How to increment a "number" in a Java 8 lambda expression in a loop?

Question

I have the following problem. I have an integer position which starts at 1 and increments every time a specific person from a txt is found at a specific position in an xml. If I use the classic iteration with the foreach for (PersonMatchInfo pmi : personMatchInfo) it works, but my senior asked me to do with the Java 8 foreach and this type of iteration works only with final variables. How can I increment the integer in the new Java 8 loop? Thank you.

int position = 1;
personMatchInfo.forEach(pmi ->{

                    if (!stopwatch1.isStarted()) {
                        stopwatch1.start();
                    } else if (stopwatch1.isStarted()) {

                    }

                    if (pmi.getPersonName().equals(e.getValue())) {

                        positionMap.put(position, positionMap.get(position) + 1);
                        break;

                    } else {

                        position++;
                    }
                });

by telling your senior: "that is not a reasonable request, it needlessly takes up time, is more complex and it is less readable. And I do not know how to do it because final blah blah blah, can you please help me!?" — luk2302
– luk2302, Commented Oct 2, 2017 at 8:41
(It would not reduce the time consumed by the code - it would not perform better than a classical foreach loop.) — Jolta
– Jolta, Commented Oct 2, 2017 at 8:44
Ask your senior if he is familiar with the term "premature optimization". — Fildor
– Fildor, Commented Oct 2, 2017 at 8:45
@Bilkokuya I think we could even find some more if we tried. — Fildor
– Fildor, Commented Oct 2, 2017 at 8:46

b4hand · Accepted Answer · 2021-09-28 18:05:01Z

20

You can use AtomicInteger, and incrementAndGet method on it.

Other solution would be int[] position = new int[]{1};

and incrementing position[0]++;

edited Sep 28, 2021 at 18:05

b4hand

9,8004 gold badges46 silver badges49 bronze badges

answered Oct 2, 2017 at 8:42

mlecz

1,02612 silver badges21 bronze badges

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6 Comments

Tayfun Omer Over a year ago

It works with the array but it is reducing the runtime whatsoever. Still the same. :(

mlecz Over a year ago

what do you mean by reducing the runtime? You will not gain performance by switching from for_each loop, to collection.forEach

Tayfun Omer Over a year ago

Right now i am trying to reduce the duration of the run in my code. My senior gave me the "hint" to use lambda foreachs because they are java 8 based and stuff.

mlecz Over a year ago

awful hint. Try sampling/profiling your code with jvisualvm

Vijay Nandwana Over a year ago

org.apache.commons.lang3.mutable.MutableInt is more appropriate.

|

Nicolas · Accepted Answer · 2017-10-02 15:39:49Z

2

You can use a static variable :

public class Poubelle {

    private static int position = 1;

    public static void setPosition (List<PersonMatchInfo> listPersonMatchInfo) {

          listPersonMatchInfo.forEach(pmi -> {
          pmi.setPosition(position++);
        });
    }
}

answered Oct 2, 2017 at 15:39

Nicolas

693 bronze badges

1 Comment

René Kåbis Over a year ago

Ironically, using a static int is what worked for me. I had a pair of nested lambda functions, and I needed to pass on the count of the inner lambda function into its contents. Since the outer lambda function needed to re-set the variable to zero every time it ticked over, I just did a position = 0; (to use your example) before the inner lambda function. Haven’t yet tested, but intellisense is not having a hissy fit.

Mark_ZuckerBerg · Accepted Answer · 2017-10-02 08:44:11Z

0

I think you senior is asking you to use streams for collections. Then use collection.stream.filter.count based on your logic that will produce a long as the value and use it.

answered Oct 2, 2017 at 8:44

Mark_ZuckerBerg

113 bronze badges

3 Comments

luk2302 Over a year ago

no the downvoter, but that sounds like an utter abuse of the filter method and only works by coincidence.

Mark_ZuckerBerg Over a year ago

Thanks bro for your answer

nuvio Over a year ago

I was initially thinking the same as Mark, and I think map filter count it's a perfectly valid case for counting instead of modifying a global state counter although I read the question again and I don't think a count operation is needed here as this is building a map with positions.

Pranay Nailwal · Accepted Answer · 2020-08-26 02:06:43Z

I know it is late, so I will just put it out here for future seekers. Here is how you can do it in a very simple way :

use indexOf(Object O) property of collections

StringBuffer messageBody = new StringBuffer();    
List<VipObj> listOfAllVips = getResponse();
    listOfAllVipsLocal.forEach(vip -> {
                            messageBody.append("\n\t" + (listOfAllVipsLocal.indexOf(vip) + 1) + ". " + vip.getBaseUrl() + "\n");
                        });

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