collect() vs forEach() in Java Streams

Both collect() and forEach() are terminal operations in Java Streams. However, they serve very different purposes.

Understanding the difference is essential for writing clean, functional-style code.

Basic Difference

collect()	forEach()
Used to gather results	Used to perform actions
Returns a value	Returns void
Functional & safe	Often side-effect based
Preferred for data transformation	Preferred for logging / printing

What is forEach()?

forEach() is used to perform an action on each element.

It does not produce a result. It is mainly used for:

Printing
Logging
Triggering side effects

Example – Printing Elements

List<String> names = List.of("A", "B", "C");

names.stream()
     .forEach(System.out::println);

What is collect()?

collect() is used to gather stream elements into a collection or another container.

It returns a result.

Example – Collecting to List

List<String> upperNames =
    names.stream().map(String::toUpperCase).collect(Collectors.toList());

✔ Returns a new List
✔ No side effects

forEach()

Performs an action
Encourages mutation
Can cause thread-safety issues in parallel streams

collect()

Follows functional programming style
Avoids shared mutable state
Safer in parallel streams
Designed for accumulation

forEach() is used to perform actions on each stream element and returns void, often causing side effects. collect() is used to accumulate stream elements into a collection and is preferred in functional-style programming, especially for parallel streams.