Java8 streams: generic logger method in stream

Question

I want to create a method that prints out information within a stream. Please take a look at my existing logger method. The method arguments have to be generic like in my example string & integer. The method is supposed to return the original object, in my case string. Can someone tell me what's wrong with my current method?

Thanks a lot in advance!

look at the logger-method

Stream<String> stream = Stream.of("A", "BC", "XYZ");

stream.map(t -> logger(t.length()))
   .map(t-> logger(t.substring(0, 2)))
   .collection(Collectors.toList());

public static <T> T logger(T t) {
    System.out.println(t);
    return t;
  }

ERROR: Cannot infer type argument(s) for map(Function)

Answer 1

Why not use Stream#peek()?

This looks like an example of where you might want to use .peek() instead of a map. It's specifically built for printing out, and you don't need to worry about returning anything!

It might look more like:

stream.peek(it -> System.out.println(it.length))
    .peek(it -> System.out.println(it.substring(0, 2))
    // presumably other functions here before collecting...
    .collect(Collectors.toList());

Take a look at the official documentation here: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#peek-java.util.function.Consumer-

Back to the Original Question

To address your previous question, you probably want to update your function so that it takes a pure example of T, and a transform for T. You can update it to look like:

public static <T, R> T logger(T t, Function<T, R> transform) {
  R logLine = transform.apply(t);
  System.out.println(logLine);
  return t;
}

and your code will look like

stream.map(element -> logger(element, element::length)) // and so on

Answer 2

The problem I face here is that the first map is return an integer, so you are trying to use Integer.substring which does not exists.

What I recommend in your case is to use peek instead of map https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#peek-java.util.function.Consumer-

import java.util.stream.Collectors;
import java.util.stream.Stream;

public class InferType {
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("A", "BC", "XYZ");

        stream.peek(t -> logger(t.length()))
                .peek(t-> logger(t.substring(0, 2)))
                .collect(Collectors.toList());
    }

    public static <T> void logger(T t) {
        System.out.println(t);
    }

}

Answer 3

This works on my end

import java.util.stream.*;

class Main {
  public static void main(String[] args) {
    Stream<String> stream = Stream.of("AX", "BC", "XYZ");


    stream.map(t -> logger(t))
      .map(t-> logger(t.substring(0, 1)))
      .collect(Collectors.toList());

    /* or this
    stream.map(t -> logger(t))
      .collect(Collectors.toList());
    */

  }

public static <T> T logger(T t) {
    System.out.println(t);
    return t;
  }
}

Have a look at what a stream() map() really does here . But basically, it says

Stream map(Function mapper) returns a stream consisting of the results of applying the > given function to the elements of this stream.

edit: @Ezequiel made a good point about Integer.substring