5 · Intermediate · lesson 29 of 41

Streams API

Declarative pipelines over collections.

Java

import java.util.stream.*;

int sum = List.of(1,2,3,4,5,6).stream()
    .filter(n -> n % 2 == 0)     // 2, 4, 6
    .mapToInt(Integer::intValue)
    .sum();                       // 12

Map<Boolean,List<Integer>> parts = IntStream.rangeClosed(1, 10)
    .boxed()
    .collect(Collectors.partitioningBy(n -> n % 2 == 0));

NOTE

Streams are lazy — nothing happens until a terminal operation (sum, collect, forEach).

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Key takeaways

✓Stages: source → intermediate ops → terminal op.
✓Streams are single-use; create a new one each time.

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