- 01Getting Started
- 02Your First Output (System.out.println)
- 03Working with Variables
- 04Conditionals (if statements)
- 05Loops (for statements)
- 06Writing a Method (Function)
- 07Working with Arrays
- 08Loops (while statements)
- 09Classes (Object-Oriented Programming)
- 10Error Handling (try-catch)
- 11Sorting an Array
- 12Recursive Functions
- 13Using the Standard Library
- 14The switch Statement
- 15The Ternary Operator
- 16Inheritance (Extending a Class)
- 17Writing Comments
- 18Logical Operators (&&, ||, !)
- 19Constants (final)
- 20Splitting and Joining Strings (split, join)
- 21Writing Null-Safe Code
- 22Searching a Collection (contains)
- 23Transforming an Array (Stream map)
- 24Two-Dimensional Arrays (Tabular Data)
- 25Writing a Custom Exception Class
- 26Method Overloading
- 27Working with HashSet (Sets)
- 28Checking Correctness with assert (Your First Step into Testing)
- 29Higher-Order Functions (Passing a Function as an Argument)
- 30Stacks and Queues (Basic Data Structures)
- 31Type Conversion (Casting) Basics
- 32Intro to Regular Expressions (Pattern Matching)
- 33The Binary Search Algorithm
- 34Building a Caesar Cipher (a Character-Shifting Cipher)
- 35Understanding How Bubble Sort Works
- 36Building and Displaying Dates (Basic Year/Month/Day Operations)
- 37Writing Multiple Test Cases Together
- 38Speeding Up Calculations with Memoization (Caching)
- 39Normalizing Strings (trim, Unifying Case)
- 40Shallow Copy vs. Deep Copy
- 41Enum (Enumerated Types) Basics
- 42Flattening a List
- 43Reversing a String and Checking for Palindromes
- 44Pairing Up Two Lists (the zip operation)
- 45Rounding Numbers (floor, ceil, round)
- 46Multi-Line Strings (Text Blocks)
- 47Functions That Return Multiple Values (record)
- 48Finding the GCD and LCM (the Euclidean Algorithm)
- 49Formatting Numbers (Padding Digits, Decimal Places)
- 50Cleanup Logic with try/catch/finally
- 51Writing Generic Functions
- 52File Reading and Writing Basics
- 53Generating Random Numbers
- 54Bitwise Operators (AND, OR, XOR, shifts)
- 55Reading Command-Line Arguments
- 56Waiting for a Fixed Amount of Time (Thread.sleep)
- 57Watch Out for Floating-Point Rounding Errors
- 58Reading from Standard Input
- 59FizzBuzz (the Classic Practice Problem)
- 60Checking Whether a Number Is Prime
- 61Set Operations (Union, Intersection, Difference)
- 62Converting Number Bases (Binary, Hex)
- 63Checking Balanced Parentheses (an Application of Stacks)
- 64Checking Whether Two Words Are Anagrams
- 65Checking Whether a Year Is a Leap Year
- 66Converting Temperature (Celsius to Fahrenheit)
- 67Prime Factorization
- 68[Applied] Build a Simple Inventory Management Tool
Flattening a List
This lesson covers flattening a nested list in Java using Stream's flatMap, so you can handle a complex data structure more simply. It's for anyone searching "Java flatMap flatten usage."
When a list contains nested lists, expanding it into a single "flat" list is called flattening. Java's Stream .flatMap() makes this possible.
The example expands a nested list into a single list with nested.stream().flatMap(List::stream).collect(Collectors.toList()). Notice the method-reference syntax, List::stream.
A common early mistake is not seeing the difference from .map(). .map() alone leaves lists nested inside lists, while .flatMap() flattens it by one level โ a distinction worth understanding.
In real projects, this is a genuinely useful, real-world technique for handling a nested data structure as a single unified list.
๐งช This site can't compile or run Java directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser โ nothing is sent anywhere).