當我嘗試打開它時，微調器活動崩潰

Question

當我打開它時，我拿着微調器的活動崩潰了，關於如何解決這個問題的任何想法？

DB helper 中使用的代碼如下所示：

public List<String> getAllNames() {
    List<String> retData = new ArrayList<String>();
    String selectQuery = "SELECT name FROM sqlite_master WHERE type IN ('table','view') AND name NOT LIKE 'sqlite_%' and name <> 'ViewWorkout' ORDER BY 1";
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);
    if (cursor.moveToFirst()) {
        do {
            retData.add(cursor.getString(0));
        } while (cursor.moveToNext());
    }
    return retData;
}

用於將返回的列表調用到微調器中的代碼如下所示：

public class SelectWorkoutActivity extends AppCompatActivity {

    DBHelper DB;
    Spinner Workout_list;

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

        DB = new DBHelper(this);
        List<String> Workouts = DB.getAllNames();

        //Set Adapter on the spinner
        Workout_list.setAdapter(new ArrayAdapter<>(SelectWorkoutActivity.this
                , android.R.layout.simple_spinner_dropdown_item,Workouts));
    }
}

任何幫助將不勝感激

Answer 1

您需要實例化微調器，否則它將是 null 導致 Null 指針異常 (NPE) 和崩潰。

所以

public class SelectWorkoutActivity extends AppCompatActivity {

    DBHelper DB;
    Spinner Workout_list;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_select_workout);
        Workout_list = this.findViewById(R.id.???????); //<<<<<<<<<< ADDED (see note)

        DB = new DBHelper(this);
        List<String> Workouts = DB.getAllNames();

        //Set Adapter on the spinner
        Workout_list.setAdapter(new ArrayAdapter<>(SelectWorkoutActivity.this
                , android.R.layout.simple_spinner_dropdown_item,Workouts));
    }
}

• 注意替換？？？？？？ 在布局 activity_select_workout/xml 中分配的 id

這是一個演示 Spinner 工作的工作：-

用於活動的布局（只有微調器）：-

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Spinner
        android:id="@+id/workout_list"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content">
    </Spinner>

</LinearLayout>

• 請注意根據android:id="@+id/workout_list"分配給微調器的 id

和活動： -

public class MainActivity extends AppCompatActivity {

    //DBHelper DB; //commented out for demo
    Spinner Workout_list;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_select_workout);
        Workout_list = this.findViewById(R.id.workout_list); //<<<<< ADDED

        //DB = new DBHelper(this); //commented out for demo
        /* List built from hardcoded items for demo */
        List<String> Workouts = Arrays.asList(new String[]{"Item1", "Item2", "Item3"});

        //Set Adapter on the spinner
        Workout_list.setAdapter(new ArrayAdapter<>(this /*<<<<< changed for demo */
                , android.R.layout.simple_spinner_dropdown_item, Workouts));

    }
}

結果：-

如果沒有 Spinner 的實例化，您將獲得 NPE（空指針異常）：-

2021-12-10 05:49:39.460 5361-5361/? D/AndroidRuntime: Shutting down VM
2021-12-10 05:49:39.462 5361-5361/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: a.a.so70292623javaspinner, PID: 5361
    java.lang.RuntimeException: Unable to start activity ComponentInfo{a.a.so70292623javaspinner/a.a.so70292623javaspinner.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setAdapter(android.widget.SpinnerAdapter)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setAdapter(android.widget.SpinnerAdapter)' on a null object reference
        at a.a.so70292623javaspinner.MainActivity.onCreate(MainActivity.java:28)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 
2021-12-10 05:49:39.466 1943-4597/system_process W/ActivityManager:   Force finishing activity a.a.so70292623javaspinner/.MainActivity

如果您查看 Android Studio 中的日志，您將看到它具有指向相應代碼的鏈接，例如

如果您單擊鏈接並在違反該行時添加一個斷點，然后使用調試選項（綠色錯誤圖標），例如

代碼會停在該行，會出現調試器window，顯示Workout_list是null：-

在https://developer.android.com/studio/debug/上查看有關調試的更多信息