  String AddressDb;
}

在我的 row.xml 中只有两个 TextView

activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/LinearLayout1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.ashu.customlist.MainActivity" >

<EditText
    android:id="@+id/nameEditText"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:hint="@string/name"
    android:ems="10" android:inputType="text">

    <requestFocus />
</EditText>

<EditText
    android:id="@+id/addressEditText"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:hint="@string/address"
    android:ems="10" />

<Button
    android:id="@+id/button1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/save" />

<ListView
    android:id="@+id/mainList"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
</ListView>
</LinearLayout>

日志

08-26 17:12:30.874: W/dalvikvm(19002): threadid=1: thread exiting with uncaught exception (group=0x41ee3d88)
08-26 17:12:30.874: E/AndroidRuntime(19002): Exception
08-26 17:12:30.874: E/AndroidRuntime(19002): FATAL EXCEPTION: main
08-26 17:12:30.874: E/AndroidRuntime(19002): Process: com.ashu.customlist, PID: 19002
08-26 17:12:30.874: E/AndroidRuntime(19002): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ashu.customlist/com.ashu.customlist.MainActivity}: java.lang.NullPointerException
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2237)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2286)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.app.ActivityThread.access$800(ActivityThread.java:145)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1243)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.os.Handler.dispatchMessage(Handler.java:102)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.os.Looper.loop(Looper.java:136)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.app.ActivityThread.main(ActivityThread.java:5127)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at java.lang.reflect.Method.invokeNative(Native Method)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at java.lang.reflect.Method.invoke(Method.java:515)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:825)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:641)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at dalvik.system.NativeStart.main(Native Method)
08-26 17:12:30.874: E/AndroidRuntime(19002): Caused by: java.lang.NullPointerException
08-26 17:12:30.874: E/AndroidRuntime(19002):    at com.ashu.customlist.Adapter.getCount(Adapter.java:31)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.widget.ListView.setAdapter(ListView.java:480)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at com.ashu.customlist.MainActivity.onCreate(MainActivity.java:55)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.app.Activity.performCreate(Activity.java:5231)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
08-26 17:12:30.874: E/AndroidRuntime(19002):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2201)
08-26 17:12:30.874: E/AndroidRuntime(19002):    ... 11 more

Answer 1

问题

您正在 Activity MainActivity 的变量声明部分初始化 DbTools dbTools = new DbTools(this);。

解决方案

将此行移至onCreate

DbTools dbTools = new DbTools(this);

此行必须位于 Activity 生命周期方法之一内，最好位于 onCreate 方法中。除非 Context 将为 null

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

        DbTools dbTools = new DbTools(this);
    }

问题

adpter.notifyDataSetChanged();
mainList.setAdapter(adpter);

这两条线不需要同时调用。

解决方案

1. 第一次设置适配器时使用此mainList.setAdapter(adpter);。

2. 当数据集发生更改时，调用 adpter.notifyDataSetChanged();。

问题

您也在变量声明部分的 Adapter 类中初始化以下行。

DbTools dbTools = new DbTools(context);

由于上下文变量为 null，因此无法初始化它。

构造函数中的另一个问题是您将空结果保存到参数rslt。其实应该是相反的。

解决方案

像这样更新适配器构造函数。

DbTools dbTools;
public Adapter(Context context,ArrayList<NameAddress> rslt){
    result = rslt; //this line is swapped
    this.context=context;
    dbTools = new DbTools(context);
    inflater =  (LayoutInflater)(this.context).getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}

也不需要在getView方法中初始化DbTools