forEach() vs forEachOrdered()

In Java Streams, both forEach() and forEachOrdered() are terminal operations used to iterate over elements.

However, their behavior differs significantly — especially when used with parallel streams.

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Basic Difference

Method	Order Guarantee	Performance	Best Used With
forEach()	❌ No order guarantee	Faster	Parallel streams
forEachOrdered()	✅ Preserves encounter order	Slightly slower	When order matters

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Using forEach() (Sequential Stream)

List<Integer> list = List.of(1, 2, 3, 4);

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

output

1
2
3
4

✔ Order preserved
✔ Single-thread execution

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Using forEach() (Parallel Stream)

list.parallelStream().forEach(System.out::println);

Possible Output:

3
1
4
2

Why?

• Elements are processed in parallel
• No guarantee of encounter order
• Faster execution

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Using forEachOrdered() (Parallel Stream)

list.parallelStream().forEachOrdered(System.out::println);

Output:

1
2
3
4

• ✔ Maintains encounter order
• ❌ Reduces parallel performance benefit

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What Is “Encounter Order”?

Encounter order means:

• The order in which elements appear in the source.

Examples:

Source	Encounter Order Exists?
List	Yes
Array	Yes
LinkedHashSet	Yes
HashSet	No (unordered)

If the source is unordered, forEachOrdered() has no meaningful order to preserve.

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Performance Impact

forEachOrdered():

• Forces synchronization
• Waits for lower-index tasks to complete
• Reduces parallel efficiency

forEach():

• Processes elements as soon as they are ready
• Maximizes parallel performance