Streams & Modern Java

The Stream API, Optional, method references, the Date and Time API and other modern Java features used in real code and interviews.

This section is being written. There are no lessons in it yet: below is the plan for what it will cover. It opens in the syllabus as soon as its first lessons are ready.

What you will be able to do

Use the built-in functional interfaces and every kind of method reference with confidence
Rewrite loops as clear stream pipelines and know when a loop is still better
Group, count and summarise data with collectors
Use Optional correctly and avoid its common misuse
Work with dates, times and time zones using java.time
Model data with records and sealed types and handle it with Java 21 pattern matching

What it will cover

8.1Functional Interfaces in Practice
The ready-made interfaces in java.util.function and how to combine them.
- Function, Predicate, Consumer and Supplier
- BiFunction, UnaryOperator and BinaryOperator
- Primitive versions like IntPredicate and ToIntFunction
- Combining with andThen, compose, and, or, negate
- Effectively final variables in lambdas
8.2Method References in Depth
All four kinds of method reference and when a lambda reads better.
- Static method references
- Bound instance references like System.out::println
- Unbound references like String::length
- Constructor references like ArrayList::new
- Choosing between a lambda and a method reference
8.3Stream Basics
What a stream is, how a pipeline runs and why streams are lazy.
- Creating streams from collections, arrays and Stream.of
- Source, intermediate and terminal operations
- Lazy evaluation and the order elements flow
- A stream can be used only once
- Loop versus stream: the same task both ways
8.4Transforming Streams
The intermediate operations you will use every day.
- filter and map
- flatMap for nested lists
- distinct, sorted with a Comparator
- limit, skip, takeWhile and dropWhile
- peek for debugging only
8.5Terminal Operations & Reduction
Get a result out of a stream: a value, a list, a check or a single combined answer.
- forEach, count, min and max
- anyMatch, allMatch, noneMatch
- findFirst and findAny
- reduce with an identity and an accumulator
- toList() versus collect(Collectors.toList())
8.6Collectors
Build maps, groups and reports from streams in one pipeline.
- toSet, toMap and handling duplicate keys
- groupingBy with downstream collectors
- partitioningBy
- counting, summingInt, averagingDouble and summarizing
- joining and mapping
- Sorted results with TreeMap and LinkedHashMap suppliers
8.7Primitive & Parallel Streams
Number streams, generated streams and an honest look at parallel streams and stream pitfalls.
- IntStream.range, sum, average and boxed
- Stream.iterate and Stream.generate
- Side effects and why they break streams
- Parallel streams: when they help and when they hurt
- Readability and performance compared with loops
8.8Optional
Return 'maybe a value' safely instead of null, and avoid the usual Optional mistakes.
- Optional.of, ofNullable and empty
- map, flatMap, filter and ifPresent
- orElse versus orElseGet versus orElseThrow
- Where Optional belongs and where it does not
- Optional results from stream operations
8.9Date & Time API
Handle dates, times, durations and time zones correctly with java.time.
- LocalDate, LocalTime and LocalDateTime
- Instant, ZonedDateTime and Indian Standard Time
- Period and Duration; adding and comparing dates
- Formatting and parsing with DateTimeFormatter
- Why the old Date and Calendar classes are avoided
8.10Records & Sealed Types in Practice
Use records for real data and sealed classes and interfaces to close a type hierarchy.
- Records as data carriers and map keys in streams
- Nested records and static factory methods
- Sealed classes and interfaces with permits
- final, sealed and non-sealed subclasses
- Modelling results and shapes with a sealed interface
8.11Pattern Matching & Newer Language Features
Java 17 to 21 features: switch patterns, record patterns, var and text blocks.
- Type patterns in switch with when guards
- Record patterns and deconstruction
- Exhaustive switch over sealed types and case null
- var: where it helps and where it hides meaning
- Text blocks for multi-line strings
- Sequenced collections in Java 21
8.12Modern Java Mini Projects
Small projects that use streams, collectors, Optional and java.time together.
- Refactor a loop-heavy program into streams
- Sales report with grouping and totals
- Attendance and leave tracker with java.time
- Order processing with sealed types and pattern matching

Start with what’s live

These sections are ready to study now.

