Java stream 中的计数和打印值

Question

I am writing a code, that runs a collection through a stream, and in the stream it filters the data with several filter functions, and then prints the result of filtering:

List<String> someList = Arrays.asList("string1", "string2", etc.);

someList.stream().filter(s -> predicate1()).filter(s -> predicate2()).forEach(System.out::println);

and what would greatly help me, would be the ability to print and count filtered elements. Since forEach() uses consumer function, and returns Void type, I cannot do anything further to the data. But if I change forEach() with count() , I receive a long value, and stream is ended this way or another. Is there any elegant way to have something like:

long counted = someList.stream().filter(s -> predicate1()).filter(s -> predicate2()).map??reduce??(printAndAlsoCountPrintedAndReturnFinalCountValue())

in one stream without having to run it 2 times, first with count() , and the second with forEach(System.out::println) ? I was thinking about some mapper function, but map() also returns Stream data, so even if I did some mapping printing-counting function, I don't see how I'd have to return counted value into a parameter.

Generally it's not a problem to run the collection through 2 streams, but I believe it's not very resource-wise, depending on how heavy the filtering functions would be, that's why I am looking for more "clever" solution, if it exists.

Thank you in advance.

Answer 1

You can use reduce function:

import java.util.Arrays;
class HelloWorld {
    public static void main(String[] args) {
        var l = Arrays.asList(1,2,3,4,5,6,7,8);
        var count= l.stream()
                    .filter(x -> x%2 == 0)
                    .reduce(0, (acc, elem) -> {
                        System.out.println(elem);
                        return acc + 1; 
                    });
        System.out.println("Total Count: " + count);
    }
}

This will print “Total Count: 4” as well as the elements themselves (2,4,6,8)

Note that this printing is a side effect so personally I would also consider writing this “as usual” with for-loops (in an imperative style) without streams at all.

Answer 2

The problem as it stated now is contradiction with the Single responsibility principle , which the first principle of SOLID.

If you aim to write clean a maintainable code, you can generate a collection as a result. And then you can do whatever you need with its contents and find out its size as well.

public List<String> foo(List<String> source) {
    
    return source.stream()
        .filter(s -> predicate1())
        .filter(s -> predicate2())
        .toList(); // for Java 16+ or collect(Collectors.toList());
}

If you need to see all the elements that passed all the filters in the pipeline on the console, you can use peek() . This method was introduces in the Stream API for debugging.

long counted = someList.stream()
    .filter(s -> predicate1())
    .filter(s -> predicate2())
    .peek(System.out::println)
    .count();

Note: that this method should be used only for debugging purposes. If let's say instead of printing you would decide to store these elements into a collection, then you need to be aware that it's discouraged by API the documentation .

And, also, pay attention to the documentation of peek()

This method exists mainly to support debugging , where you want to see the elements as they flow past a certain point in a pipeline

In cases where the stream implementation is able to optimize away the production of some or all the elements (such as with short-circuiting operations like findFirst , or in the example described in count() ), the action will not be invoked for those elements .

That means when have in mind something more important, than printing elements on the console, then you have to stick with the first approach.

Answer 3

It's not really intended to be used for that, but you could use an AtomicLong .

    List<String> someList  = Arrays.asList("string1", "string2", "s1", "s2");
    AtomicLong count = new AtomicLong();
    someList .stream()
            .filter(s -> s.contains("string"))
            .forEach(s -> {
                System.out.println(s);
                count.incrementAndGet();
            });
    System.out.println(count.get());