Conditional extract of value from Java Optional

问题

I am trying to find the most concise (and meaningful) way of using Java Optional, to read the first value off a Optional<String> and return the String if exists, or return "NOT_FOUND". Here is the code I am working with:

public static String getValue(Optional<String> input) {
    return input.ifPresent(val -> val.get()).orElse("NOT_FOUND")
}

The methods of Optional apparently have very specific purposes but the API has left me confused.

Update (4/13/2018): The code in my question is incorrect, because if I regarded val as the value inside the Optional, then val.get() does not make any sense. Thanks for pointing that out, @rgettman.

Also, I added another part to my question in the accepted answer's comments, i.e. I needed a way to manipulate the String value, if present, before returning. The orElse("NOT_FOUND") is still applicable, if the Optional does not contain a value. So what is an acceptable use of the Optional API to achieve the following?

public static String getValue(Optional<String> input) {
    return input.isPresent() ? input.get().substring(0,7).toUpperCase() : "NOT_FOUND";
}

@Aominè's answer and follow up comments addressed both parts of this question.

回答1:

All you have to do is change your return statement to:

return input.orElse("NOT_FOUND");

This will return the object in the Optional if present else returns "NOT_FOUND".

That said, I'd avoid using Optional's as parameters. see here.

回答2:

If you need to manipulate the string value, if it is present before returning it, use map method:

public static String getValue(Optional<String> input) {
    return input.map(s -> s.substring(0,7).toUpperCase()).orElse("NOT_FOUND");
}

If input is empty the method returns default value - "NOT_FOUND", otherwise capitalized part of the string is returned.

getValue(Optional.ofNullable(null));
$6 ==> "NOT_FOUND"

getValue(Optional.of("some long string"));
$7 ==> "SOME LO"

来源：https://stackoverflow.com/questions/49807204/conditional-extract-of-value-from-java-optional

标签

java

optional