Stream (Stream API)

A Stream is a declarative, composable pipeline for processing a sequence of elements. Introduced in Java 8, streams let you express data transformations as a chain of operations (filter, map, reduce, collect) instead of explicit loops.

Example pipeline

List<String> names = List.of("Ada", "Grace", "Linus", "Ken");

long count = names.stream()
    .filter(name -> name.length() > 3)
    .map(String::toUpperCase)
    .count();
// count = 2 ("GRACE" and "LINUS")

Stream vs collection

A collection stores data; a stream describes a computation. Streams are:

• Lazy — intermediate operations (filter, map) only execute when a terminal operation (collect, count, forEach) runs.
• One-shot — a stream can be consumed once. Re-running requires creating a new stream.
• Possibly parallel — list.parallelStream() splits the work across the common ForkJoinPool.

Common operations

stream.filter(pred)            // keep matching
      .map(fn)                  // transform each element
      .flatMap(fn)              // map and flatten
      .distinct()               // remove duplicates
      .sorted(comparator)       // sort
      .limit(n)                 // take first n
      .skip(n)                  // drop first n
      .reduce(identity, op)     // fold to a single value
      .collect(Collectors.toList()) // materialise as collection

Streams != java.io.InputStream

Note the naming clash: java.util.stream.Stream (this topic) is unrelated to java.io.InputStream (byte-level I/O). Always say "Stream API" or "java.util.stream" to disambiguate in conversation.