See the whole roadmap

What you will be able to do

Use the built-in functional interfaces and every kind of method reference with confidence

Rewrite loops as clear stream pipelines and know when a loop is still better

Group, count and summarise data with collectors

Use Optional correctly and avoid its common misuse

Work with dates, times and time zones using java.time

Model data with records and sealed types and handle it with Java 21 pattern matching

What it will cover

8.1Functional Interfaces in Practice

The ready-made interfaces in java.util.function and how to combine them.

Function, Predicate, Consumer and Supplier
BiFunction, UnaryOperator and BinaryOperator
Primitive versions like IntPredicate and ToIntFunction
Combining with andThen, compose, and, or, negate
Effectively final variables in lambdas

8.2Method References in Depth

All four kinds of method reference and when a lambda reads better.

Static method references
Bound instance references like System.out::println
Unbound references like String::length
Constructor references like ArrayList::new
Choosing between a lambda and a method reference

8.3Stream Basics

What a stream is, how a pipeline runs and why streams are lazy.

Creating streams from collections, arrays and Stream.of
Source, intermediate and terminal operations
Lazy evaluation and the order elements flow
A stream can be used only once
Loop versus stream: the same task both ways

8.4Transforming Streams

The intermediate operations you will use every day.

filter and map
flatMap for nested lists
distinct, sorted with a Comparator
limit, skip, takeWhile and dropWhile
peek for debugging only

8.5Terminal Operations & Reduction

Get a result out of a stream: a value, a list, a check or a single combined answer.

forEach, count, min and max
anyMatch, allMatch, noneMatch
findFirst and findAny
reduce with an identity and an accumulator
toList() versus collect(Collectors.toList())

8.6Collectors

Build maps, groups and reports from streams in one pipeline.

toSet, toMap and handling duplicate keys
groupingBy with downstream collectors
partitioningBy
counting, summingInt, averagingDouble and summarizing
joining and mapping
Sorted results with TreeMap and LinkedHashMap suppliers

8.7Primitive & Parallel Streams

Number streams, generated streams and an honest look at parallel streams and stream pitfalls.

IntStream.range, sum, average and boxed
Stream.iterate and Stream.generate
Side effects and why they break streams
Parallel streams: when they help and when they hurt
Readability and performance compared with loops

8.8Optional

Return 'maybe a value' safely instead of null, and avoid the usual Optional mistakes.

Optional.of, ofNullable and empty
map, flatMap, filter and ifPresent
orElse versus orElseGet versus orElseThrow
Where Optional belongs and where it does not
Optional results from stream operations

8.9Date & Time API

Handle dates, times, durations and time zones correctly with java.time.

LocalDate, LocalTime and LocalDateTime
Instant, ZonedDateTime and Indian Standard Time
Period and Duration; adding and comparing dates
Formatting and parsing with DateTimeFormatter
Why the old Date and Calendar classes are avoided

8.10Records & Sealed Types in Practice

Use records for real data and sealed classes and interfaces to close a type hierarchy.

Records as data carriers and map keys in streams
Nested records and static factory methods
Sealed classes and interfaces with permits
final, sealed and non-sealed subclasses
Modelling results and shapes with a sealed interface

8.11Pattern Matching & Newer Language Features

Java 17 to 21 features: switch patterns, record patterns, var and text blocks.

Type patterns in switch with when guards
Record patterns and deconstruction
Exhaustive switch over sealed types and case null
var: where it helps and where it hides meaning
Text blocks for multi-line strings
Sequenced collections in Java 21

8.12Modern Java Mini Projects

Small projects that use streams, collectors, Optional and java.time together.

Refactor a loop-heavy program into streams
Sales report with grouping and totals
Attendance and leave tracker with java.time
Order processing with sealed types and pattern matching