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Collections beginner

Typed stream toArray

Convert streams to typed arrays with a method reference.

Code Comparison
✕ Pre-Streams 
List<String> list = getNames();
String[] arr = new String[list.size()];
for (int i = 0; i < list.size(); i++) {
    arr[i] = list.get(i);
}
✓ Java 8+ 
String[] arr = getNames().stream()
    .filter(n -> n.length() > 3)
    .toArray(String[]::new);
Why the modern way wins
🎯

Type-safe

No Object[] cast — the array type is correct.

🔗

Chainable

Works at the end of any stream pipeline.

📏

Concise

One expression replaces the manual loop.

Old Approach
Manual Array Copy
Modern Approach
toArray(generator)
Since JDK
8
Difficulty
beginner
JDK Support
Typed stream toArray
Available

Widely available since JDK 8 (March 2014)

How it works

The toArray(IntFunction) method creates a properly typed array from a stream. The generator (String[]::new) tells the stream what type of array to create.

Related patterns

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+ →
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 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+ →
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