无法从List中刷新Listview

Question

无论我在这个列表视图中尝试做什么，并尝试在按下按钮时动态更新它，它都不起作用并始终抛出NPE。

代码如下，我真的很乐意提供帮助。我已经尝试过requery，我已经尝试过notifyDataSetChanged，但我会说实话，这只是我的第二个项目，第一个使用数据库或列表视图，我会苦苦挣扎。

DatabaseHelper

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper; 
import android.provider.BaseColumns;
import android.util.Log;

public class DatabaseHelper extends SQLiteOpenHelper {

private static final String TAG = DatabaseHelper.class.getSimpleName();

public static final String DATABASE_NAME = "reordermymeds.db";
public static final int DATABASE_VERSION = 3;
public static final String TABLE_NAME = "Medicines";
public static final String C_ID = BaseColumns._ID;
public static final String C_MED_NAME = "med_name"; 
public static final String TABLE_NAME_SURG = "Surgeries";
public static final String SURGERY_NAME = "sName";
public static final String SURGERY_TEL = "sTel";
public static final String SURGERY_MAIL = "sMail";



public DatabaseHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    // TODO Auto-generated constructor stub
}

@Override
public void onCreate(SQLiteDatabase db) {
    // TODO Auto-generated method stub
    String sql = String.format("CREATE TABLE %s (%s INTEGER PRIMARY KEY, %s     TEXT)", TABLE_NAME, C_ID, C_MED_NAME);
    String sql_surg = String.format(
            "create table %s (%s int primary key, %s TEXT, %s TEXT, %s TEXT)", 
            TABLE_NAME_SURG, C_ID, SURGERY_NAME, SURGERY_TEL, SURGERY_MAIL);

    Log.d(TAG, "onCreate sql: "+sql);

    db.execSQL(sql);
    db.execSQL(sql_surg);



}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // TODO Auto-generated method stub
    db.execSQL("drop table if exists " + TABLE_NAME);
    db.execSQL("drop table if exists " + TABLE_NAME_SURG);
    this.onCreate(db);
}

}

和

活动

import android.app.ListActivity;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.SimpleCursorAdapter;
import android.widget.Toast;



public class pills extends ListActivity {

public static final String C_MED_NAME = "med_name";
public SimpleCursorAdapter ladapter;

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.pills);

    try {

            DatabaseHelper DBHelper= new DatabaseHelper(this);
            SQLiteDatabase db = DBHelper.getReadableDatabase();


            Cursor cur = db.query("Medicines", null, null, null, null, null, null);
            startManagingCursor(cur);

            String [] columns = new String[] {C_MED_NAME};
            int [] to = new int[] {R.id.meditem};

            SimpleCursorAdapter ladapter = new SimpleCursorAdapter(this, R.layout.meditem, cur, columns, to);

            this.setListAdapter(ladapter);
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }



    Button saveMed = (Button) findViewById(R.id.pills_newmedsubmit);
    saveMed.setOnClickListener(new OnClickListener() {



        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub

            final EditText newMed = (EditText) findViewById(R.id.pills_newmedtxtbox);

            if (newMed.getText().toString().equals("")) {

                    Context context = getApplicationContext();
                    CharSequence text = "Please type the name of a new prescription item.";
                    int duration = Toast.LENGTH_SHORT;

                    Toast toast= Toast.makeText(context, text, duration);
                    toast.show();
            }
            else
            {

                    String medName = newMed.getText().toString();

                    insertNewPrescriptionItem(medName);

                    EditText clearTextBox = (EditText) findViewById(R.id.pills_newmedtxtbox);
                    clearTextBox.setText("");

                    Context context = getApplicationContext();
                    CharSequence text = "Saved";
                    int duration = Toast.LENGTH_SHORT;

                    Toast toast = Toast.makeText(context, text, duration);
                    toast.show();


            }

        setListAdapter(ladapter);
        ladapter.notifyDataSetChanged();

        }


    });}

    private void insertNewPrescriptionItem (String medName){

        DatabaseHelper DbHelper = new DatabaseHelper(this);

        SQLiteDatabase db = DbHelper.getWritableDatabase();

        ContentValues cv = new ContentValues();

            cv.put(DatabaseHelper.C_MED_NAME, medName);

            db.insert(DatabaseHelper.TABLE_NAME, DatabaseHelper.C_MED_NAME, cv);
            db.close();
            }


