[英]Externally configurable filtering using Java lambdas
Lets say I have an external JSON as: 假设我有一个外部JSON:
[{
"condition": "equals",
"field": "name",
"value": "jack"
},
{
"condition": "greater",
"field": "age",
"value": 25
}
]
What this means is I want to check, in a list of Person objects for people with name as "jack"
and age > 25
. 这意味着我想在名为
"jack"
且age > 25
人的Person对象列表中进行检查。 It is fairly simple with Java 8 filtering(example shown is filtering just on name). 使用Java 8过滤相当简单(显示的示例仅在名称上过滤)。
However, I want to make this filter configurable and also apply multiple filters. 但是,我想使此过滤器可配置,并且还应用多个过滤器。 Assuming the Person pojo below (which is self-explanatory), takes name and age, how can I make the filters dynamic or configurable?
假设下面的Person pojo(这是不言自明的),取名字和年龄,我怎样才能使过滤器动态或可配置?
public class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
// Accessors and Mutators
}
List<Person> persons = Arrays.asList(
new Person("mkyong", 30),
new Person("jack", 20),
new Person("lawrence", 40)
);
Person result1 = persons.stream() // Convert to steam
.filter(x -> "jack".equals(x.getName())) // we want "jack" only
.findAny() // If 'findAny' then return found
.orElse(null);
I am expecting a list of Person object which satisfy the criterion as a result. 我期待一个满足条件的Person对象列表。
That has little to do with streams
per-se as far as I can tell. 据我所知,这与
streams
本身没什么关系。 That filter method is just a Predicate
which can be supplied as an instance, for example from a factory method that you could create. 该过滤方法只是一个
Predicate
,可以作为实例提供,例如从您可以创建的工厂方法。
static Predicate<Person> fromName(String value){
return p -> p.getName().equals(value);
}
Suppose you have another method: 假设你有另一种方法:
static Predicate<Person> fromAge(int value) {
return p -> p.getAge() == value;
}
Because Predicate.and
, Predicate.or
exists you could do: 因为
Predicate.and
, Predicate.or
存在,你可以这样做:
fromAge(12).and(fromName("jack"));
thus chaining your predicates 从而链接你的谓词
There are two approaches to this. 这有两种方法。
Either you just chain call the desired filters inline: 您可以直接串联调用所需的过滤器:
Person result1 = persons.stream()
.filter(x -> "jack".equals(x.getName()))
.filter(x -> 25 > x.getAge()) // new filter
.findAny()
.orElse(null);
Or, you have your own class implementing Predicate<Person>
. 或者,您有自己的类实现
Predicate<Person>
。
In which case, you may use a builder pattern to build your predicate step-by-step. 在这种情况下,您可以使用构建器模式逐步构建谓词。
The difficulties you'll have to face are as follows: 您将面临的困难如下:
&&
vs ||
) &&
vs ||
) (a && b) || c
is not a && (b || c)
) (a && b) || c
不是a && (b || c)
) Note that Predicate
has handy and
and or
methods to chain predicates. 请注意,
Predicate
具有链接谓词的方便and
/ or
方法。
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