Home / Collections / Immutable set creation
Collections Beginner

Immutable set creation

Create immutable sets with a single factory call.

Code Comparison
✕ Java 8 
Set<String> set =
    Collections.unmodifiableSet(
        new HashSet<>(
            Arrays.asList("a", "b", "c")
        )
    );
✓ Java 9+ 
Set<String> set =
    Set.of("a", "b", "c");
See a problem with this code? Let us know.
Why the modern way wins
📏

Concise

One line instead of three nested calls.

🚫

Detects duplicates

Throws if you accidentally pass duplicate elements.

🔒

Immutable

No add/remove possible after creation.

Old Approach
Verbose Wrapping
Modern Approach
Set.of()
Since JDK
9
Difficulty
Beginner
JDK Support
Immutable set creation
Available

Widely available since JDK 9 (Sept 2017)

How it works

Set.of() creates a truly immutable set that rejects nulls and duplicate elements at creation time. No more wrapping mutable sets.

Related Documentation
Set.of() ↗ Collections Factory Methods (JEP 269) ↗
Proof
View proof source ↗
Related patterns
Collections Beginner

Copying collections immutably

Java 8
List<String> copy =
    Collections.unmodifiableList(
        new ArrayList<>(original)
    );
Java 10+
List<String> copy =
    List.copyOf(original);
Hover to see modern ➜
JDK 10+ →
Collections Intermediate

Collectors.teeing()

Java 8
long count = items.stream().count();
double sum = items.stream()
    .mapToDouble(Item::price)
    .sum();
var result = new Stats(count, sum);
Java 12+
var result = items.stream().collect(
    Collectors.teeing(
        Collectors.counting(),
        Collectors.summingDouble(Item::price),
        Stats::new
    )
);
Hover to see modern ➜
JDK 12+ →
Streams Beginner

Stream.toList()

Java 8
List<String> result = stream
    .filter(s -> s.length() > 3)
    .collect(Collectors.toList());
Java 16+
List<String> result = stream
    .filter(s -> s.length() > 3)
    .toList();
Hover to see modern ➜
JDK 16+ →