Java 8 Lambda Expressions

Question

I've been stuck with one lambda expression and the Comparator class using Comparator.comparing(...).thenComparing(...) methods to sum up two way of sorting a Stream.

Both of my methods are working, but when I put them together nothing is working at all.

Here is the link if you want to try and validate the exercise:

http://codecheck.it/codecheck/files?repo=heigvdcs1&problem=poo3e

And here is what you have to do:

For each word in a stream, determine the “vowelness”, i.e. the number of vowels - the number of consonants. Produce the n words with the highest vowelness paired with the vowelness value. Sort first by vowelness, then by the string. Complete this program. This time, you have a hidden static method long Words.vowels(String w) at your disposal that yields the number of vowels in w, including duplicates.

For now i have managed to do this:

import java.util.*;
import java.util.stream.*;

public class Streams
{
   List<Pair<String, Long>> wordsWithManyVowels(Stream<String> words, int n)
   {
      return words
         .map( w -> Pair.of( w , ( Words.vowels(w) - ( w.length() - Words.vowels(w)))))
         .sorted(Comparator.comparingLong(f1 -> -f1.second())
         //This part is working without the first comparing
         //.thenComparing(f2 -> f2.first().length()))
         .limit(n)
         .collect(Collectors.toList());
   }
}

The Pair class:

import java.util.Objects;

public class Pair<T, S> 
{
   private T first;
   private S second;

   public Pair(T firstElement, S secondElement)
   {
      first = firstElement;
      second = secondElement;
   }

   /*
      Use Pair.of(x, y) instead of new Pair<...,...>(x, y)
      so you get the type inferred
   */

   public static <T, S> Pair<T, S> of(T firstElement, S secondElement)
   {
      return new Pair<T, S>(firstElement, secondElement);
   }

   public T first() { return first; }
   public S second() { return second; }

   public String toString() { return "(" + first + "," + second + ")"; }

   public boolean equals(Object otherObject)
   {
      if (this == otherObject) 
         return true;
      if (otherObject == null || !(otherObject instanceof Pair)) 
         return false;
      @SuppressWarnings("unchecked") Pair<T, S> other = (Pair<T, S>) otherObject;
      return Objects.equals(first, other.first) &&
         Objects.equals(second, other.second);
   }
}

What does "nothing is working at all" mean? What happens? Exceptions, compile errors, wrong sort order? — Malte Hartwig
– Malte Hartwig, Commented Jun 14, 2017 at 14:28
Your f2 -> f2.first().length()) function is an int extractor, thus shouldn't the method be : thenComparingInt ? — Jeremy Grand
– Jeremy Grand, Commented Jun 14, 2017 at 14:42
@MalteHartwig It means that method are not working togather. Although the are working separately — 1020rpz
– 1020rpz, Commented Jun 14, 2017 at 15:41
Just a side note: don’t use negation to reverse the order. In case of Long.MIN_VALUE, it doesn’t work due to overflow, the same applies to the minimum values of the other integer types. It’s tempting to say that this will never happen for a particular property, but it will cost you a lot of time once this assumption doesn’t hold anymore and you don’t remember that you made this assumption somewhere in your code. You may just use Comparator.comparingLong(Pair<String, Long>::second) .reversed() .thenComparingInt(p -> p.first().length()) — Holger
– Holger, Commented Jun 14, 2017 at 16:26

Andriy Kryvtsun · Accepted Answer · 2017-06-14 15:11:34Z

3

Solution 1

Extracting comparing methods as static works like this

public class Streams {
    List<Pair<String, Long>> wordsWithManyVowels(Stream<String> words, int n) {
        return words
            .map(w -> Pair.of(w, (Words.vowels(w) - (w.length() - Words.vowels(w)))))
            .sorted(Comparator.comparingLong(Streams::vowelness).thenComparingInt(Streams::length))
            .limit(n)
            .collect(Collectors.toList());
    }

    static int length(Pair<String, Long> p) {
        return p.first().length();
    }

    static long vowelness(Pair<String, Long> p) {
        return -p.second();
    }
}

Solution 2

Use this Comparator implementation without static methods

Comparator
    .comparingLong((Pair<String, Long> p) -> -p.second())
    .thenComparingInt((Pair<String, Long> p) -> p.first().length())

Note: see how thenComparingInt is used at the end of the composite comparator in the both solutions.

Solution 3

The problem of the source code is static Comparator methods need info about processing elements' type in a chain. By default Object type is used. So, it's possible to specify it this way (simpler than in Solution 2):

Comparator.<Pair<String, Long>>comparingLong(p -> -p.second()).thenComparing(p -> p.first().length())

edited Jun 14, 2017 at 15:11

answered Jun 14, 2017 at 14:38

Andriy Kryvtsun

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5 Comments

Malte Hartwig Over a year ago

Andriy, could you explain what exactly the error was in Thomas' code?

Andriy Kryvtsun Over a year ago

@MalteHartwig pls, see Solution 3: here is compilation error as compiler can't understand needed Pair type in chained static methods.

1020rpz Over a year ago

@AndriyKryvtsun Thank you for your help, I'm understanding better now. But you solutions are working find although they are sorting string by length and i need them to be alphabeticaly sorted. That was, why i wasn't using a thenCampingInt(). But thank you i will find how to do it from that point

Andriy Kryvtsun Over a year ago

@ThomasL if you need to compare strings in alphabetical order just use this clause .thenComparing(Pair::first). So the whole comparator can looks like this Comparator.<Pair<String, Long>>comparingLong(Pair::second).reversed().thenComparing(Pair::first)

1020rpz Over a year ago

@AndriyKryvtsun Thanks i do it like this: Comparator.comparingLong((Pair<String, Long> p) -> -p.second()) .thenComparing((Pair<String, Long> p) -> p.first())

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