应用功能后获取第一个元素并返回 [英] Getting first element and returning after apply a function

问题描述

我是Java 8的新手，我想创建一个方法，该方法在应用函数后获取匹配的第一个元素并返回

I am new in Java 8, I want to make a method that gets the first element that matched and returning after apply a function

public void test() {
    List<String> features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");

    String str = features
            .stream()
            .filter(s -> "Lambdas".equals(s))
            .findFirst()
            .ifPresent(this::toLowerCase);
}

private String toLowerCase (String str) {
    return str.toLowerCase();
}

但是我遇到了不兼容的类型错误.

but I got an Incompatible types error.

推荐答案

Optional.ifPresent接受Consumer，并且不返回任何值.使用map:

Optional.ifPresent accepts a Consumer, and doesn't return any value. Use map:

String str =
    features.stream()
            .filter(s -> "Lambdas".equals(s))
            .findFirst()
            .map(this::toLowerCase)
            .orElse(null); // default value or orElseThrow

或者按照Holger的建议，您可以将map步骤移至流管道中:

Or, as Holger suggested, you can move the map step into the stream pipeline:

String str =
    features.stream()
            .filter(s -> "Lambdas".equals(s))
            .map(this::toLowerCase)
            .findFirst()
            .orElse(null); // default value or orElseThrow

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