Java 8流过滤:IN子句 [英] Java 8 stream filtering: IN clause

问题描述

List<Y> tmp= new DATA<Y>().findEntities();
List<X> tmp1 = new DATA<X>().findEntities().stream().filter(
                        IN (tmp) ???
                ).collect(Collectors.toList());

如何使用谓词模拟典型的IN子句(如mysql或JPA)?

How to simulate a tipical IN clause (like in mysql or JPA) using a Predicate ?

推荐答案

我决定将评论更新为答案.您请求的Predicate<Y>(其中Y应该是具体类型)的lambda表达式如下:

I decided to update my comment to an answer. The lambda expression for your requested Predicate<Y> (where Y should be a concrete type) looks as following:

element -> tmp.contains(element)

由于集合的contains方法与谓词的test方法具有相同的签名，因此可以使用方法引用(此处为实例方法引用):

Because the collection's contains method has the same signature as the predicate's test method, you can use a method reference (here an instance method reference):

tmp::contains

完整示例:

List<Number> tmp = Arrays.asList(1, 2, 3);
List<Integer> tmp1 = Arrays
    .stream(new Integer[] { 1, 2, 3, 4, 5 })
    .filter(tmp::contains)
    .collect(Collectors.toList());
System.out.println(tmp1);

此打印

[1, 2, 3]

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