java 如何否定 lambda 谓词？

How can I negate a lambda Predicate?

提问by Jin Kwon

Lets say I have a Stream of Strings.

假设我有一个字符串流。

final Stream<String> stream = ...;

I want to filter out each empty string after trimmed.

我想在修剪后过滤掉每个空字符串。

stream
    .filter(Objects::nonNull)
    .map(String::trim)
    .filter(v -> !v.isEmpty());

Is there any way to apply Predicate#negate()for replacing v -> !v.isEmpty()part?

有没有办法应用Predicate#negate()来替换v -> !v.isEmpty()部分？

.filter(((Predicate) String::isEmpty).negate()) // not compile

回答by Misha

You would have to do .filter(((Predicate<String>) String::isEmpty).negate())

你必须做 .filter(((Predicate<String>) String::isEmpty).negate())

If you want, you can define

如果你愿意，你可以定义

static<T> Predicate<T> not(Predicate<T> p) {
    return t -> !p.test(t);
}

and then

接着

.filter(not(String::isEmpty))

but I would just stick with v -> !v.isEmpty()

但我会坚持下去 v -> !v.isEmpty()

回答by tobias_k

It seems like you have to cast to Predicate<String>for this to work:

似乎你必须投射到Predicate<String>这个才能工作：

.filter(((Predicate<String>) (String::isEmpty)).negate())

Of course, it would be much shorter to use a lambda in this case:

当然，在这种情况下使用 lambda 会更短：

.filter(s -> ! s.isEmpty())

回答by Cherry

Instead of creating classes by yourself use durianutility library. It has a lot of methods: and, orand notof course:

而不是自己创建类使用榴莲实用程序库。它有很多的方法：and，or和not当然的：

import static com.diffplug.common.base.Predicates.not;

stream
.filter(Objects::nonNull)
.map(String::trim)
.filter(not(String::isEmpty));