Home / Collections / Unmodifiable collectors
Collections intermediate

Unmodifiable collectors

Collect directly to unmodifiable collections.

Code Comparison
✕ Java 8 
List<String> list = stream.collect(
    Collectors.collectingAndThen(
        Collectors.toList(),
        Collections::unmodifiableList
    )
);
✓ Java 10+ 
List<String> list = stream.collect(
    Collectors.toUnmodifiableList()
);
Why the modern way wins
📏

Direct

One collector instead of wrapping with collectingAndThen.

🔒

Immutable

Result cannot be modified — no accidental mutations.

🚫

Null-safe

Rejects null elements during collection.

Old Approach
collectingAndThen
Modern Approach
toUnmodifiable*()
Since JDK
10
Difficulty
intermediate
JDK Support
Unmodifiable collectors
Available

Widely available since JDK 10 (March 2018)

How it works

Java 10 added toUnmodifiableList(), toUnmodifiableSet(), and toUnmodifiableMap() collectors. No more wrapping with collectingAndThen.

Related patterns

Collections beginner

Immutable map creation

Java 8
Map<String, Integer> map = new HashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
map = Collections.unmodifiableMap(map);
Java 9+
Map<String, Integer> map =
    Map.of("a", 1, "b", 2, "c", 3);
Hover to see modern ➜
JDK 9+ →
Collections beginner

Immutable set creation

Java 8
Set<String> set =
    Collections.unmodifiableSet(
        new HashSet<>(
            Arrays.asList("a", "b", "c")
        )
    );
Java 9+
Set<String> set =
    Set.of("a", "b", "c");
Hover to see modern ➜
JDK 9+ →
Collections beginner

Map.entry() factory

Java 8
Map.Entry<String, Integer> e =
    new AbstractMap.SimpleEntry<>(
        "key", 42
    );
Java 9+
var e = Map.entry("key", 42);
Hover to see modern ➜
JDK 9+ →
Share 𝕏 🦋 in ⬡