How to check hashset for an existing element and then adding a count

Question

I'm a beginner in Java and is starting to learn how to use hashsets. I have a problem with my practice exercise. The goal is to remove the duplicate description and then adding the count of similar descriptions.

For example I have,

Computer Science - 15
Engineering - 20
Dentistry - 10
Architecture - 11
Computer Science - 25
Dentistry - 7

then the output should only be:

Computer Science - 40
Engineering - 20
Dentistry - 17
Architecture - 11

import java.util.ArrayList;
import java.util.Date;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.Arrays;

public class Main {
  public static void main(String[] args) {
    //printing summary report
    ArrayList<String>   summaryReport=  new ArrayList<String>();

    final String[] CourseDesc           = {"Computer Science", "Architecture", "Dentistry", "Computer Science"};
    final int[] CourseCount         = {15, 10, 10, 25};

    for (String element: CourseDesc) {
        for(int el: CourseCount){
             summaryReport.add(element + " "+ el);
        }
     }
   System.out.println(summaryReport);
  }
}

I'm just stuck on how to add the count of similar descriptions. Thanks!

But you haven't written "thousands lines" have you? Please share the code you have written. It's hard to see why you need a HashSet here, a HashMap looks like a much more suitable solution for the problem. — Joakim Danielson, Mar 05 '19 at 17:09
You should share an working example oft your issue, so wie can reproduce it. — Samuel Philipp, Mar 05 '19 at 17:12
I already copied a part of the full code that I have an issue on — Xiaoren, Mar 05 '19 at 17:54
How is your current code relevant to HashSet and to the problem description? Now it is even more confusing what you are asking about. — Joakim Danielson, Mar 05 '19 at 19:42

score 2 · Answer 1 · answered Mar 05 '19 at 20:00

A smart way to solve you problem would be to use Java Streams:

final String[] courseDesc = {"Computer Science", "Engineering", "Dentistry", "Architecture", "Computer Science", "Dentistry"};
final int[] courseCount = {15, 20, 10, 11, 25, 7};

Map<String, Integer> summaryReport = IntStream.range(0, courseDesc.length).boxed()
    .collect(Collectors.toMap(
        index -> courseDesc[index], index -> courseCount[index],
        (count0, count1) -> count0 + count1, LinkedHashMap::new
    ));

This creates a Map with the course as key and the sum as value. If the map contains an course and the same course is collected again the lambda expression (count0, count1) -> count0 + count1 handles it by summing the values. The LinkedHashMap::new is to maintain the order.

Now you can print the summary like this:

summaryReport.forEach((course, count) -> System.out.println(course + " - " + count));

Result:

Computer Science - 40
Engineering - 20
Dentistry - 17
Architecture - 11

J-Alex · Answer 2 · 2019-03-05T18:37:41.663

From OP I can see that you need an association key-value container, so the most obvious way will be to use Map, check if the value present as a key, if not - add it with 1 counter, if present - add + 1:

void test() {
    Map<String, Integer> summaryReport = new HashMap<>();
    String curDesc = "whatever";

    Integer count = summaryReport.get(curDesc);
    if (count == null) {
        summaryReport.put(curDesc, 1);
    } else {
        summaryReport.put(curDesc, count + 1);
    }
}

Doing in more tricky way, you can use mutable AtomicInteger as a counter:

void test() {
    Map<String, AtomicInteger> summaryReport = new HashMap<>();
    AtomicInteger counter = summaryReport.putIfAbsent("whatever", new AtomicInteger(0));
    if (counter != null) {
        counter.incrementAndGet();
    }
}

How to check hashset for an existing element and then adding a count

2 Answers2