java如何使用JAVA8流从类中获取所需的值

Question

我有列表<RecipeDto>食谱。我只想使用stream从RecipeDto类中获取关键字和ingredients。此代码工作不正常

List<String> keywordsAndIngredientsStream = 
recipes.stream().forEach(recipeDto -> {
            recipeDto.getIngredients().forEach(ingredient -> ingredient.toLowerCase());
            recipeDto.getKeywords().forEach(keywords -> keywords.toLowerCase());})
           .collect(Collectors.toList());

Answer 1

如果您想要成分和关键字的列表，只需执行以下操作：

ArrayList<RecipeDTO> recipes = new ArrayList<RecipeDTO>() {{

    add(new RecipeDTO(Arrays.asList("onion", "rice"), Arrays.asList("yummy", "spicy")));
    add(new RecipeDTO(Arrays.asList("garlic", "tomato"), Arrays.asList("juicy", "salty")));

}};

List<String> ingredientsAndKeywords = recipes.stream()
        .flatMap(recipe -> Stream.concat(recipe.getIngredients().stream(), recipe.getKeywords().stream()))
        .map(String::toLowerCase)
        .collect(toList());

for (String ingredientsAndKeyword : ingredientsAndKeywords) {
    System.out.println(ingredientsAndKeyword);
}

输出

onion
rice
yummy
spicy
garlic
tomato
juicy
salty

更新

考虑到新的要求，只需执行以下操作：

List<String> ingredientsAndKeywords = recipes.stream()
                .map(recipe -> Stream.concat(recipe.getIngredients().stream(), recipe.getKeywords().stream())
                        .map(String::toLowerCase).collect(joining(" ")))
                .collect(toList());

        for (String ingredientsAndKeyword : ingredientsAndKeywords) {
            System.out.println(ingredientsAndKeyword);
        }

输出

onion rice yummy spicy
garlic tomato juicy salty

Answer 2

如果您真的要将成分和关键字流（如变量名称所示）收集成一个映射为小写的流，您可以concat它们如下：

Stream<String> keywordsAndIngredientsStream = recipes.stream()
        .flatMap(rec -> Stream.concat(rec.getIngredients().stream(), rec.getKeywords().stream())
                .map(String::toLowerCase));

此外，如果您想将其收集到List<String>作为：

List<String> keywordsAndIngredientsList = keywordsAndIngredientsStream.collect(Collectors.toList());