Java Streams Mastery 🚀

Companion repository for the Medium article: "Java Streams: From forEach to Production-Grade Pipelines"

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What's in this repo?

A production-inspired e-commerce order analytics system built entirely around Java Streams. Every class maps to a section in the article and is heavily commented to explain the theory and how it's implemented in the code.

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Project Structure

src/main/java/com/streamsmastery/
│
├── model/
│   ├── Order.java               # Core domain record
│   ├── OrderItem.java           # Line item record
│   ├── OrderStatus.java         # PENDING / SHIPPED / DELIVERED / CANCELLED
│   ├── OrderSummary.java        # Result DTO for reports
│   └── OrderDataFactory.java    # 20-order test dataset
│
├── pipeline/
│   └── StreamPipelineFundamentals.java
│       ├── Demo 1: Lazy evaluation (proven with a counter)
│       ├── Demo 2: Operation order and performance impact
│       ├── Demo 3: flatMap for nested collections
│       └── Demo 4: Method references vs lambdas
│
├── collectors/
│   ├── AdvancedCollectorsDemo.java
│   │   ├── Demo 1: groupingBy() — the SQL GROUP BY of Java
│   │   ├── Demo 2: Multi-level groupingBy() (nested maps)
│   │   ├── Demo 3: partitioningBy() — boolean splits
│   │   ├── Demo 4: toMap() with merge function
│   │   └── Demo 5: Collectors.teeing() (Java 12+)
│   │
│   └── OrderSummaryCollector.java
│       └── Full custom Collector<Order, Accumulator, OrderSummary>
│           with supplier / accumulator / combiner / finisher / characteristics
│
├── parallel/
│   └── ParallelStreamsDemo.java
│       ├── Demo 1: The shared mutable state bug (and fix)
│       ├── Demo 2: forEach vs forEachOrdered
│       ├── Demo 3: Measuring actual CPU-bound speedup
│       ├── Demo 4: Custom ForkJoinPool
│       └── Demo 5: Safe parallel reduction patterns
│
└── realworld/
    └── OrderReportEngine.java
        ├── Report 1: Executive dashboard (custom collector)
        ├── Report 2: Regional performance matrix
        ├── Report 3: Category revenue breakdown (flatMap + groupingBy)
        ├── Report 4: Customer Lifetime Value (toMap + merge)
        ├── Report 5: Fulfillment rate by region (teeing as downstream)
        ├── Report 6: Recent high-value order alerts (parallel)
        └── Report 7: Pending inventory demand (flatMap + toMap)

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Running the Project

Prerequisites

• Java 17+
• Maven 3.8+

Run all demos

# Run the complete pipeline fundamentals demo
mvn exec:java -Dexec.mainClass="com.streamsmastery.pipeline.StreamPipelineFundamentals"

# Run the advanced collectors demo
mvn exec:java -Dexec.mainClass="com.streamsmastery.collectors.AdvancedCollectorsDemo"

# Run the custom collector demo
mvn exec:java -Dexec.mainClass="com.streamsmastery.collectors.OrderSummaryCollector"

# Run the parallel streams demo
mvn exec:java -Dexec.mainClass="com.streamsmastery.parallel.ParallelStreamsDemo"

# Run the full report engine (recommended starting point)
mvn exec:java -Dexec.mainClass="com.streamsmastery.realworld.OrderReportEngine"

Run the tests

mvn test

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Key Concepts by Class

Concept	Where to look
Lazy evaluation (proven)	StreamPipelineFundamentals.demonstrateLazyEvaluation()
Operation order performance	StreamPipelineFundamentals.demonstrateOperationOrder()
flatMap mental model	StreamPipelineFundamentals.demonstrateFlatMap()
groupingBy + downstream	AdvancedCollectorsDemo.demonstrateGroupingBy()
Multi-level grouping	AdvancedCollectorsDemo.demonstrateMultiLevelGrouping()
teeing()	AdvancedCollectorsDemo.demonstrateTeeing()
Custom Collector (all 5 components)	OrderSummaryCollector.java
Parallel stream bug	ParallelStreamsDemo.demonstrateParallelBugAndFix()
Custom ForkJoinPool	ParallelStreamsDemo.demonstrateCustomForkJoinPool()
Production pipeline composition	OrderReportEngine.java

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Article

Read the full article on Medium: Java Streams: From forEach to Production-Grade Pipelines

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License

MIT — use freely, attribution appreciated.