Module 4: Stream API – Core Concepts

Target Audience

• Java developers new to Streams
• Developers comfortable with Lambdas and Method References
• Anyone preparing for real-world Java 8 data processing

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1. Objective of This Module

By the end of this module, you will:

• Understand what Streams are
• Learn how Streams differ from Collections
• Understand the Stream pipeline
• Learn intermediate vs terminal operations
• Grasp lazy evaluation

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2. What Is a Stream?

A Stream is:

• A sequence of elements
• Supporting functional-style operations
• Processed declaratively, not imperatively

👉 A Stream does not store data.
👉 It processes data from a source (Collection, Array, IO, etc.).

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3. Stream vs Collection

Aspect	Collection	Stream
Storage	Stores data	Does not store data
Iteration	External	Internal
Reusability	Can be reused	Cannot be reused
Execution	Immediate	Lazy

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4. Creating Streams

From a Collection

List<String> list = List.of("A", "B", "C");
Stream<String> stream = list.stream();

From Values

Stream<Integer> numbers = Stream.of(1, 2, 3);

From Arrays

Stream<String> stream = Arrays.stream(array);

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5. Stream Pipeline

A Stream pipeline consists of:

1. Source
2. Intermediate operations
3. Terminal operation

list.stream()
    .filter(s -> s.startsWith("A"))
    .map(String::toUpperCase)
    .forEach(System.out::println);

Nothing happens until the terminal operation is called.

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6. Intermediate Operations

• Return a Stream
• Are lazy
• Can be chained

Examples:

• filter
• map
• flatMap
• sorted
• distinct

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7. Terminal Operations

• Trigger stream execution
• Produce a result or side effect

Examples:

• forEach
• collect
• count
• findFirst
• anyMatch

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8. Lazy Evaluation

Streams use lazy evaluation:

• Intermediate operations are not executed immediately
• Execution starts only with a terminal operation
• Operations run element-by-element

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9. Stateless vs Stateful Operations

Stateless

• map, filter

Stateful

• sorted, distinct

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10. Single-Use Streams

Streams cannot be reused.

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11. Hands-On Exercises

• Filter names starting with a letter
• Identify stream pipeline parts
• Predict output before execution

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12. Summary

• Streams are lazy
• They process data, not store it
• Correct understanding prevents misuse

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13. What’s Next?

➡️ Phase 2 – Core Skills ➡️ Module 5: Stream API – Practical Operations