Android - 检查 Not Null 未按预期工作

Question

我遇到了一个奇怪的问题，即我的 not Null 检查不起作用。我正在尝试检查值是否为空，如果是，则弹出一个警告，上面写着“成分需要一个名称”我不知道为什么，但我似乎无法弄清楚这应该相当简单。

我还是 Kotlin 的新手，所以我怀疑我的 Java 到 Kotlin 的思维过程在这里存在缺陷。

任何帮助和建议将不胜感激！

我遇到问题的部分：

// Check that the name is not null
    (if (values != null) values else throw KotlinNullPointerException()).getAsString(IngredientEntry.COLUMN_INGREDIENT_NAME) ?: throw IllegalArgumentException("Ingredient requires a name")

完整的课程。我倾向于评论所有的废话，所以希望它是有道理的：

import android.content.ContentProvider
import android.content.ContentUris
import android.content.ContentValues
import android.content.UriMatcher
import android.database.Cursor
import android.database.sqlite.SQLiteDatabase
import android.net.Uri
import android.util.Log
import 
import kotlin.properties.Delegates

class DbContentProvider : ContentProvider() {

// Database helper object
private var mDbHelper: DbHelper by Delegates.notNull()


// Order the of ingredients in the list view
private val ingredientSortBy = IngredientEntry.COLUMN_INGREDIENT_NAME

override fun onCreate(): Boolean {
    mDbHelper = DbHelper(context)
    return true
}

override fun query(uri: Uri, projection: Array<String>?, selection: String?, selectionArgs: Array<String>?,
                   sortOrder: String?): Cursor? {

    // Get readable database
    val database: SQLiteDatabase = mDbHelper.readableDatabase

    // This cursor will hold the result of the query
    val cursor: Cursor

    // Figure out if the URI matcher can match the URI to a specific code
    val match = sUriMatcher.match(uri)
    when (match) {
        INGREDIENTS ->
            // For the INGREDIENTS code, query the ingredients table directly with the given
            // projection, selection, selection arguments, and sort order. The cursor
            // could contain multiple rows of the ingredients table.
            // Query is uri, projection, selection, selectionArgs, sortOrder and orderBy
            cursor = database.query(IngredientEntry.TABLE_NAME, projection, selection, selectionArgs, null, null, ingredientSortBy)
        INGREDIENTS_ID -> {
            // For the INGREDIENTS_ID code, extract out the ID from the URI.
            // For an example URI such as "content://com.example.android.ingredients/ingredients/3",
            // the selection will be "_id=?" and the selection argument will be a
            // String array containing the actual ID of 3 in this case.
            //
            // For every "?" in the selection, we need to have an element in the selection
            // arguments that will fill in the "?". Since we have 1 question mark in the
            // selection, we have 1 String in the selection arguments' String array.
            val mSelection = IngredientEntry._ID + "=?"
            val mSelectionArgs = arrayOf(ContentUris.parseId(uri).toString())

            // This will perform a query on the ingredients table where the _id equals 3 to return a
            // Cursor containing that row of the table.
            // Query is uri, projection, selection, selectionArgs, sortOrder and orderBy
            cursor = database.query(IngredientEntry.TABLE_NAME, projection, mSelection, mSelectionArgs, null, null, ingredientSortBy)
        }
        else -> throw IllegalArgumentException("Cannot query unknown URI " + uri)
    }

    // Set notification URI on the Cursor,
    // so we know what content URI the Cursor was created for.
    // If the data at this URI changes, then we know we need to update the Cursor.
    cursor.setNotificationUri(context.contentResolver, uri)

    // Return the cursor
    return cursor
}

override fun insert(uri: Uri, contentValues: ContentValues?): Uri? {
    val match = sUriMatcher.match(uri)
    when (match) {
        INGREDIENTS -> return insertIngredient(uri, contentValues)
        else -> throw IllegalArgumentException("Insertion is not supported for " + uri)
    }
}

/**
 * Insert an ingredient into the database with the given content values. Return the new content URI
 * for that specific row in the database.
 */
private fun insertIngredient(uri: Uri, contentValues: ContentValues?): Uri? {
    // Check that the name is not null or empty
    val ingredientName = contentValues?.getAsString(IngredientEntry.COLUMN_INGREDIENT_NAME)
    if (ingredientName == null || ingredientName.isEmpty()) {
        throw IllegalArgumentException("Ingredient requires valid category")
    }

    // Check that the category is valid
    val category = contentValues.getAsInteger(IngredientEntry.COLUMN_INGREDIENT_CATEGORY)
    if (category == null || !IngredientEntry.isValidCategory(category)) {
        throw IllegalArgumentException("Ingredient requires valid category")
    }

    // Check that the measurement is valid
    val measurement = contentValues.getAsInteger(IngredientEntry.COLUMN_INGREDIENT_MEASUREMENT)
    if (measurement == null || !IngredientEntry.isValidMeasurement(measurement)) {
        throw IllegalArgumentException("Ingredient requires valid measurement")
    }

    // No need to check the description, any value is valid (including null).

