When does the stream execution start in Java?

Stream execution begins when a terminal operation is invoked.

Terminal operations:

• forEach / forEachOrdered
• toArray
• reduce
• collect
• count
• anyMatch / allMatch / noneMatch
• findFirst / findAny
• min / max

Intermediate operations, such as filter, map, sorted, are lazy and only describe the data processing pipeline without actually performing any actions until a terminal operation is called.

Example:

// Data source: list of numbers
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

// Intermediate operations define the pipeline but are not executed
Stream<Integer> processedStream = numbers.stream()
    .filter(n -> {
        // This code will not execute until a terminal operation is invoked
        System.out.println("Filtering: " + n);
        return n % 2 == 0;
    })
    .map(n -> {
        // This code will not execute until a terminal operation is invoked
        System.out.println("Mapping: " + n);
        return n * 2;
    });

// Terminal operation - triggers the pipeline execution
List<Integer> result = processedStream.collect(Collectors.toList());

// Output will be:
// Filtering: 1
// Filtering: 2
// Mapping: 2
// Filtering: 3
// Filtering: 4
// Mapping: 4
// Filtering: 5