Java 8 collect() 仅 isPresent() 可选值
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Java 8 collect() only isPresent() Optional values
提问by ?????????s
Is there a more elegant way of practically achieving this in Java 8?
在 Java 8 中是否有更优雅的方法来实际实现这一点？
list.stream()
.map(e -> myclass.returnsOptional(e))
.filter(Optional::isPresent)
.map(Optional::get)
.collect(Collectors.toList());
I'm talking about filter(Optional::isPresent)followed by map(Optional::get), I want to elegantly collect in a list only Optionalresults which have a value.
我说的是filter(Optional::isPresent)后跟map(Optional::get)，我想优雅地将仅Optional具有值的结果收集在列表中。
回答by Amin J
Not sure if its so different but you could just filter based on your optional instead of getting the optional and filtering next. Something like this?
不确定它是否如此不同，但您可以根据您的可选项进行过滤，而不是接下来获取可选项和过滤。像这样的东西？
list.stream()
.filter(e -> myclass.returnsOptional(e).isPresent())
.collect(Collectors.toList());
Note: This will only work if returnsOptional returns the same object type as your original list item types.
注意：这仅在returnsOptional 返回与原始列表项类型相同的对象类型时才有效。
回答by eGoLai
In your case you can use one flatMapinstead of combinations of mapfilterand again map.
To Do that it's better to define a separate function for creating a Stream: public private static Stream<Integer> createStream(String e)to not have several lines of code in lambda expression.
你的情况，你可以使用一个flatMap替代的组合mapfilter和再次map。要做到这一点，最好定义一个单独的函数来创建一个流：public private static Stream<Integer> createStream(String e)在 lambda 表达式中不要有几行代码。
Please see my full Demo example:
请参阅我的完整演示示例：
public class Demo{
public static void main(String[] args) {
List<String> list = Arrays.asList("1", "2", "Hi Stack!", "not", "5");
List<Integer> newList = list.stream()
.flatMap(Demo::createStream)
.collect(Collectors.toList());
System.out.println(newList);
}
public static Stream<Integer> createStream(String e) {
Optional<Integer> opt = MyClass.returnsOptional(e);
return opt.isPresent() ? Stream.of(opt.get()) : Stream.empty();
}
}
class MyClass {
public static Optional<Integer> returnsOptional(String e) {
try {
return Optional.of(Integer.valueOf(e));
} catch (NumberFormatException ex) {
return Optional.empty();
}
}
}
in case returnsOptional cannot be static you will need to use "arrow" expression instead of "method reference"
如果returnsOptional不能是静态的，您将需要使用“箭头”表达式而不是“方法引用”
