Stream: flatMap

Java Streams, Lambda & Functional ProgrammingTopic 8 of 307 questions

Q1map() vs flatMap(): one-to-one vs one-to-many + flatten

map transforms one-to-one (wraps), flatMap transforms one-to-many and flattens (unwraps): use flatMap when map gives you Stream<List<T>> but you want Stream<T>.

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Q2flatMap use cases: flatten nested collections, split values, Optional::stream, nested data

flatMap has five core use cases: flatten nested collections, split values, extract Optional values, merge streams, and multi-level drill-down through nested data structures.

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Q3flatMapToInt/Long/Double: flatten to primitive stream, vs flatMap+mapToInt

flatMapToInt maps each element to an IntStream and flattens all resulting primitive streams into one, avoiding the boxing that flatMap+mapToInt would create.

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Q4Stream.mapMulti (Java 16): push 0..N elements imperatively, vs flatMap

Stream.mapMulti pushes 0 to N elements to a Consumer imperatively instead of creating a Stream per element like flatMap, avoiding per-element stream allocation overhead.

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Q5Flatten a List of departments, each holding a List of employees, into one Stream of employees using flatMap

flatMap maps each department to a stream of its employees and then concatenates all those streams into one flat Stream<Employee>, which map alone cannot do because map keeps the nesting.

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Q6Given a List<Department> where each department has a List<Employee>, flatten to a single List<Employee>

`flatMap` opens up each nested collection and pours all their elements into one single stream.

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Q7Given a Map<String, List<Employee>> (department → employees), flatten all employees into one list

Stream the map's values and `flatMap` each inner list to pour all employees into one flat list.

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