Question

我正在尝试创建一个应用程序，该应用程序需要用户输入主题名称和子代码，并且该子代码将用作字段的名称来创建数据库Table.But我得到一个即使正在保存共享首选项，创建数据库时也会出现空指针异常。

这不是A＆＃34; context.getSharedPreference问题＆＃34;请看一下。

public class Check_regularHelper extends SQLiteOpenHelper {

    Context mcontext=null;
    public static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = "Check_regular.db";
    SharedPreferences pref;

    private String createCMD(){
        String s = "";
        int total=pref.getInt("sno",0);
        for(int i=1;i<=total;i++){
            s += pref.getString(String.valueOf(i),"") + " INTEGER DEFAULT 0";
            if(i!=total){
                s+=", ";
            }
        }
        return s;
    }

    public String SQL_CREATE_ENTRIES = "CREATE TABLE "+ SubContract.Check_Regular.TABLE_NAME
            + " (" + SubContract.Check_Regular.DATE + " TEXT PRIMARY KEY," + createCMD() +");";

    public Check_regularHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
        mcontext=context;
        this.pref = context.getSharedPreferences(subjectFill.Subjects,Context.MODE_PRIVATE);
        if(pref==null) Log.e(context.toString(),"fine-----------------");
        if(pref!=null) Log.e(context.toString(),"wrong-----------------");
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(SQL_CREATE_ENTRIES);

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
}

08-24 19:21:06.184 8226-8226/com.creator.innov.check_regular E/AndroidRuntime: FATAL EXCEPTION: main
                                                                               Process: com.creator.innov.check_regular, PID: 8226
                                                                               java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.creator.innov.check_regular/com.creator.innov.check_regular.DailyTimeTable.Check_regularSubject}: java.lang.NullPointerException: Attempt to invoke interface method 'int android.content.SharedPreferences.getInt(java.lang.String, int)' on a null object reference
                                                                                   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2334)
                                                                                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2483)
                                                                                   at android.app.ActivityThread.access$900(ActivityThread.java:153)
                                                                                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1349)

即使我试图检查sharedPreferences是否为null，它也没有显示任何东西。请帮忙

Answer 1

在初始化pref之前评估赋值字符串SQL_CREATE_ENTRIES，这就是pref为空的原因。

尝试这种方式：

    public String SQL_CREATE_ENTRIES ;

   public Check_regularHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
        mcontext=context;
        this.pref = context.getSharedPreferences(subjectFill.Subjects,Context.MODE_PRIVATE);
        if(pref==null) Log.e(context.toString(),"fine-----------------");
        if(pref!=null)  {
           Log.e(context.toString(),"wrong-----------------");

           SQL_CREATE_ENTRIES = "CREATE TABLE "+ SubContract.Check_Regular.TABLE_NAME
            + " (" + SubContract.Check_Regular.DATE + " TEXT PRIMARY KEY," + createCMD() +");";
           }
    }

Answer 2

尝试使用app context来获取共享首选项：

public class App extends Application {

    public static Application application;

    @Override
    public void onCreate() {
        super.onCreate();
        application = this;

    }

    public static Application get(){
        return application;
    }
}

将Manifest添加到应用程序标签android：name =“。App”

public class Helper extends SQLiteOpenHelper {

                private static final String DB_NAME = "sample.db";
                private static final int VERSION = 1;
                private String CREATE =
                        "CREATE TABLE " + "%s"
                                + "("
                                + BaseColumns._ID + " INTEGER PRIMARY KEY AUTOINCREMENT,"
                                + "%s TEXT,"
                                + "%s TEXT"
                                + ");";

                public Helper(Context context) {
                    super(context, DB_NAME, null, VERSION);
                }

                @Override
                public void onCreate(SQLiteDatabase sqLiteDatabase) {
                    Context appContext = App.get();
                    SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(appContext);

                    String tableName = sharedPreferences.getString("TABLE_NAME", "TABLE1");

                    String column1 = sharedPreferences.getString("COLUMN1", "COLUMN1");
                    String column2 = sharedPreferences.getString("COLUMN2", "COLUMN2");

                    String formatted = String.format(CREATE, tableName, column1, column2);

                    sqLiteDatabase.execSQL(formatted);
                }

                @Override
                public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

                }
            }

如何通过从共享首选项中获取字段来创建数据库？

2 个答案: