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Immutable map creation

Create immutable maps inline without a builder.

Code Comparison

✕ 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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Why the modern way wins

📏

Inline creation

No temporary mutable map needed.

🔒

Immutable result

The map cannot be modified after creation.

🚫

No null keys/values

Null entries are rejected immediately.

Old Approach

Map Builder Pattern

Modern Approach

Map.of()

Since JDK

Difficulty

Beginner

JDK Support

Immutable map creation

Available

Widely available since JDK 9 (Sept 2017)

How it works

Map.of() accepts key-value pairs inline and returns an immutable map. For more than 10 entries, use Map.ofEntries() with Map.entry() pairs.

Copying collections immutably

Java 8

List<String> copy =
    Collections.unmodifiableList(
        new ArrayList<>(original)
    );

Java 10+

List<String> copy =
    List.copyOf(original);

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JDK 10+ →

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+ →

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();

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JDK 16+ →