Stream

Learn

Existing chapters for this concept.

• Streams & Optional

  Core Java

  💡 Streams are single-use — create a new stream for a second pipeline. Milestone: You completed Core APIs (chapters 16–20).

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See it in code

Existing Java examples.

• core 22 Streams Demo

  Core Java · pkg1core/core22StreamsDemo.java · pkg1core · core22StreamsDemo.java

  - Streams are LAZY: intermediate ops (filter/map/sorted) build a pipeline; a terminal op (collect/reduce/forEach/count) triggers execution. - Streams don't…

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• io 2 Byte Streams

  I/O · pkg9io/io2ByteStreams.java · pkg9io · io2ByteStreams.java

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• io 3 Character Streams

  I/O · pkg9io/io3CharacterStreams.java · pkg9io · io3CharacterStreams.java

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• core23OptionalDemo

  Core Java · pkg1core/core23OptionalDemo.java · pkg1core · core23OptionalDemo.java

  - Optional is a container that holds a value or nothing. - Prefer map/filter/flatMap + orElse/orElseThrow over isPresent()/get(). - Use it as a RETURN type;…

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Practice

Existing LeetCode material in JavaForge.

• Find Median from Data Stream

  Blind 75 · Heap · pkg5leetcode/blind75/blind75_LC295FindMedianFromDataStream.java · pkg5leetcode · blind75_LC295FindMedianFromDataStream.java

  Two heaps: max-heap lower half, min-heap upper half balanced. addNum O(log n), findMedian O(1)

  Open practice →
• Find Median from Data Stream

  Blind 75 · Heap · pkg5leetcode/blind75/blind75_LC295FindMedianFromDataStream.java · pkg5leetcode · blind75_LC295FindMedianFromDataStream.java

  Two heaps: max-heap lower half, min-heap upper half balanced. addNum O(log n), findMedian O(1)

  Open practice →

Prepare

Existing interview questions.

• Stream from collection?

  Collections Framework — Interview Questions (120+)

  By access pattern: order, uniqueness, sorting, concurrency.

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• parallelStream pool?

  Concurrency & Multithreading — Interview Questions (150+)

  Prefer immutability and high-level concurrency utilities over low-level locks.

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• Functional Programming & Streams — Interview Questions (100+)

  Functional Programming & Streams — Interview Questions (100+)

  Keep lambdas pure, stateless, and non-interfering.

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• What is a Stream and how does it differ from a Collection?

  Functional Programming & Streams — Interview Questions (100+)

  A pipeline for processing data; not storage.

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• What does "lazy evaluation" mean for streams?

  Functional Programming & Streams — Interview Questions (100+)

  Work happens only when a terminal op runs, element-by-element.

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• When should you use parallel streams?

  Functional Programming & Streams — Interview Questions (100+)

  Large, CPU-bound, stateless, easily-splittable data—after measuring.

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• Why are streams single-use?

  Functional Programming & Streams — Interview Questions (100+)

  A stream is consumed by its terminal op.

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• How do you avoid side effects in streams?

  Functional Programming & Streams — Interview Questions (100+)

  Use pure functions and collectors, not external mutation.

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• What are primitive streams and why use them?

  Functional Programming & Streams — Interview Questions (100+)

  IntStream/LongStream/DoubleStream avoid boxing.

  Open question →
• lines Stream?

  Strings & Performance — Interview Questions (82+)

  NPE if reference null on instance concat.

  Open question →
• Java 9 — Stream takeWhile/dropWhile?

  Java 9–21 Features — Interview Questions (95+)

  Short-circuit on sorted/predicate prefix; not same as filter.

  Open question →
• reactive streams in JDK?

  Java 9–21 Features — Interview Questions (95+)

  infers raw-ish; specify generic.

  Open question →
• Byte streams vs character streams?

  I/O, NIO & Serialization — Interview Questions (82+)

  / for bytes; / for chars with encoding.

  Open question →
• DirectoryStream?

  I/O, NIO & Serialization — Interview Questions (82+)

  Use virtual threads executor Java 21.

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• Stream not reusable?

  Effective Java & Best Practices — Interview Questions (82+)

  Named constants.

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• Primitive streams?

  Performance & Tuning — Interview Questions (60+)

  Profile, fix the biggest bottleneck, verify, repeat.

  Open question →

Java versions

• Streams API

  Java 8

  Java 8. modern, immutable date/time API.

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• Stream API

  Final

  Stream API. A stream is a pipeline of intermediate operations and one terminal operation. Intermediate work stays lazy until the terminal operation runs.

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• Stream additions

  Final

  Stream additions. takeWhile, dropWhile, and a new iterate overload arrive on Stream. The shared example collects with Collectors.toList(), which already…

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• Stream.toList()

  Final

  Stream.toList(). Stream gains toList(), which returns an unmodifiable list. The Java 9–11 example does not call it. It keeps Collectors.toList() so that file…

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• Stream Gatherers (Preview)

  JEP 461 · Preview

  Stream Gatherers (Preview). A gatherer is a preview intermediate stream operation that can do more than the built-in operations.

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• Stream Gatherers (Second Preview)

  JEP 473 · Preview

  Stream Gatherers (Second Preview). Stream gatherers are previewed a second time.

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• Stream Gatherers

  JEP 485 · Final

  Stream Gatherers. Gatherers are a permanent intermediate stream operation. Gatherers.windowFixed is one of the built-in gatherers.

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