Java 8：简化代码并将其转换为流 API

Question

Code structure

public class Article {
   public String name;   
   public String description;
   public List<Translation> translations;
}

public class Translation { 
   public String field;
   public String value;
   public String language;
}

There is a method to transform and return specific result

List<Article> returnArticleswithCorrectTranslation(List<Article> articles, String language) {
    List<Article> result = new ArrayList<>();
    for (Article article: articles) {
        String languageCriteria = language;
        boolean isTranslationFound = false;
        for (Translation translation : article.translations) {
            if (translation.language.equals(language)) {
              isTranslationFound = true;
            }
        }

        if (!isTranslationFound) {
            languageCriteria = "en";
        }
        List<Translation> resultTranslations = new ArrayList<>();
        for (Translation translation : article.translations) {
            if (translation.language.equals(languageCriteria)) {
                resultTranslations.add(translation);
            }
        }
        article.translations = resultTranslations;
        result.add(article);

    }
    return result;
}

Need an advice how can I convert that logic to Java 8 stream api ?

Answer 1

Such questions are more suitable for CodeReview , but ok, I'll answer.

First, even in previous Java versions there's no need to create the result list in the loop. The result is actually the same List as the input articles list as you don't create new objects, you modify the existing ones. Thus if you want to copy the original list (for whatever reason), you could write simply:

return new ArrayList<>(articles);

But probably you may omit the copying and return simply articles . Or even better change the return type to void . This way it will be more clear that your method actually changes the passed objects instead of creating the new ones.

To search whether the Collection has element matching the given predicate use Stream.anyMatch() :

String languageCriteria = 
        article.translations.stream().anyMatch(t -> t.language.equals(language)) 
        ? language : "en";

To filter the elements according to the given predicate use Stream.filter() :

article.translations = article.translations.stream()
        .filter(t -> t.language.equals(languageCriteria))
        .collect(Collectors.toList());

Though it's likely that the original article.translations list is not used anywhere alse, and you can perform the filtering in-place using Collection.removeIf() instead of creating the Stream and separate List :

article.translations.removeIf(t -> !t.language.equals(languageCriteria));

So your method might look like this:

void filterArticleTranslations(List<Article> articles, String language) {
    articles.forEach(article -> {
        String languageCriteria = 
                article.translations.stream().anyMatch(t -> t.language.equals(language)) 
                ? language : "en";

        article.translations.removeIf(t -> !t.language.equals(languageCriteria));
    });
}

Further to encapsulate the Article modifications I would extract the loop body to the method of Article class:

public static class Article {
    String name;
    String description;
    List<Translation> translations;

    public void filterTranslations(String language) {
        String languageCriteria = 
                translations.stream().anyMatch(t -> t.language.equals(language)) 
                ? language : "en";

        translations.removeIf(t -> !t.language.equals(languageCriteria));
    }
}

Now your original method may look like this:

void filterArticleTranslations(List<Article> articles, String language) {
    articles.forEach(article -> article.filterTranslations(language));
}