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Collections beginner

Copying collections immutably

Create an immutable copy of any collection in one call.

Code Comparison
✕ Java 8 
List<String> copy =
    Collections.unmodifiableList(
        new ArrayList<>(original)
    );
✓ Java 10+ 
List<String> copy =
    List.copyOf(original);
Why the modern way wins
⚡

Smart copy

Skips the copy if the source is already immutable.

📏

One call

No manual ArrayList construction + wrapping.

🛡️

Defensive copy

Changes to the original don't affect the copy.

Old Approach
Manual Copy + Wrap
Modern Approach
List.copyOf()
Since JDK
10
Difficulty
beginner
JDK Support
Copying collections immutably
Available

Widely available since JDK 10 (March 2018)

How it works

List.copyOf(), Set.copyOf(), and Map.copyOf() create immutable snapshots of existing collections. If the source is already an immutable collection, no copy is made.

Related patterns

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+ →
Collections beginner

Immutable list creation

Java 8
List<String> list =
    Collections.unmodifiableList(
        new ArrayList<>(
            Arrays.asList("a", "b", "c")
        )
    );
Java 9+
List<String> list =
    List.of("a", "b", "c");
Hover to see modern ➜
JDK 9+ →
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