    // Get writable database
    val database = mDbHelper.writableDatabase

    // Insert the new ingredient with the given values
    val id = database.insert(IngredientEntry.TABLE_NAME, null, contentValues)
    // If the ID is -1, then the insertion failed. Log an error and return null.
    if (id == (-1).toLong()) {
        Log.e(LOG_TAG, "Failed to insert row for " + uri)
        return null
    }

    // Notify all listeners that the data has changed for the ingredient content URI
    context.contentResolver.notifyChange(uri, null)

    // Return the new URI with the ID (of the newly inserted row) appended at the end
    return ContentUris.withAppendedId(uri, id)
}

override fun update(uri: Uri, contentValues: ContentValues?, selection: String?,
                    selectionArgs: Array<String>?): Int {

    val match = sUriMatcher.match(uri)
    return when (match) {
        INGREDIENTS -> updateIngredient(uri, contentValues, selection, selectionArgs)
        INGREDIENTS_ID -> {
            // For the INGREDIENTS_ID code, extract out the ID from the URI,
            // so we know which row to update. Selection will be "_id=?" and selection
            // arguments will be a String array containing the actual ID.
            val mSelection = IngredientEntry._ID + "=?"
            val mSelectionArgs = arrayOf(ContentUris.parseId(uri).toString())
            updateIngredient(uri, contentValues, mSelection, mSelectionArgs)
        }
        else -> throw IllegalArgumentException("Update is not supported for " + uri)
    }
}

/**
 * Update ingredients in the database with the given content contentValues. Apply the changes to the rows
 * specified in the selection and selection arguments (which could be 0 or 1 or more ingredients).
 * Return the number of rows that were successfully updated.
 */
private fun updateIngredient(uri: Uri, contentValues: ContentValues?, selection: String?, selectionArgs: Array<String>?): Int {
    // If the {@link IngredientEntry#COLUMN_INGREDIENT_NAME} key is present,
    // check that the name value is not null.
    if ((if (contentValues != null) contentValues else throw KotlinNullPointerException()).containsKey(IngredientEntry.COLUMN_INGREDIENT_NAME)) {
        contentValues.getAsString(IngredientEntry.COLUMN_INGREDIENT_NAME) ?: throw IllegalArgumentException("Ingredient requires a name")
    }

    // If the {@link IngredientEntry#COLUMN_INGREDIENT_CATEGORY} key is present,
    // check that the category value is valid.
    if (contentValues.containsKey(IngredientEntry.COLUMN_INGREDIENT_CATEGORY)) {
        val category = contentValues.getAsInteger(IngredientEntry.COLUMN_INGREDIENT_CATEGORY)
        if (category == null || !IngredientEntry.isValidCategory(category)) {
            throw IllegalArgumentException("Ingredient requires valid category")
        }
    }

    // If the {@link IngredientEntry#COLUMN_INGREDIENT_MEASUREMENT} key is present,
    // check that the measurement value is valid.
    if (contentValues.containsKey(IngredientEntry.COLUMN_INGREDIENT_MEASUREMENT)) {
        val measurement = contentValues.getAsInteger(IngredientEntry.COLUMN_INGREDIENT_MEASUREMENT)
        if (measurement == null || !IngredientEntry.isValidMeasurement(measurement)) {
            throw IllegalArgumentException("Ingredient requires valid measurement")
        }
    }

    // No need to check the description, any value is valid (including null).

    // If there are no contentValues to update, then don't try to update the database
    if (contentValues.size() == 0) {
        return 0
    }

    // Otherwise, get writable database to update the data
    val database = mDbHelper.writableDatabase

    // Perform the update on the database and get the number of rows affected
    val rowsUpdated = database.update(IngredientEntry.TABLE_NAME, contentValues, selection, selectionArgs)

    // If 1 or more rows were updated, then notify all listeners that the data at the
    // given URI has changed
    if (rowsUpdated != 0) {
        context.contentResolver.notifyChange(uri, null)
    }

    // Return the number of rows updated
    return rowsUpdated
}

override fun delete(uri: Uri, selection: String?, selectionArgs: Array<String>?): Int {

