我正在开发我的第一个“非教程”应用程序，以发展和加强我的 Android 开发技能。
我一直在使用大量 Java 泛型来增强可重用性和调试性，特别是因为我的许多 fragment 都对 Question 类的子类执行相同的操作。
我刚遇到一个对我来说很陌生的传统模式，想知道我是否可以将它应用于 Java 中的泛型类。
根据文本，应在 Fragment 类中创建一个 newInstance(args,...) 方法来处理 Intent Extras 到 Fragment 的转换参数。
例子:
SomeActivity.class
@Override
protected Fragment createFragment() {
return new ObjectFragment();
UUID objectId = (UUID)getIntent()
.getSerializableExtra(ObjectFragment.EXTRA_OBJECT_ID);
return ObjectFragment.newInstance(objectId);
}
ObjectFragment.class
public static ObjectFragment newInstance(UUID objectId) {
Bundle args = new Bundle();
args.putSerializable(EXTRA_CRIME_ID, objectId);
ObjectFragment fragment = new ObjectFragment();
fragment.setArguments(args);
return fragment;
}
摘自:Brian Hardy。 “Android 编程:大 Nerd 牧场指南。”
但是使用 Java 泛型的情况呢？
我正在处理的代码:
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，我正在从 QuestionListFragment 的 onCreate() 方法中获取 Extras。我知道如果过渡到 newInstance() 约定应该与 Java 泛型一起使用，我将删除这段代码。
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 fragment 和 Java 泛型的最佳实践是什么？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()，请通过提供正确的声明语法来帮助我修复错误:
// Attempting to refactor to newInstance(Bundle args)
return QuestionListFragment<MultipleAnswerQuestion>.newInstance(getIntent().getExtras()); // ERROR
最佳答案
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;
}
然后调用它:
QuestionListFragment.<MultipleAnswerQuestion>newInstance(getIntent().getExtras());
关于java - 带有自定义 Java Generic 的 newInstance()？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24380930/