如何通过Java 8谓词仅过滤特定元素? [英] How to filter only specific elements by Java 8 Predicate?

问题描述

我有Foo个元素的集合List<Foo>:

class Foo {
  private TypeEnum type;
  private int amount;

  //getters, setters ...
}

Foo类型可以是TypeEnum.A和TypeEnum.B.

我只想从列表中获取那些Foo元素，如果该元素具有type == TypeEnum.B，则amount大于零(amount > 0).

I would like to get only those Foo elements from list which if the element have type == TypeEnum.B then amount is greater than zero (amount > 0).

如何通过Java 8 Streams filter()方法做到这一点?

How can I do it by Java 8 Streams filter() method?

如果我使用:

List<Foo> l = list.stream()
    .filter(i -> i.getType().equals(TypeEnum.B) && i.getAmount() > 0)
    .collect(Collectors.<Foo>toList());

我得到具有TypeEnum.B但没有TypeEnum.A的Foo元素.

I get Foo elements with TypeEnum.B but without TypeEnum.A.

推荐答案

尝试如下操作:

List<Foo> l = list.stream()
        .filter(i -> i.getType().equals(TypeEnum.B) ? i.getAmount() > 0 : true)
        .collect(Collectors.<Foo>toList());

仅在type等于TypeEnum.B时才检查i.getAmount() > 0.

在上一次尝试中，只有当type为TypeEnum.B并且amount大于0时，您的谓词才为true.这就是为什么您仅得到TypeEnum.B的原因.

In your previous attempt your predicate was true only if type was TypeEnum.B and amount was greater than 0 - that's why you got only TypeEnum.B in return.

您还可以在评论部分中查看 Holger (与他分享一些积分)提出的建议，使用更短的表达式:

you can also check a suggestion made by Holger (share some credits with him) in the comments section and use even shorter version of the expression:

!i.getType().equals(TypeEnum.B) || i.getAmount()>0

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