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Java Stream API Guide

This document provides a comprehensive guide to the Java Stream API and Collectors, detailing methods with examples and corner cases. It explains the structure of streams, including intermediate and terminal operations, and highlights the use of Collectors for transforming stream elements. Key points include handling corner cases and the importance of merge functions in certain operations.

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5 views2 pages

Java Stream API Guide

This document provides a comprehensive guide to the Java Stream API and Collectors, detailing methods with examples and corner cases. It explains the structure of streams, including intermediate and terminal operations, and highlights the use of Collectors for transforming stream elements. Key points include handling corner cases and the importance of merge functions in certain operations.

Uploaded by

techbuzz3934
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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Java Stream API & Collectors —

Complete Guide
Introduction
This document explains the Java Stream API and Collectors in detail. Each method is covered
with examples, corner cases, and explanations. Frequently used methods are highlighted.

Streams Overview
A Stream is a sequence of elements supporting aggregate operations. It consists of a pipeline
of: Source → Intermediate Operations → Terminal Operation.

// Example: Stream pipeline


List<String> result = words.stream()
.filter(w -> w.length() > 3)
.map(String::toUpperCase)
.sorted()
.toList();

Intermediate Operations
Stateless: filter, map, flatMap, peek, mapMulti

Stateful: distinct, sorted, limit, skip, takeWhile, dropWhile

// Flatten nested lists and remove duplicates


List<String> tags = posts.stream()
.flatMap(p -> p.getTags().stream())
.distinct()
.sorted()
.toList();

Terminal Operations
forEach, toArray, reduce, collect, min, max, count, anyMatch, allMatch, findFirst, findAny,
toList
// Reduce vs Collect
int sum = numbers.stream().reduce(0, Integer::sum);
int sum2 = numbers.stream().collect(Collectors.summingInt(x -> x));

Collectors
Collectors help transform stream elements into collections, maps, strings, or perform
aggregations.

toMap and groupingBy

// toMap with merge function


Map<String,Integer> scores = entries.stream()
.collect(Collectors.toMap(
e -> e.getName(),
e -> e.getScore(),
Integer::max
));

// groupingBy
Map<String, Long> deptCounts = emps.stream()
.collect(Collectors.groupingBy(
Emp::getDepartment,
Collectors.counting()
));

Corner Cases
• Empty min/max returns Optional — must handle.
• toMap throws IllegalStateException on duplicates unless merge function provided.
• peek doesn’t execute without a terminal op.
• Stream.toList() (Java 16+) returns unmodifiable list.

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