你不能在ifPresent之后调用orElse，原因是，orElse是在可选对象上调用的，而ifPresent返回void。所以实现的最好方法是ifpresenttorelse。 它可以是这样的:

op.ifPresentOrElse( 
            (value) 
                -> { System.out.println( 
                         "Value is present, its: "
                         + value); }, 
            () 
                -> { System.out.println( 
                         "Value is empty"); }); 

如果你想存储这个值:

Pair.of<List<>, List<>> output = opt.map(details -> Pair.of(details.a, details.b))).orElseGet(() -> Pair.of(Collections.emptyList(), Collections.emptyList()));

另一个解决方案是:

下面是它的用法:

    final Opt<String> opt = Opt.of("I'm a cool text");
    opt.ifPresent()
        .apply(s -> System.out.printf("Text is: %s\n", s))
        .elseApply(() -> System.out.println("no text available"));

或者如果你。如果相反的用例是真的:

    final Opt<String> opt = Opt.of("This is the text");
    opt.ifNotPresent()
        .apply(() -> System.out.println("Not present"))
        .elseApply(t -> /*do something here*/);

这些是成分:

稍微修改了函数界面，只是为了“elseApply”方法 可选增强 一点点卷曲:-)

“美化”增强的功能界面。

@FunctionalInterface
public interface Fkt<T, R> extends Function<T, R> {

    default R elseApply(final T t) {
        return this.apply(t);
    }

}

和用于增强的Optional包装器类:

public class Opt<T> {

    private final Optional<T> optional;

    private Opt(final Optional<T> theOptional) {
        this.optional = theOptional;
    }
    
    public static <T> Opt<T> of(final T value) {
        return new Opt<>(Optional.of(value));
    }

    public static <T> Opt<T> of(final Optional<T> optional) {
        return new Opt<>(optional);
    }
    
    public static <T> Opt<T> ofNullable(final T value) {
        return new Opt<>(Optional.ofNullable(value));
    }
    
    public static <T> Opt<T> empty() {
        return new Opt<>(Optional.empty());
    }

    private final BiFunction<Consumer<T>, Runnable, Void> ifPresent = (present, notPresent) -> {
        if (this.optional.isPresent()) {
            present.accept(this.optional.get());
        } else {
            notPresent.run();
        }
        return null;
    };

   private final BiFunction<Runnable, Consumer<T>, Void> ifNotPresent = (notPresent, present) -> {
        if (!this.optional.isPresent()) {
            notPresent.run();
        } else {
            present.accept(this.optional.get());
        }
        return null;
    };

    public Fkt<Consumer<T>, Fkt<Runnable, Void>> ifPresent() {
        return Opt.curry(this.ifPresent);
    }

    public Fkt<Runnable, Fkt<Consumer<T>, Void>> ifNotPresent() {
        return Opt.curry(this.ifNotPresent);
    }

    private static <X, Y, Z> Fkt<X, Fkt<Y, Z>> curry(final BiFunction<X, Y, Z> function) {
        return (final X x) -> (final Y y) -> function.apply(x, y);
    }
}

这应该可以解决问题，并且可以作为处理此类需求的基本模板。

The basic idea here is following. In a non functional style programming world you would probably implement a method taking two parameter where the first is a kind of runnable code which should be executed in case the value is available and the other parameter is the runnable code which should be run in case the value is not available. For the sake of better readability, you can use curring to split the function of two parameter in two functions of one parameter each. This is what I basically did here.

提示:Opt还提供了另一个用例，在该用例中，您希望执行一段代码，以防该值不可用。这也可以通过Optional.filter.stuff完成，但我发现这更可读。

希望有帮助!

额外的信息:

还有另一种表达if then else的方法:

public static <X, Y> Function<Predicate<X>, Function<Function<X, Y>, Function<Function<X, Y>, Y>>> ifThenElse(X input) {
  return (final Predicate<X> pred) -> (final Function<X, Y> ifPresent) -> (final Function<X, Y> ifNotPresent) -> pred.test(input) ? ifPresent.apply(input) : ifNotPresent.apply(input);
}

这样你就可以说:

final String result = ifThenElse("fancy")
  .apply(input -> input.contains("fancy")) /* test      */
  .apply(input -> input.toUpperCase())     /* if-case   */
  .apply(input -> input.toLowerCase());    /* else-case */

如果你只能使用Java 8或更低版本:

1)如果你没有弹簧数据，到目前为止最好的方法是:

opt.<Runnable>map(param -> () -> System.out.println(param))
      .orElse(() -> System.out.println("no-param-specified"))
      .run();

现在我知道对某些人来说，它不是那么可读，甚至很难理解，但对我个人来说，它看起来还不错，我想不出另一种流畅的方法来处理这种情况。

2)如果你足够幸运，你可以使用弹簧数据，最好的方法是 可选# ifPresentOrElse:

Optionals.ifPresentOrElse(opt, System.out::println,
      () -> System.out.println("no-param-specified"));

如果你能使用Java 9，你绝对应该使用:

opt.ifPresentOrElse(System.out::println,
      () -> System.out.println("no-param-specified"));

如果你使用Java 9+，你可以使用ifPresentOrElse()方法:

opt.ifPresentOrElse(
   value -> System.out.println("Found: " + value),
   () -> System.out.println("Not found")
);

另一种解决方案是使用如下所示的高阶函数

opt.<Runnable>map(value -> () -> System.out.println("Found " + value))
   .orElse(() -> System.out.println("Not Found"))
   .run();