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Collections Beginner

Typed stream toArray

Filter a collection and collect the results to a typed array using a single stream expression.

Code Comparison
✕ Pre-Streams 
List<String> list = getNames();
List<String> filtered = new ArrayList<>();
for (String n : list) {
    if (n.length() > 3) {
        filtered.add(n);
    }
}
String[] arr = filtered.toArray(new String[0]);
✓ Java 8+ 
String[] arr = getNames().stream()
    .filter(n -> n.length() > 3)
    .toArray(String[]::new);
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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

No intermediate list — one expression replaces the manual loop and copy.

Old Approach
Manual Filter + 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

When you need to filter a collection before converting it to a typed array, streams let you chain the operations without an intermediate list. The toArray(IntFunction) generator (String[]::new) creates the correctly typed array directly at the end of the pipeline, eliminating the manual loop and temporary ArrayList.

Related Documentation
Stream.toArray() ↗
Proof
View proof source ↗
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+ →