newInstance()与自定义Java Generic？

Question

我正在开发我的第一个"非教程"应用程序来增强和增强我的Android开发技能.

我一直在使用大量的Java Generics来增强可重用性和调试性,特别是因为我的许多片段对类的子类做了同样的事情Question.

我刚刚遇到了一个对我来说很新的传统模式,并且想知道我是否可以将它应用于Java中的通用类.

根据文本,newInstance(args,...)应该在Fragment类中创建一个方法来处理Intent Extras 到 Fragment Arguments的转换.

例:

SomeActivity.class

@?O?v?e?r?r?i?d?e?
p?r?o?t?e?c?t?e?d? ?F?r?a?g?m?e?n?t? ?c?r?e?a?t?e?F?r?a?g?m?e?n?t?(?)? ?{?
 ? ? ? ?r?e?t?u?r?n? ?n?e?w? ?ObjectF?r?a?g?m?e?n?t?(?)?;?

 ? ? ? ?U?U?I?D? ?object?I?d? ?=? ?(?U?U?I?D?)?g?e?t?I?n?t?e?n?t?(?)?
 ? ? ? ? ? ? ? ?.?g?e?t?S?e?r?i?a?l?i?z?a?b?l?e?E?x?t?r?a?(Object?F?r?a?g?m?e?n?t?.?E?X?T?R?A?_?OBJECT_?I?D?)?;?

 ? ? ? ?r?e?t?u?r?n? ?Object?F?r?a?g?m?e?n?t?.?n?e?w?I?n?s?t?a?n?c?e?(object?I?d?)?;?
}?

ObjectFragment.class

p?u?b?l?i?c? ?s?t?a?t?i?c? ObjectF?r?a?g?m?e?n?t? ?n?e?w?I?n?s?t?a?n?c?e?(?U?U?I?D? ?object?I?d?)? ?{?
 ? ? ? ?B?u?n?d?l?e? ?a?r?g?s? ?=? ?n?e?w? ?B?u?n?d?l?e?(?)?;?
 ? ? ? ?a?r?g?s?.?p?u?t?S?e?r?i?a?l?i?z?a?b?l?e?(?E?X?T?R?A?_?C?R?I?M?E?_?I?D?,? ?object?I?d?)?;?

 ? ? ? ?ObjectF?r?a?g?m?e?n?t? ?f?r?a?g?m?e?n?t? ?=? ?n?e?w? ?ObjectF?r?a?g?m?e?n?t?(?)?;?
 ? ? ? ?f?r?a?g?m?e?n?t?.?s?e?t?A?r?g?u?m?e?n?t?s?(?a?r?g?s?)?;?

 ? ? ? ?r?e?t?u?r?n? ?f?r?a?g?m?e?n?t?;?
}?

摘录自:Brian Hardy."Android编程:大书呆子牧场指南."

但是使用Java Generics的情况呢？

我正在编写的代码:

QuestionListActivity.class

public class QuestionListActivity extends SingleFragmentActivity {


    // CONSTANTS
    public static final String EXTRA_FRAGMENT_TYPE = "com.renaissanceartsmedia.flashcard.editquestionactivity.fragment";
    public static final String EXTRA_ACTIVITY_TITLE = "ListQuestionActivity.EXTRA_ACTIVITY_TITLE";
    public static final String TAG = "QuestionListActivity";

    // Member Properties
    QuestionType mFragmentType;

    @Override
    protected Fragment createFragment() {

        mFragmentType = (QuestionType) getIntent().getSerializableExtra(EXTRA_FRAGMENT_TYPE);
        System.out.println("mFragmentType: " + mFragmentType);

        // Switch on Enumeration
        switch (mFragmentType) {

            case MULTIPLE_ANSWER_QUESTION:
            case MULTIPLE_CHOICE_QUESTION:
            case TRUE_FALSE_QUESTION:

                // PREVIOUS METHOD
                //return new QuestionListFragment<MultipleAnswerQuestion>();

                // Attempting to refactor to newInstance(Bundle args)
                return QuestionListFragment<MultipleAnswerQuestion>.newInstance(getIntent().getExtras()); // ERROR

            case MATCHING_QUESTION:
                return new QuestionListFragment<MatchingQuestion>();

            case BLANK_QUESTION:
                //return new BQFragment();
                return new QuestionListFragment<BlankQuestion>();
            default:
                return new QuestionListFragment<Question>();
        }

    }
}

目前,我从QuestionListFragment的onCreate()方法中获取Extras.我知道如果转换到newInstance()约定应该与Java Generics一起使用,我将删除此代码.