}

当然，最后但并非最不重要的是，堆栈跟踪。我可以看到错误的行，但我尝试使用谷歌的每一个结果都是相同的。任何人都可以表明我一定是傻了吗？

堆栈跟踪

08-11 01:21:26.095: ERROR/AndroidRuntime(15327): FATAL EXCEPTION: main
08-11 01:21:26.095: ERROR/AndroidRuntime(15327): java.lang.NullPointerException
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at com.asurya.reordmymeds.pills$1.onClick(pills.java:90)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at android.view.View.performClick(View.java:2485)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at android.view.View$PerformClick.run(View.java:9080)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at android.os.Handler.handleCallback(Handler.java:587)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at android.os.Handler.dispatchMessage(Handler.java:92)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at android.os.Looper.loop(Looper.java:130)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at android.app.ActivityThread.main(ActivityThread.java:3683)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at java.lang.reflect.Method.invokeNative(Native Method)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at java.lang.reflect.Method.invoke(Method.java:507)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
08-11 01:21:26.095: ERROR/AndroidRuntime(15327):     at dalvik.system.NativeStart.main(Native Method)
08-11 01:21:26.107: WARN/ActivityManager(110):   Force finishing activity com.asurya.reordmymeds/.pills

糟糕。如果我让大家都知道，而不是自己计算它们会有所帮助:)

导致NPE的代码行是：

ladapter.notifyDataSetChanged();  

此外，如果我只是注释掉这一行，应用程序运行正常，但您必须刷新活动以查看数据库中的更新数据。谢谢大家。

Answer 1

    import android.app.ListActivity;
    import android.content.ContentValues;
    import android.content.Context;
    import android.database.Cursor;
    import android.database.sqlite.SQLiteDatabase;
    import android.os.Bundle;
    import android.view.View;
    import android.view.View.OnClickListener;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.SimpleCursorAdapter;
    import android.widget.Toast;

    public class pills extends ListActivity {

    public static final String C_MED_NAME = "med_name";
    public SimpleCursorAdapter ladapter;
    Context context;
    Cursor cur;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.pills);
        context=this;
        try {

                DatabaseHelper DBHelper= new DatabaseHelper(this);
                SQLiteDatabase db = DBHelper.getReadableDatabase();


                cur = db.query("Medicines", null, null, null, null, null, null);
                startManagingCursor(cur);

                String [] columns = new String[] {C_MED_NAME};
                int [] to = new int[] {R.id.meditem};

                ladapter = new SimpleCursorAdapter(this, R.layout.meditem, cur, columns, to);

                this.setListAdapter(ladapter);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }



        Button saveMed = (Button) findViewById(R.id.pills_newmedsubmit);
        saveMed.setOnClickListener(new OnClickListener() {



            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub

                final EditText newMed = (EditText) findViewById(R.id.pills_newmedtxtbox);

                if (newMed.getText().toString().equals("")) {

    //                    Context context = getApplicationContext();
                        CharSequence text = "Please type the name of a new prescription item.";
                        int duration = Toast.LENGTH_SHORT;

                        Toast toast= Toast.makeText(context, text, duration);
                        toast.show();
                }
                else
                {

                        String medName = newMed.getText().toString();

                        insertNewPrescriptionItem(medName);

                        EditText clearTextBox = (EditText) findViewById(R.id.pills_newmedtxtbox);
                        clearTextBox.setText("");

    //                  Context context = getApplicationContext();
                        CharSequence text = "Saved";
                        int duration = Toast.LENGTH_SHORT;

                        Toast toast = Toast.makeText(context, text, duration);
                        toast.show();


                }

            refresh();

            }


        });}

        private void insertNewPrescriptionItem (String medName){

            DatabaseHelper DbHelper = new DatabaseHelper(this);

            SQLiteDatabase db = DbHelper.getWritableDatabase();

            ContentValues cv = new ContentValues();

                cv.put(DatabaseHelper.C_MED_NAME, medName);

                db.insert(DatabaseHelper.TABLE_NAME, DatabaseHelper.C_MED_NAME, cv);
                db.close();
                }


    }
public void refresh(){
cur = db.query("Medicines", null, null, null, null, null, null);
ladapter.notifyDataSetChanged();
}

尝试这个，我认为你的上下文是空的，这就是它发生的原因

Answer 2

设置好适配器后，您无需执行.setListAdapter()。在您的类中声明适配器为全局变量（就像您在开始时所做的那样），然后在适配器数据发生变化的位置调用adapter.notifyDataSetChanged()。如果它不起作用，请尝试重新启用适配器，然后调用notifyDataSetChanged()。

在尝试删除适配器的类型定义时，您已经public SimpleCursorAdapter ladapter;只需在try中提供数据。