Question

我有一个数据库表，其中有一个名为“notes”的列。此列的数据仅为“是”或“否”，具体取决于特定行是否为我最喜欢的行（添加到收藏夹）。

我正在尝试使用以下代码更新此列的数据：

public void updateFavorite(long id) 
   {
       String txt;
       System.out.println("first");
       Cursor c = database.query("recipes", null, "_id=" + id, null, null, null, null);
       c.moveToFirst();
       System.out.println("second");
       int notesIndex = c.getColumnIndex("notes");
       System.out.println("third");
       String fav=c.getString(notesIndex);
       System.out.println("fourth");

       System.out.println(fav);

       if (fav=="yes") {txt="no";}
       else {txt="yes";}

       ContentValues editEsoda = new ContentValues();

       editEsoda.put("notes", txt);

       open(); // open the database
       database.update("recipes", editEsoda, "_id=" + id, null);
       close(); // close the database
       c.close();
   }

尝试检索Cursor c时出现错误：

Cursor c = database.query("recipes", null, "_id=" + id, null, null, null, null);

我想读取此行和列的字符串。如果是“是”则将其变为“否”。否则，如果是“否”，则将其变为“是”。

为什么我在那里收到错误？提前谢谢

这是我的LogCat：

11-26 19:00:25.404: D/dalvikvm(30378): GC_FOR_MALLOC freed 4678 objects / 265896 bytes in 79ms
11-26 19:00:25.444: I/System.out(30378): first
11-26 19:00:25.444: D/AndroidRuntime(30378): Shutting down VM
11-26 19:00:25.444: W/dalvikvm(30378): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
11-26 19:00:25.454: E/AndroidRuntime(30378): FATAL EXCEPTION: main
11-26 19:00:25.454: E/AndroidRuntime(30378): java.lang.IllegalStateException: Could not execute method of the activity
11-26 19:00:25.454: E/AndroidRuntime(30378):    at android.view.View$1.onClick(View.java:2072)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at android.view.View.performClick(View.java:2408)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at android.view.View$PerformClick.run(View.java:8816)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at android.os.Handler.handleCallback(Handler.java:587)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at android.os.Handler.dispatchMessage(Handler.java:92)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at android.os.Looper.loop(Looper.java:123)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at android.app.ActivityThread.main(ActivityThread.java:4627)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at java.lang.reflect.Method.invokeNative(Native Method)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at java.lang.reflect.Method.invoke(Method.java:521)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at dalvik.system.NativeStart.main(Native Method)
11-26 19:00:25.454: E/AndroidRuntime(30378): Caused by: java.lang.reflect.InvocationTargetException
11-26 19:00:25.454: E/AndroidRuntime(30378):    at development.nk.cretanrecipes.ViewRecipes.favorite(ViewRecipes.java:235)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at java.lang.reflect.Method.invokeNative(Native Method)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at java.lang.reflect.Method.invoke(Method.java:521)
11-26 19:00:25.454: E/AndroidRuntime(30378):    at android.view.View$1.onClick(View.java:2067)
11-26 19:00:25.454: E/AndroidRuntime(30378):    ... 11 more
11-26 19:00:25.454: E/AndroidRuntime(30378): Caused by: java.lang.NullPointerException
11-26 19:00:25.454: E/AndroidRuntime(30378):    at development.nk.cretanrecipes.DatabaseConnector.updateFavorite(DatabaseConnector.java:93)
11-26 19:00:25.454: E/AndroidRuntime(30378):    ... 15 more
11-26 19:00:29.094: I/Process(30378): Sending signal. PID: 30378 SIG: 9

Answer 1

首先，fav=="yes"不正确。您应该使用："yes".equals(fav)
其次，如果您只有两个可能的值，为什么不将您的值存储为数据库中的布尔值？您将节省存储空间。
第三，你得到哪个错误?? !!

Answer 2

我在尝试检索Cursor c时遇到错误 Cursor c = database.query("recipes", null, "_id=" + id, null, null, null, null);

这意味着database为null，您需要在此行前使用database = helper.getWritableDatabase()，并将helper替换为您的SQLiteOpenHelper变量。我猜这是open()的作用。

与其他人所说的一样，让这个方法更快一点：

public void updateFavorite(long id) 
{
    open(); // open the database here!
    ContentValues editEsoda = new ContentValues();

    // Only fetch the columns that you are going to work with
    Cursor c = database.query("recipes", new String[] {"notes"}, "_id=" + id, null, null, null, null);

    // If a row exists with the given id
    if(c.moveToFirst()) {
        // Toggle the value of "notes", but this should be a boolean
        if(c.getString(0).equals("yes")
            editEsoda.put("notes", "no");
        else
            editEsoda.put("notes", "yes");

        c.close();
        database.update("recipes", editEsoda, "_id=" + id, null);
    }
    else {
        // No data has this id
    }
    close(); // close the database
}

SQLite数据库更新行时的游标错误

2 个答案: