Java Functional Programming rules

Identify and apply functional programming principles in Java to improve immutability, expressiveness, and maintainability.

What is covered in this Skill?

• Immutable objects and Records (JEP 395)
• Pure functions free of side effects
• Functional interfaces: Function, Predicate, Consumer, Supplier, custom @FunctionalInterface
• Lambda expressions and method references
• Stream API: filter/map/reduce pipelines, parallel streams, toUnmodifiable* collectors
• Optional idiomatic usage: map/flatMap/filter/orElse* over isPresent()+get()
• Function composition: andThen/compose
• Higher-order functions: memoization, currying, partial application
• Pattern Matching for instanceof and switch (Java 21)
• Sealed classes and interfaces (Java 17) for exhaustive domain hierarchies
• Switch Expressions (Java 14), Stream Gatherers (JEP 461)
• Effect-boundary separation: side effects at edges, pure core logic
• Immutable collections: List.of(), Collectors.toUnmodifiableList()