How to remove duplicates from a list based on a cu

Question

Before I post this question, I found somehow similar question posted here. But the answer was based on a String. However, I have a different situation here. I am not trying to remove String but another object called AwardYearSource. This class has an int attribute called year. So I want to remove duplicates based on the year. i.e if there is year 2010 mentioned more than once, I want to remove that AwardYearSource object. How can I do that?

Answer 1

If your AwardYearSource class overrides equals and hashcode methods (Eclipse can generate both), then you can add them to a Set. The Set will not contain any duplicates.

public class AwardYearSource
{
    private final int year;

    public AwardYearSource(int year)
    {
        this.year = year;
    }

    @Override
    public int hashCode()
    {
        final int prime = 31;
        int result = 1;
        result = prime * result + year;
        return result;
    }

    @Override
    public boolean equals(Object obj)
    {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        AwardYearSource other = (AwardYearSource) obj;
        if (year != other.year)
            return false;
        return true;
    }

    @Override
    public String toString()
    {
        return String.valueOf(year);
    }


    public static void main(String[] args)
    {
        Set<AwardYearSource> set = new HashSet<AwardYearSource>();
        set.add(new AwardYearSource(2000));
        set.add(new AwardYearSource(2000));
        set.add(new AwardYearSource(2000));
        set.add(new AwardYearSource(2000));

        System.out.println(set);
    }
}

The output is [2000]. Only one item in the set.

Answer 2

Create a HashMap object with int as the key type and your class as the value type. Then iterate over the list and insert each element to the map using:

mymap.put(source.year, source);

Then remove all elements from the origianl list and iterate over the map and insert each element to the list.

Answer 3

The simplest way to remove elements based on a field is as follows (preserving order):

Map<Integer, AwardYearSource> map = new LinkedHashMap<>();
for (AwardYearSource ays : list) {
  map.put(ays.getYear(), ays);
}
list.clear();
list.addAll(map.values());

Answer 4

Another way would be to override hashCode() and equals(Object obj) for your object. Since it just has one field you want to use to determine equality, this is pretty straightforward. Something like:

public boolean equals(Object obj) {
  if (obj == null || !(obj instanceof AwardYearSource)) {
    return false;
  }
  return (this.year == ((AwardYearSource)obj).year);
}
public int hashCode() {
  return this.year;
}

Then you can just stick all of the objects into a Set to remove duplicates:

Set<AwardYearSource> set = new Set<AwardYearSource>();

set.add(new AwardYearSource(2011));
set.add(new AwardYearSource(2012));
set.add(new AwardYearSource(2011));

for (AwardYearSource aws : set) {
  System.out.println(aws.year);
}

Answer 5

Fairly simply. Although something bugs me about the map versions (not that I doubt they'd work, it just seems like overkill, somehow - although this version isn't necessarily any better in that regard).
Answer is functional, and threadsafe (assuming AwardYearSource is immutable).

public static List<AwardYearSource> removeDuplicateYears(
                                          final Collection<AwardYearSource> awards) {
    final ArrayList<AwardYearSource> input = new ArrayList<AwardYearSource>(awards);
    // If there's only one element (or none), guaranteed unique.
    if (input.size() <= 1) {
        return input;
    }
    final HashSet<Integer> years = new HashSet<Integer>(input.size(), 1);
    final Iterator<AwardYearSource> iter = input.iterator();
    while(iter.hasNext()) {
        final AwardYearSource award = iter.next();
        final Integer year = award.getYear();
        if (years.contains(year)) {
            iter.remove();
        } else {
            years.add(year);
        }
    }
    return input;       

}

Answer 6

You could use a map and store your objects with the year as a key:

Map<Integer, AwardYearSource> map = new HashMap<Integer, AwardYearSource>();
map.put(someAwardYearSource1.getYear(), someAwardYearSource1);
map.put(someAwardYearSource2.getYear(), someAwardYearSource2);

etc.

At the end the map will contain unique values by year, which you can call with the values method:

Collection<AwardYearSource> noDups = map.values();