QuestionListFragment.class

    public class QuestionListFragment<E extends Question> extends ListFragment implements QuestionDialogInterface {

        // Constants
        public static final String TAG = "QuestionListFragement";
        public static final String DIALOG_TITLE = "QuestionListFragment.DIALOG_TITLE";
        public static final String DIALOG_MESSAGE = "QuestionListFragment.DIALOG_MESSAGE";
        public static final String QUESTION_TYPE = "QUESTION_TYPE";
        private static final int DIALOG_FRAGMENT = 1;

        // Member Properties
        Flashcard mFlashcard;
        QuestionType mQuestionType;
        String mActivityTitle;
        ArrayList<E> mQuestions;
        DialogFragment mDialogFragment;

// SOMETHING LIKE THIS???
        @SuppressWarnings({ "unchecked", "rawtypes" })
        public static QuestionListFragment<? extends Question> newInstance(Bundle args) {
            // Create a new instance of QuestionListFragment<? extends Question>
            QuestionListFragment<? extends Question> fragment = new QuestionListFragment();

            // Set the arguments
            fragment.setArguments(args);

            // Return the Fragment
            return fragment;
        }

        @SuppressWarnings("unchecked")
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            Log.d(TAG,"Enter onCreate(Bundle savedInstanceState)");
            // Enable Options Menu
            setHasOptionsMenu(true);

            // Create the ActionBar 'UP' Button
            getActivity().getActionBar().setDisplayHomeAsUpEnabled(true);

            // The Intent Extras
            Bundle extras = getActivity().getIntent().getExtras();

            // Extract the Flashcard from the extras
            UUID flashcardId = (UUID) extras.getSerializable(Flashcard.EXTRA_FLASHCARD_ID);
            mFlashcard = FlashcardStore.get(getActivity()).getFlashcard(flashcardId);
            mQuestionType = (QuestionType) extras.getSerializable(EditQuestionActivity.EXTRA_FRAGMENT_TYPE);
            mActivityTitle = extras.getString(QuestionListActivity.EXTRA_ACTIVITY_TITLE);


            // Get a Container of Multiple Answer Questions
            mQuestions = (ArrayList<E>) mFlashcard.getQuestions(mQuestionType);

            // Set the Title of the Fragment's Activity
            getActivity().setTitle(mActivityTitle);

            // Create a list 
            ListItemLayoutAdapter adapter = new ListItemLayoutAdapter(mQuestions);

            // Set the adapter for the list
            setListAdapter(adapter);
            Log.d(TAG,"Exit onCreate(Bundle savedInstanceState)");
        }
    ....
    }

关于Android Fragments和Java Generics的最佳实践是什么？有人可以描述它们是什么,为什么要使用它们.如果应该使用newInstance(),请通过提供正确的语法来帮助我修复错误:

// Attempting to refactor to newInstance(Bundle args)
 return QuestionListFragment<MultipleAnswerQuestion>.newInstance(getIntent().getExtras()); // ERROR

Answer 1

    public static <T extends Question> QuestionListFragment<T> newInstance(Bundle args) {
        // Create a new instance of QuestionListFragment<? extends Question>
        QuestionListFragment<T> fragment = new QuestionListFragment<T>();

        // Set the arguments
        fragment.setArguments(args);

        // Return the Fragment
        return fragment;
    }

然后称它为:

    QuestionListFragment.<MultipleAnswerQuestion>newInstance(getIntent().getExtras()??);