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Separate first occurrences from repeats in one pass
A Set cannot contain duplicate elements, as Oracle’s Java Collections tutorial explains. When scanning a list, HashSet.add(value) tells you whether adding that value changed the set: it returns true for a first occurrence and false when the set already contains an equal value. The input list remains unchanged.
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import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class CategoryDuplicates {
public static void main(String[] args) {
List<String> categories = List.of("Books", "Games", "Books", "Music", "Games");
Set<String> seen = new HashSet<>();
List<String> firstOccurrences = new ArrayList<>();
List<String> repeatedOccurrences = new ArrayList<>();
for (String category : categories) {
if (seen.add(category)) {
firstOccurrences.add(category);
} else {
repeatedOccurrences.add(category);
}
}
System.out.println("First occurrences: " + firstOccurrences);
System.out.println("Repeated occurrences: " + repeatedOccurrences);
}
}For this input, the lists contain [Books, Games, Music] and [Books, Games], respectively. The lists preserve scan order; the set is only used for membership checks. List.of is available in Java 9 and later. On an older JDK, use Arrays.asList("Books", "Games", "Books", "Music", "Games") and import java.util.Arrays.
Decide what “unique” means for your output
“Unique” can mean either one representative of each distinct category or categories whose total frequency is exactly one. The example’s firstOccurrences list gives one representative of each category, even if that category appears again later. Its repeatedOccurrences list includes every occurrence after the first. Neither list, by itself, answers which categories appeared exactly once overall.
Keep categories that appear exactly once
For an exact frequency check, count values first, then select those with a count of one. A map makes that meaning explicit:
import java.util.HashMap;
import java.util.Map;
Map<String, Integer> counts = new HashMap<>();
for (String category : categories) {
counts.merge(category, 1, Integer::sum);
}
List<String> appearingOnce = new ArrayList<>();
for (String category : categories) {
if (counts.get(category) == 1 && !appearingOnce.contains(category)) {
appearingOnce.add(category);
}
}Iterating the original list in the second pass preserves its order in appearingOnce. The contains check prevents a category with count one from being added more than once; alternatively, collect qualifying keys from the map if their order does not matter.
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Choose a collection based on ordering and counting needs
| Collection | When it fits | Ordering and trade-off |
|---|---|---|
HashSet |
Membership checks and duplicate detection when display order is irrelevant. | No iteration-order guarantee. Basic operations are expected to take constant time when hash values disperse elements properly, according to the Java SE 26 API. |
LinkedHashSet |
Keep one copy of each category in first-insertion order. | Retains insertion order, with a modest cost compared with HashSet, according to the Oracle collections implementations tutorial. |
TreeSet |
Keep distinct categories in sorted order. | Orders values rather than preserving input order; it is substantially slower than HashSet, according to the same Oracle tutorial. |
| Frequency map | Find counts or categories whose frequency is exactly one. | Stores a count per distinct value; choose an ordered map or separately preserve input order if the output needs a particular order. |
If you need first-seen and repeated values in separate outputs, the lists in the example are the simplest way to preserve both sequences. Iterating a HashSet to produce output does not preserve the original list order: Oracle’s Java SE 26 API explicitly makes no iteration-order guarantee.
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Make equality match the meaning of “same category”
A set decides whether values are duplicates using equality and hash codes, not by comparing every field automatically. Strings already have value-based equality, so equal text is treated as the same category. For a custom category class, implement equals and hashCode using the same fields that define category identity—for example, a stable category ID if names can vary.
- Two objects that represent the same category should compare equal and return the same hash code.
- A hash collision alone does not make two values equal; equality is also considered.
- Do not change fields used by
equalsorhashCodewhile an object is in a set. Doing so can make membership checks behave unexpectedly.
A HashSet permits one null element; any later null is considered already present. Decide whether null is valid input and handle it consistently with your category rules.
What the usual performance claim does—and does not—mean
Oracle describes HashSet as the best-performing general-purpose Set implementation and documents expected constant-time basic operations when the hash function disperses elements properly in its Java SE 26 API. This is a conditional expectation, not a guarantee that every input or custom hashCode implementation will perform equally well. The one-pass duplicate scan does one set insertion attempt per input element; the frequency-map approach also processes the input once to build counts, then makes a further pass to select the desired results.
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