转换成 Java 8 Lambda 表达式

Question

I am very new to lambda. Can someone please help in converting following nested loops in lambda -

for (Question question : questionList) {
        formattedText += NEW_LINE + localizeString(QUESTION_STRING_ID) + question.getQuestionText();
        formattedText += NEW_LINE + localizeString(ANSWER_STRING_ID);
        for (Answer answer : question.getAnswers()) {
            formattedText += answer.getAnswerText() + NEW_LINE;
        }
    }

Solution I tried

for (Question question : questionList) {
        formattedText += NEW_LINE + localizeString(QUESTION_STRING_ID) + question.getQuestionText();
        formattedText += NEW_LINE + localizeString(ANSWER_STRING_ID);
        question.getAnswers().forEach(answer -> {formattedText += answer.getAnswerText() + NEW_LINE;});
}

Not sure how can I convert both loops into single lambda expression? Also, this solution is not working since it needs formattedText to be declared as final.

Answer 1

It may be implemented using Collectors.joining collector of Stream API:

String prefixQ =  NEW_LINE + localizeString(QUESTION_STRING_ID);
String suffixQ =  NEW_LINE + localizeString(ANSWER_STRING_ID);
String result = questionList.stream()
    .map(q -> 
            prefixQ + q.getQuestionText() + suffixQ 
            + q.getAnswers().stream()
                    .map(Answer::getAnswerText)
                    .collect(Collectors.joining(NEW_LINE))
    )
    .collect(Collectors.joining(NEW_LINE));

Answer 2

You're looking for a Collector . For example,

for (Question question : questionList) {
        formattedText += NEW_LINE + localizeString(QUESTION_STRING_ID) + question.getQuestionText();
        formattedText += NEW_LINE + localizeString(ANSWER_STRING_ID);
        formattedText +=
                question.getAnswers()
                        .stream()
                        .map(Answer::getAnswerText)
                        .collect(Collectors.joining(NEW_LINE));
        formattedText += NEW_LINE;
}

That supposes that NEW_LINE represents a String . If instead it is a character then you could use String.valueOf(NEWLINE) as the argument to Collectors.joining() .

Do note, however, that there is not actually any lambda in that at all. The one place where one might have been used, I use a method reference instead.