java - Guava:将 List> 转换为 List 只保留当前值

Question

是否有使用 Guava 将可选列表转换为现值列表的优雅方法？

例如，从

ImmutableList.of(
    Optional.of("Tom"), Optional.<String>absent(), Optional.of("Dick"),
    Optional.of("Harry"), Optional.<String>absent()
)

一个只包含

的列表

["Tom", "Dick", "Harry"]

一种方法是:

List<T> filterPresent(List<Optional<T>> inputs) {
    return FluentIterable.from(inputs)
            .filter(new Predicate<Optional<T>>() {
                @Override
                public boolean apply(Optional<T> optional) {
                    return optional.isPresent();
                }
            }).transform(new Function<Optional<T>, T>() {
                @Override
                public T apply(Optional<T> optional) {
                    return optional.get();
                }
            }).toList();
}

但这很冗长。

不幸的是，Java 8 不是一个选项。

Answer 1

Guava 中实际上有内置方法:presentInstances in Optional :

Returns the value of each present instance from the supplied optionals, in order, skipping over occurrences of absent(). Iterators are unmodifiable and are evaluated lazily.

例子:

List<Optional<String>> optionalNames = ImmutableList.of(
    Optional.of("Tom"), Optional.<String>absent(), Optional.of("Dick"),
    Optional.of("Harry"), Optional.<String>absent());

Iterable<String> presentNames = Optional.presentInstances(optionalNames); // lazy

// copy to List if needed
List<String> presentNamesList = ImmutableList.copyOf(presentNames);
System.out.println(presentNamesList); // ["Tom", "Dick", "Harry"]