When exactly do the operations in a Stream method chain in Java occur?

In Java, operations in a Stream method chain are divided into intermediate and terminal operations. Intermediate operations are lazy transformations that are not executed immediately but only form a description of the data processing pipeline.

The execution of operations occurs only when a terminal operation is called, which initiates data processing and returns a result or side effect.

For example:

List<String> list = Arrays.asList("a", "b", "c");
Stream<String> stream = list.stream()
    .filter(s -> s.startsWith("a"))  // intermediate operation
    .map(String::toUpperCase);        // intermediate operation

List<String> result = stream.collect(Collectors.toList()); // terminal operation

Here, filtering and transformation are not performed until collect() is called. The terminal operation triggers the entire chain.