游标崩溃:请求了索引0，大小为0

Question

我不知道为什么我的光标是空的。我读了其他4-5个问题，其中包含相同的信息，但我无法解决我的问题。我忘了什么？

有了这个代码，我得到了消息："光标为空"：

    public void onClick(View arg0) {
        String txt_edit_hw = edit_hw.getText().toString(); 
        ContentValues args = new ContentValues();
        args.put("hw", txt_edit_hw);
        String test = "SELECT _id FROM tbl_homework where hw='"+ txt_edit_hw +"';";
        Cursor mCursor = db.rawQuery(test, null); 
        startManagingCursor(mCursor);
        if (mCursor.moveToFirst()){
        String hw = mCursor.getString(mCursor.getColumnIndex("hw"));

        doMessage(hw);
        db.update("tbl_homework", args, "_id=34", null);
        fillData();
        } else {
            doMessage("Cursor is empty");
        }
    }

有了这个代码，我得到了crach。。。android索引0请求，大小为0:

    public void onClick(View arg0) {
        String txt_edit_hw = edit_hw.getText().toString(); 
        ContentValues args = new ContentValues();
        args.put("hw", txt_edit_hw);
        String test = "SELECT _id FROM tbl_homework where hw='"+ txt_edit_hw +"';";
        Cursor mCursor = db.rawQuery(test, null); 
        startManagingCursor(mCursor);
        mCursor.moveToFirst();
        String hw = mCursor.getString(mCursor.getColumnIndex("hw"));
        doMessage(hw);
        db.update("tbl_homework", args, "_id=34", null);
        fillData();
    }

同样的错误是这样的：它在字符串（我评论了它）上崩溃

        public void onClick(View arg0) {
        String txt_edit_hw = edit_hw.getText().toString(); 
        txt_edit_hw.trim();
        ContentValues args = new ContentValues();
        args.put("hw", txt_edit_hw);
        String test = "SELECT _id FROM tbl_homework where hw='"+ txt_edit_hw +"';";
        Cursor mCursor = db.rawQuery(test, null); 
        startManagingCursor(mCursor);
        mCursor.moveToFirst();
        /**It crashes here**/String hwl = mCursor.getString(mCursor.getColumnIndex("_id"));
        doMessage(hwl);
        db.update("tbl_homework", args, "_id=34", null);
        fillData();
    }
02-06 12:13:49.789: I/Process(6112): Sending signal. PID: 6112 SIG: 9
02-06 12:13:55.582: D/dalvikvm(6140): GC_EXTERNAL_ALLOC freed 37K, 49% free 2785K/5379K, external 5211K/5218K, paused 23ms
02-06 12:13:58.785: D/AndroidRuntime(6140): Shutting down VM
02-06 12:13:58.785: W/dalvikvm(6140): threadid=1: thread exiting with uncaught exception (group=0x40091568)
02-06 12:13:58.796: E/AndroidRuntime(6140): FATAL EXCEPTION: main
02-06 12:13:58.796: E/AndroidRuntime(6140): android.database.CursorIndexOutOfBoundsException: Index 0 requested, with a size of 0
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.database.AbstractCursor.checkPosition(AbstractCursor.java:580)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.database.AbstractWindowedCursor.checkPosition(AbstractWindowedCursor.java:214)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.database.AbstractWindowedCursor.getString(AbstractWindowedCursor.java:41)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at test.marco.notenha.homework$3.onClick(homework.java:227)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.view.View.performClick(View.java:2486)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.view.View$PerformClick.run(View.java:9130)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.os.Handler.handleCallback(Handler.java:587)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.os.Handler.dispatchMessage(Handler.java:92)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.os.Looper.loop(Looper.java:130)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at android.app.ActivityThread.main(ActivityThread.java:3703)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at java.lang.reflect.Method.invokeNative(Native Method)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at java.lang.reflect.Method.invoke(Method.java:507)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
02-06 12:13:58.796: E/AndroidRuntime(6140):     at dalvik.system.NativeStart.main(Native Method)

表：行：_id、hwdate、hwVal:"34"，"05/02/12"，"hy，xsdhagsah"

Answer 1

简单地说，您的Cursor没有任何值，但您仍在尝试从Cursor中读取。因此，检查cursor.getCount(); 的值总是更好的

Log.d("Count",String.valueOf(cursor.getCount())); 
if(cursor.getCount() > 0){
// get values from cursor here
}

Answer 2

您选择了_id列，但使用了mCursor.getColumnIndex("hw")。您的选择中没有列hw，只有_id。getColumnIndex()返回0，会得到相应的错误。

UPD:无需使用getCount()。让我分享一些代码片段。首先，让我们假设我们只需要光标的第一个值：

Cursor cursor = //query here
if (cursor.moveToFirst()) {
    // retrieve values from the cursor
}

如果我们需要多个值：

Cursor cursor = //query here
while (cursor.moveToNext()) {
    // retrieve next set of values from the cursor
}

光标的初始位置总是-1，这就是为什么上面的代码可以工作并处理所有条目的原因。