Getting first element and returning after apply a function

问题

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();
}

but I got an Incompatible types error.

回答1:

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

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

回答2:

 String str =
        features
            .stream()
            .filter("Lambdas"::equals)
            .findFirst()
            .map(this::toLowerCase)
            .orElse("AnythingElse");

findFirst returns an Optional, as such use some method on that Optional, like orElse that would return some String instance.

Look closely at what Optional::ifPresent takes as input - it's a Consumer, thus read it as "take that String as input, do something with it and return nothing".

回答3:

You can map the string if found orElse assign null to it:

List<String> features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
String str = features
        .stream()
        .filter(s -> "Lambdas".equals(s))
        .findFirst()
        .map(String::toLowerCase)
        .orElse(null);

来源：https://stackoverflow.com/questions/56965311/getting-first-element-and-returning-after-apply-a-function