当我尝试打开它时，微调器活动崩溃

Question

当我打开它时，我拿着微调器的活动崩溃了，关于如何解决这个问题的任何想法？

DB helper 中使用的代码如下所示：

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;
}

用于将返回的列表调用到微调器中的代码如下所示：

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));
    }
}

任何帮助将不胜感激

Answer 1

您需要实例化微调器，否则它将是 null 导致 Null 指针异常 (NPE) 和崩溃。

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));
    }
}

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

这是一个演示 Spinner 工作的工作：-

用于活动的布局（只有微调器）：-

<?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>

• 请注意根据android:id="@+id/workout_list"分配给微调器的 id

和活动： -

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));

    }
}

结果：-

如果没有 Spinner 的实例化，您将获得 NPE（空指针异常）：-

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，您将看到它具有指向相应代码的链接，例如

如果您单击链接并在违反该行时添加一个断点，然后使用调试选项（绿色错误图标），例如

代码会停在该行，会出现调试器window，显示Workout_list是null：-

在https://developer.android.com/studio/debug/上查看有关调试的更多信息