Java Functional Programming & Stream API Training Roadmap

This is a 3-week hands-on training roadmap for mastering Java Functional Programming and Stream API. Designed for 2–3 hours/day of practice, progressing from basics to advanced concepts with mini-projects.

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Week 1: Foundations of Functional Programming in Java

Goal: Understand lambdas, functional interfaces, method references, and core FP concepts.

Day 1–2: Functional Interfaces & Lambdas

• Learn:

  • @FunctionalInterface
  • Lambda syntax (params) -> expression

• Practice:

  • Implement Runnable, Comparator, and a custom functional interface.
  • Example: Calculator interface for add, subtract, multiply.

Day 3: Method References

• Learn:

  • ClassName::staticMethod
  • instance::method
  • ClassName::new (constructor references)

• Practice:

  • Convert existing lambdas to method references.

  • Example:

    List<String> names = List.of("Alice","Bob");
    names.forEach(System.out::println);

Day 4: Predicates, Consumers, and Suppliers

• Learn:

  • Predicate<T> → test conditions
  • Consumer<T> → accept values
  • Supplier<T> → provide values
  • Function<T,R> → transform values

• Practice:

  • Filter strings by length using Predicate
  • Generate random numbers with Supplier
  • Print values with Consumer

Day 5: Optional & Null Safety

• Learn:

  • Optional<T> to avoid NullPointerException

• Practice:

  • Wrap variables in Optional
  • Use ifPresent(), orElse(), map()

Day 6–7: Mini Project

• Build a functional utility class with:

  • Filtering, mapping, aggregating, and null-safe methods.

• Practice chaining functional interfaces.

• Example:

  UserUtils.filterActiveUsers(users)
           .map(User::getEmail)
           .forEach(System.out::println);

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Week 2: Stream API Basics

Goal: Process collections in a functional style.

Day 8: Creating Streams

• Learn: stream(), parallelStream(), of(), Arrays.stream()

• Practice:

  • Convert list/array to stream and print elements.

Day 9: Filter, Map, Peek

• Learn: filter(), map(), peek()

• Practice:

  • Filter even numbers, square them, print intermediate results.

Day 10: Reduce & Collect

• Learn: reduce() → aggregate, collect() → gather results

• Practice:

  • Sum, multiply, max, min
  • Collect to List, Set, or Map

Day 11: Sorting & Distinct

• Learn: sorted(), distinct(), limit(), skip()

• Practice:

  • Sort numbers descending
  • Remove duplicates from a list

Day 12: Grouping & Partitioning

• Learn: Collectors.groupingBy(), Collectors.partitioningBy()

• Practice:

  • Group Person objects by age
  • Partition even and odd numbers

Day 13: FlatMap & Nested Streams

• Learn: Flatten nested collections

• Practice:

  • Flatten List<List<Integer>> into List<Integer>
  • Flatten a List<Person> each having List<Address>

Day 14: Mini Project

• Task: Build a data processor:

  • Input: List of Employees (name, dept, salary)

  • Output:

    • Employees per department
    • Highest salary per department
    • List of names with salary > 5000

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Week 3: Advanced Stream & Functional Patterns

Goal: Apply FP & Stream API in real scenarios.

Day 15: Parallel Streams

• Learn: parallelStream(), spliterator()

• Practice:

  • Compare stream() vs parallelStream() performance on large lists

Day 16: Advanced Collectors

• Learn: joining(), summarizingInt(), mapping(), toMap()

• Practice:

  • Create comma-separated names
  • Count employees per department

Day 17: Combining Streams

• Learn: Stream concatenation, Stream from Optional

• Practice:

  • Merge two lists, filter, sort

Day 18: Functional Programming Patterns

• Learn: Immutability, pure functions, higher-order functions

• Practice:

  • Create pure transformation functions for lists

Day 19: Exception Handling in Streams

• Learn: Wrap checked exceptions in lambdas

• Practice:

  • Read file lines with Files.lines().stream() and handle exceptions functionally

Day 20–21: Capstone Mini Project

• Task: Employee Analytics App

  • Input: List<Employee> (id, name, dept, salary)

  • Features:

    • Filter by salary range
    • Group by department and calculate avg salary
    • Find top 3 highest-paid employees
    • Flatten nested projects per employee
    • Produce summary report

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Tips for Maximum Learning

1. Use IntelliJ IDEA or Eclipse for functional shortcuts.
2. Try to chain operations instead of writing loops.
3. Keep a practice notebook of examples.
4. Refactor imperative code to functional style wherever possible.
5. Check JavaDocs for Stream API and Collectors – they have ready examples.