Question

How to convert a list of objects to a Map in Java 8 by handling duplicate keys?

Answer 1

public class Notes {

    private int id;
    private String tagName;
    private long tagId;

    public Notes(int id, String tagName, long tagId) {
        Id = id;
        this.tagName = tagName;
        this.tagId = tagId;
    }

   // getters and setters
}public class TestNotes {

    public static void main(String[] args) {

        List<Notes> noteLst = new ArrayList<>();
        noteLst.add(new Notes(1, "note1", 11));
        noteLst.add(new Notes(2, "note2", 22));
        noteLst.add(new Notes(3, "note3", 33));
        noteLst.add(new Notes(4, "note4", 44));
        noteLst.add(new Notes(5, "note5", 55));

        noteLst.add(new Notes(6, "note4", 66));

//use third mergeFunction argument (oldValue, newValue) -> oldValue solved the duplicated key issue by considering old value
        Map<String, Long> notesRecords = noteLst.stream().collect(
                Collectors.toMap(Notes::getTagName, Notes::getTagId,
                        (oldValue, newValue) -> oldValue
                )
        );

        System.out.println("Notes : " + notesRecords);
    }
}

//Output - for (oldValue, newValue) -> oldValue, it took old value note4=44
Notes : {note1=11, note2=22, note3=33, note4=44, note5=55}

//Output - for (oldValue, newValue) -> newValue, it took new value note4=66
Notes : {note1=11, note2=22, note3=33, note4=66, note5=55}