How does a stream process elements — one by one or in batches?

The Java Stream API processes elements one by one, lazily and sequentially, unless specified otherwise. This means that elements are not loaded and processed in batches, but pass through a chain of operations one at a time, which helps save memory and perform computations as needed.

For example, when calling stream().map(...).filter(...).collect(...), each element goes through all operations in sequence, rather than transforming the entire collection first and then filtering.

However, if a parallel stream (parallelStream()) is used, elements can be processed in multiple threads simultaneously, but still one at a time in each thread.

If batch processing is required, it must be implemented explicitly, for example, by grouping elements into batches within the stream.