CODEWITHSUNDEEP
javajava-8java-streamcollectors
i_rezic
i_rezic

Reputation: 372

How to get Map<Integer,Integer> using Collectors.toMap?

I have List<StudentRecord> records containing StudentRecord instances.

public class StudentRecord {

private String lastName;
private String firstName;
private int mark;

//constructor + getters
}

How do I make Map<Integer,Integer> that as key has mark and as value, number of mark occurances in records list? Note: I have to use exactly this method toMap.

I have tried this myself: Map<Integer,Integer>mapaPoOcjenama2= records.stream() .collect(Collectors.toMap(StudentRecord::getMark, Collectors.counting(), mergeFunction));

But I am now sure how Collectors.counting() works and dont know what to write as merge function.

Upvotes: 4

Views: 2496

Answers (1)

Eugene
Eugene

Reputation: 120968

that's fairly easy to with toMap:

collect(Collectors.toMap(StudentRecord::getMark, 
        s -> 1, 
        (left, right) -> left + right));

The first argument is a Function that maps the Key in the map.

The second is a Function that maps the Value in the map. Since you need to count them, it will always return 1.

And the third is a BiFunction that says how to merge two keys in case they are the same. Since you want to count, you will increment by one all the time.

Upvotes: 4

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