Functional Programming

Java 8 added lambdas, the Streams API, and functional interfaces — transforming how Java developers write and read data-processing code. Mastering the stream pipeline, understanding lazy evaluation, and knowing when to prefer functional style over imperative loops are skills expected at every seniority level.

What You'll Find Here

Topic	Description
Lambdas	(params) -> expression syntax, effectively-final capture, this behavior.
Functional Interfaces	@FunctionalInterface, Function, Predicate, Consumer, Supplier.
Method References	Static, instance, constructor references — when to prefer over lambdas.
Streams API	Pipeline anatomy — source, intermediate ops (filter, map, flatMap), terminal ops.
Collectors	toList, groupingBy, joining, counting, custom collectors.
Parallel Streams	When parallel helps vs. hurts — ordering, side effects, ForkJoin pool.
Optional Deep Dive	Correct usage patterns; anti-patterns (field types, method parameters).

Learning Path

1. Lambdas — understand syntax and effectively-final capture before going further.
2. Functional Interfaces — Function<T,R>, Predicate<T>, and Consumer<T> are the backbone of the Streams API.
3. Method References — the concise alternative to lambdas; know all four kinds.
4. Streams API — the pipeline model (lazy evaluation + one terminal op) is the core concept.
5. Collectors — groupingBy and toMap are the collectors most likely to appear in interviews.
6. Optional Deep Dive — return type design and safe value retrieval patterns.
7. Parallel Streams — study AFTER understanding sequential streams; parallelism introduces correctness risks.

Related Domains

• Java Type System — lambdas rely on generic functional interfaces and wildcard inference.
• Collections Framework — streams operate on collections; know both approaches.
• Multithreading & Concurrency — parallel streams use the ForkJoin pool under the hood.