java - 如何摆脱 Toast 警告中的 "Argument ' context' might be null"

Question

我正在尝试在非 Activity 类(class)中使用 Toast。为此，我正在尝试以下操作:

//Method to get all books from database
public List<Book> getAllBooks() {
    List<Book> books = new LinkedList<Book>();
    Context context = null;
    // 1. build the query
    String query = "SELECT  * FROM " + TABLE_BOOKS;

    // 2. get reference to writable DB
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(query, null);

    // 3. go over each row, build book and add it to list
    Book book = null;
    if (cursor.moveToFirst()) {
        do {
            book = new Book();
            book.setId(Integer.parseInt(cursor.getString(0)));
            book.setTitle(cursor.getString(1));
            book.setAuthor(cursor.getString(2));

            // Add book to books
            books.add(book);
        } while (cursor.moveToNext());
    } else {
        Toast.makeText(context, "No books in database", Toast.LENGTH_LONG).show();
    }
    ....

但是 context 上有一个黄色警告标记，表示Argument 'context' might be null

这是预期的，因为它被初始化为 null。在这种情况下如何使用上下文而不导致程序崩溃？该方法是从 Activity 的 onCreate() 方法调用的。我需要显示此消息。

Answer 1

如果您从 Activity 的 onCreate 调用 getAllBooks，则只需传递 Activity 实例即可作为上下文。

public List<Book> getAllBooks(Context context) {
    ....
}

在 Activity 类中:

public void onCreate(Bundle savedInstanceState)
{
    ... getAllBooks(this); ....
}