🚦 What are Terminal Operations in Streams?

🔗 Definition

A terminal operation is a final operation that: ✅ Triggers the actual processing of the stream pipeline.
✅ Produces a result (a value, a collection, or a side effect like printing).
✅ Closes the stream — after a terminal operation, the stream cannot be reused.

---

📊 Stream Pipeline Structure

Stream Source → Intermediate Operations (filter, map, etc.) → Terminal Operation

• Intermediate operations are lazy (they only prepare transformations).
• The terminal operation triggers the actual work.

---

✅ Examples of Terminal Operations

Terminal Operation	Purpose	Example
collect()	Gather elements into a collection (List, Set, Map)	.collect(Collectors.toList())
forEach()	Perform an action on each element	.forEach(System.out::println)
reduce()	Combine elements into a single result	.reduce(0, Integer::sum)
count()	Count the number of elements	.count()
anyMatch()	Check if any element matches a condition	.anyMatch(x -> x > 10)
allMatch()	Check if all elements match a condition	.allMatch(x -> x > 0)
noneMatch()	Check if no element matches a condition	.noneMatch(x -> x < 0)
findFirst()	Get the first element (optional)	.findFirst()
findAny()	Get any element (useful with parallel streams)	.findAny()
toArray()	Convert to array	.toArray()

---

🔗 Example in Action

List<String> names = List.of("Alice", "Bob", "Charlie");

long count = names.stream()
    .filter(name -> name.startsWith("A"))  // Intermediate
    .count();                               // Terminal

System.out.println("Count: " + count);  // Triggers actual work

✅ Without count(), nothing actually happens — the stream is just prepared.
✅ count() forces the pipeline to execute, processing each element.

---

❗️ What Happens After Terminal Operation?

The stream becomes consumed and cannot be reused:

Stream<String> stream = names.stream();
stream.filter(name -> name.startsWith("A")).count();
stream.filter(name -> name.length() > 3).count();  // ❌ IllegalStateException (stream already closed)

🏁 Key Takeaways

Concept	Explanation
Intermediate Ops	Prepare the pipeline (filter, map)
Terminal Op	Triggers actual work (collect, count)
One-time use	Stream cannot be reused after terminal operation
Functional Style	Terminal operation is where you “finalize” your intention