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Collections Beginner

Immutable list creation

Create immutable lists in one clean expression.

Code Comparison

✕ Java 8

List<String> list =
    Collections.unmodifiableList(
        new ArrayList<>(
            Arrays.asList("a", "b", "c")
        )
    );

✓ Java 9+

List<String> list =
    List.of("a", "b", "c");

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Why the modern way wins

📏

One call

Replace three nested calls with a single factory method.

🔒

Truly immutable

Not just a wrapper — the list itself is immutable.

🛡️

Null-safe

Rejects null elements at creation time, failing fast.

Old Approach

Verbose Wrapping

Modern Approach

List.of()

Since JDK

Difficulty

Beginner

JDK Support

Immutable list creation

Available

Widely available since JDK 9 (Sept 2017)

How it works

List.of() creates a truly immutable list — no wrapping, no defensive copy. It's null-hostile (rejects null elements) and structurally immutable. The old way required three nested calls.

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);

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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");

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JDK 9+ →

Collections Beginner

Sequenced collections

Java 8

// Get last element
Object last = list.get(list.size() - 1);
// Get first
Object first = list.get(0);
// Reverse iteration: manual

Java 21+

var last = list.getLast();
var first = list.getFirst();
var reversed = list.reversed();

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JDK 21+ →