Is there a way to combine Java8 Optional returning a value with printing a message on null?

问题

I'm starting with this code:

String startingValue = getMyValue();
String finishingValue = "";
if (startingValue != null) {
    finishingValue = startingValue;
} else {
    System.out.println("Value was null");
}

I want to transform it using Java 8 options to something like this:

Optional<String> startingOptional = getMyOptional();
String finishingValue =
        startingOptional
                .map(value -> value)
                .orElse(System.out.println("value not found"));

My question is: Is there a way to combine Java8 Optional returning a value with printing a message on null?

回答1:

Use orElseGet:

Optional<String> startingOptional = getMyOptional();
String finishingValue = startingOptional.orElseGet(() -> {
    System.out.println("value not found");
    return "";
});

Using .map(value -> value) is useless: transforming a value into itself doesn't change anything.

回答2:

If you are using Java 9 or later and you don't want to use default stub for empty value but instead handle empty and non-empty cases separately, you can also use Optional#ifPresentOrElse(Consumer, Runnable) method whose second argument is the handler for empty value. For your case it would look like:

Optional<String> startingOptional = getMyOptional();
finishingValue =
        startingOptional
                .map(value -> value)
                .ifPresentOrElse(
                        value -> {/* do something with your value */},
                        () -> System.out.println("value not found")
                );

or, ommiting the local variable:

getMyOptional()
        .map(value -> value)
        .ifPresentOrElse(
                value -> {/* do something with your value */},
                () -> System.out.println("value not found")
        );

来源：https://stackoverflow.com/questions/59517743/is-there-a-way-to-combine-java8-optional-returning-a-value-with-printing-a-messa

标签

java

java-8

null

optional