Stream.mapMulti()

Code Comparison

stream.flatMap(order ->
    order.items().stream()
        .map(item -> new OrderItem(
            order.id(), item)
        )
);

stream.<OrderItem>mapMulti(
    (order, downstream) -> {
        for (var item : order.items())
            downstream.accept(
                new OrderItem(order.id(), item));
    }
);

Why the modern way wins

⚡

Less allocation

No intermediate Stream created per element.

🎯

Imperative style

Use loops and conditionals directly.

📐

Flexible

Emit zero, one, or many elements with full control.

Old Approach

flatMap + List

Modern Approach

mapMulti()

JDK Support

Stream.mapMulti()

Available

Widely available since JDK 16 (March 2021)

How it works

mapMulti() is an imperative alternative to flatMap that avoids creating intermediate Stream objects for each element. It's more efficient when the mapping produces a small number of elements.