Java stream assign filter predicate from element method

Question

I am trying to use an object's method in a Java stream to set the filter. For example:

//Full of Test objects that have a method getPredicate() that returns a valid predicate (ex d -> d.getName.equals("Test")). 
 ArrayList<Test> testArray = new ArrayList<Test>(); 

testArray
 .stream()  
 .filter(*CURRENTELEMENT*.getPredicate())  //Goal
 .forEach(System.out::println);

The return lambda from getPredicate() could be anything as the goal of this is to have a dynamic filter that can be set by the Test objects in this case.

Thanks in advance for any help! This is my first post so I hope I explained myself ok.

Edit/Update: Here is what the getPredicate() method looks like for a Test object:

    public Predicate<Test> getPredicate(String name, String id) {

    List<Predicate<Test>> allFilters = Arrays.asList();
    Predicate<Test> aggregateFilters;

        allFilters.add(d -> d.getName().equals(name));
        allFilters.add(d -> d.getId().equals(id));

        //Chain all filter predicates together using "or" method. 
            aggregateFilters= allFilters
                    .stream()
                    .reduce(d -> false, Predicate::or);

            // Returns a valid filter lambda expression
            // If I wasn't trying to get the aggregateFilters variable from this method, I could 
            // statically assign it and plug it right in and it works. ex. .filter(aggregateFilters)
            return aggregateFilters; 
}

Thanks again for your time and help.

What does getPredicate() return? It sounds like you just want e -> e.getPredicate().test(e). — Louis Wasserman
– Louis Wasserman, Commented Apr 3, 2018 at 20:57
It’s a bit odd for an instance to return a predicate; it may be simpler to have a method that returns boolean and use a method reference — Bohemian
– Bohemian ♦, Commented Apr 3, 2018 at 21:16
you need to provide more details here, like how does Test look like at least and why do you think you need this dynamic setting of a Predicate. this might be an interesting question, but its just poorly written. — Eugene
– Eugene, Commented Apr 4, 2018 at 6:57
The return lambda from getPredicate() could be anything. No it can't. It can only have a type Predicate if you want to use that return value as a predicate in the filter above. Furthermore, since each Test object is returning a predicate, then each time that object is tested against the predicate, it will always be true...wat? — smac89
– smac89, Commented Apr 4, 2018 at 15:43

y.luis.rojo · Accepted Answer · 2018-04-04 18:29:36Z

1

You can do:

testArray.stream().filter(e -> e.getPredicate("some_name", "some_id").test(e)).forEach(System.out::println);

where Test is defined as in your question.

Why I do not see is the point of getPredicate method to be instance member of Test class since it does not depends on any instance member. Instead you should consider it as a static method, that way you will have:

testArray.stream().filter(e -> Test.getPredicate("some_name", "some_id").test(e)).forEach(System.out::println);

Note: a list initialized as Arrays.asList() will throw a UnsupportedOperationException in add method since it is an immutable object. Instead you can initialize it as new ArrayList<>().

edited Apr 4, 2018 at 18:29

answered Apr 3, 2018 at 21:48

y.luis.rojo

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3 Comments

Ousmane D. Over a year ago

maybe you didn't see it but the OP did put "Test objects that have a method getPredicate() that returns a valid predicate (ex d -> d.getName.equals("Test"))." sounds like getPredicate returns a function which when evaluated will return a boolean as opposed to it being boolean getPredicate().

Eugene Over a year ago

@Aominè it seems that the goal here is to somehow distinguish lambda expressions, which is simply not possible

Ousmane D. Over a year ago

@Eugene Right, I see. completely agree with your comment under the question.

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