如何查看数据库中的所有数据？

Question

这是我的数据库辅助类：

public class DatabaseHelper extends SQLiteOpenHelper { 
    public static final String DATABASE_NAME="assignment.db"; 
    public static final String TABLE_NAME="assignment_data"; 
    public static final int DATABASE_VERSION=1; 
    public static final String COL_1="ID"; 
    public static final String COL_2="ASSIGNMENT_NAME"; 
    public static final String COL_3="SUBMISSION_DATE"; 
    public static final String COL_4="INFORMATION"; 
    public static final String COL_5="REMARKS"; 

    public DatabaseHelper(Context context) { 
    super(context, DATABASE_NAME, null, DATABASE_VERSION); 
    } 

    @Override 
    public void onCreate(SQLiteDatabase db) { 
    String create="create table "+TABLE_NAME+"("+COL_1+" INTEGER PRIMARY KEY AUTOINCREMENT,"+COL_2+" TEXT NOT NULL,"+COL_3+" TEXT NOT NULL,"+COL_4+" TEXT NOT NULL,"+COL_5+" TEXT NOT NULL"+")"; 
    db.execSQL(create); 
    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int i, int i1) { 
    String upgrade="Drop table if exists "+TABLE_NAME; 
    db.execSQL(upgrade); 
    onCreate(db); 
    } 

    public boolean addAssignment(Assignment x){ 
    SQLiteDatabase myDB=this.getWritableDatabase(); 
    ContentValues content=new ContentValues(); 
    content.put(COL_2,x.getName()); 
    content.put(COL_3,x.getDate()); 
    content.put(COL_4,x.getInfo()); 
    content.put(COL_5,x.getRemarks()); 
    long result=myDB.insert(TABLE_NAME,null,content); 
    if(result == -1){ 
     return false; 
    } 
    else { 
     return true; 
    } 
    } 

    public Cursor viewAll(){ 
    SQLiteDatabase MyDB=this.getReadableDatabase(); 
    Cursor res=myDB.rawQuery("SELECT * FROM "+DatabaseHelper.TABLE_NAME,null); 
    return res; 
    } 

    /*public Cursor view(){ 
    Cursor cursor=myDB.rawQuery("SELECT * FROM "+TABLE_NAME+" WHERE ID=?",new String[]{COL_1+""}); 
    return cursor; 
    }*/ 
} 

这是我的视图类：

public class View extends Activity { 
    Button viewAll,show; 
    EditText id; 
    DatabaseHelper dbHelper; 
    int id1; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_view); 
    viewAll = (Button)findViewById(R.id.viewAll); 
    show = (Button)findViewById(R.id.show); 
    id = (EditText)findViewById(R.id.id); 
    loadData(); 
    //showData(); 
    } 

    public void loadData(){ 
    viewAll.setOnClickListener(new android.view.View.OnClickListener() { 
     @Override 
     public void onClick(android.view.View view) { 
     Cursor cursor = dbHelper.viewAll(); 
     if(cursor.getCount() == 0){ 
      Toast.makeText(getApplicationContext(),"No Data for any Assignment found!",Toast.LENGTH_SHORT).show(); 
     } 
     StringBuffer s = new StringBuffer(""); 
     while(cursor.moveToNext()){ 
      s.append("ID:"+cursor.getString(0)+"\n"); 
      s.append("ASSIGNMENT NAME:"+cursor.getString(1)+"\n"); 
      s.append("SUBMISSION DATE:"+cursor.getString(2)+"\n"); 
      s.append("INFORMATION:"+cursor.getString(3)+"\n"); 
      s.append("REMARKS:"+cursor.getString(4)+"\n\n"); 
     } 
     showMessage("ASSIGNMENT DETAILS:",s.toString()); 
     } 
    }); 
    } 

    /*public void showData(){ 
    show.setOnClickListener(new android.view.View.OnClickListener() { 
     @Override 
     public void onClick(android.view.View view) { 
      id1=Integer.parseInt(id.getText().toString()); 
      Cursor m=dbHelper.view(id1); 
      if(m.getCount()==0){ 
       Toast.makeText(View.this,"No Data for the Assignment found!",Toast.LENGTH_SHORT).show(); 
      } 
      StringBuffer sb= new StringBuffer(); 
      while(m.moveToNext()){ 
       sb.append("ID:"+m.getString(0)+"\n"); 
       sb.append("ASSIGNMENT NAME:"+m.getString(1)+"\n"); 
       sb.append("SUBMISSION DATE:"+m.getString(2)+"\n"); 
       sb.append("INFORMATION:"+m.getString(3)+"\n"); 
       sb.append("REMARKS:"+m.getString(4)+"\n\n"); 
      } 
      showMessage("ASSIGNMENT DETAILS:",sb.toString()); 
     } 
    }); 

    }*/ 

    public void showMessage(String title,String mess){ 
    AlertDialog.Builder x=new AlertDialog.Builder(View.this); 
    x.setCancelable(true); 
    x.setTitle(title); 
    x.setMessage(mess); 
    x.show(); 
    } 
} 

，这是我的错误：

07 -10 08：51：45.497 27027-27027/com.dutt.rishabh.assignmentbuddy E/AndroidR untime：FATAL EXCEPTION：main 进程：com.dutt.rishabh.assignmentbuddy，PID：27027 java.lang.NullPointerException at com.dutt.rishabh.assignmentbuddy.View $ 1.onClick（View.java:36） at android .view.View.performClick（View.java:4482） at android.view.View $ PerformClick.run（View.java:18787） at android.os.Handler.handleCallback（Handler.java:808） at android .os.Handler.dispatchMessage（Handler.java:103） at android.os.Looper.loop（Looper.java:193） at android.app.ActivityThread.main（ActivityThread.java:5345） at java.lang.reflect.Method.invokeNative（Native Method） at java.lang.reflect.Method.invoke（Method.java:515） at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit的.java：828） 在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:644） 在dalvik.system.NativeStart.main（本机方法）

Answer 1

上传整个logcat的消息

Answer 2

在你的onClick方法进入前while循环添加以下行：

cursor.moveToFirst(); 

然后读取光标。如果这没有解决您的问题，请之前while(cursor.moveToNext())

Answer 3

呼叫cursor.moveToPrevious()我没有看到你的DatabaseHelper实例（dbHelper）成气候初始化。

你需要获取的所有记录这样的事情之前初始化dbHelper：

dbHelper = new DatabaseHelper(this);