    // Get writable database
    val database = mDbHelper.writableDatabase

    // Track the number of rows that were deleted
    val rowsDeleted: Int

    val match = sUriMatcher.match(uri)
    rowsDeleted = when (match) {
        INGREDIENTS ->
            // Delete all rows that match the selection and selection args
            database.delete(IngredientEntry.TABLE_NAME, selection, selectionArgs)
        INGREDIENTS_ID -> {
            // Delete a single row given by the ID in the URI
            val mSelection = IngredientEntry._ID + "=?"
            val mSelectionArgs = arrayOf(ContentUris.parseId(uri).toString())
            database.delete(IngredientEntry.TABLE_NAME, mSelection, mSelectionArgs)
        }
        else -> throw IllegalArgumentException("Deletion is not supported for " + uri)
    }

    // If 1 or more rows were deleted, then notify all listeners that the data at the
    // given URI has changed
    if (rowsDeleted != 0) {
        context.contentResolver.notifyChange(uri, null)
    }

    // Return the number of rows deleted
    return rowsDeleted
}

override fun getType(uri: Uri): String? {
    val match = sUriMatcher.match(uri)
    return when (match) {
        INGREDIENTS -> IngredientEntry.CONTENT_LIST_TYPE
        INGREDIENTS_ID -> IngredientEntry.CONTENT_ITEM_TYPE
        else -> throw IllegalStateException("Unknown URI $uri with match $match")
    }
}

companion object {

    /** Tag for the log messages  */
    val LOG_TAG: String? = ContentProvider::class.java.simpleName

    /** URI matcher code for the content URI for the ingredients table  */
    private val INGREDIENTS = 100

    /** URI matcher code for the content URI for a single ingredient in the ingredients table  */
    private val INGREDIENTS_ID = 101

    /**
     * UriMatcher object to match a content URI to a corresponding code.
     * The input passed into the constructor represents the code to return for the root URI.
     * It's common to use NO_MATCH as the input for this case.
     */
    private val sUriMatcher = UriMatcher(UriMatcher.NO_MATCH)

    // Static initializer. This is run the first time anything is called from this class.
    init {
        // The calls to addURI() go here, for all of the content URI patterns that the provider
        // should recognize. All paths added to the UriMatcher have a corresponding code to return
        // when a match is found.

        // The content URI of the form "content://com.example.android.ingredients/ingredients" will map to the
        // integer code {@link #INGREDIENTS}. This URI is used to provide access to MULTIPLE rows
        // of the ingredients table.
        sUriMatcher.addURI(DbContract.CONTENT_AUTHORITY, DbContract.PATH_INGREDIENTS, INGREDIENTS)

        // The content URI of the form "content://com.example.android.ingredients/ingredients/#" will map to the
        // integer code {@link #INGREDIENTS_ID}. This URI is used to provide access to ONE single row
        // of the ingredients table.
        //
        // In this case, the "#" wildcard is used where "#" can be substituted for an integer.
        // For example, "content://com.example.android.ingredients/ingredients/3" matches, but
        // "content://com.example.android.ingredients/ingredients" (without a number at the end) doesn't match.
        sUriMatcher.addURI(DbContract.CONTENT_AUTHORITY, DbContract.PATH_INGREDIENTS + "/#", INGREDIENTS_ID)
    }
}
}

我更新了我的方法，现在它捕获 null 或空。但是，如果有人将名称字段留空，它显然会使我的应用程序崩溃。关于如何让用户知道必须填充名称字段，而不是让我的应用程序崩溃并停止创建成分的任何想法？谢谢！

// Check that the name is not null or empty
    val ingredientName = contentValues?.getAsString(IngredientEntry.COLUMN_INGREDIENT_NAME)
    if (ingredientName == null || ingredientName.isEmpty()) {
        throw IllegalArgumentException("Ingredient requires valid category")
    }

我制作的另一个版本捕获了异常并且不会使应用程序崩溃，但仍然允许用户创建没有名称的成分。呸！

// Check that the name EditText field is not null or empty
    val ingredientName = contentValues?.getAsString(IngredientEntry.COLUMN_INGREDIENT_NAME)
    if (ingredientName == null || ingredientName.isEmpty()) {
        try {
            throw Exception ("Ingredient requires a name")
        } catch (exception: Exception) {
            System.out.println(exception)
        }
    }

Answer 1

您可以省略 if 语句并改为这样做

values?.getAsString(IngredientEntry.COLUMN_INGREDIENT_NAME) ?: throw IllegalArgumentException("Ingredient requires a name")

您的 elvis 运算符不起作用，因为您的 if 语句从不返回 null，它要么是字符串，要么是 NPE。

您实际上可以通过使用 let 块来改进您的代码。像这样的。

values?.let{
    //logic here that uses value, refer to value as it
} ?: IllegalArgumentException()

编辑

values?.let{
    if(it.getAsString(IngredientEntry.COLUMN_INGREDIENT_NAME).isEmpty()) throw IllegalArgumentException()
    else // do your logic
}