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Java
Optional Chains — flatMap, filter
`flatMap` lets you chain Optional-returning methods without nested `if`. `filter` short-circuits to empty if the predicate fails. The functional equivalent of safe-navigation `?.`.
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Rust
flat_map and Iterator::flatten
`flat_map(f)` is `map(f).flatten()` — map each item to an iterator, then concatenate. Indispensable for working with nested structures.
Kotlin
flatMap and Flattening
`flatMap` maps each item to a collection, then concatenates. Use it when "for each X, produce N Y's, collect all Y's". `flatten()` is the no-mapping version.