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Java

732 problems50 sections📅 2026 curriculum

Learn Java by typing real, idiomatic code.

Type through modern Java: classes and interfaces, generics, the Stream API, records, and lambda expressions. Each snippet is idiomatic Java that compiles, and an AI tutor explains every line — how a stream pipeline evaluates lazily, why an interface beats inheritance here — so the verbosity turns into fluency and your typing keeps up.

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Run Java on your machine

1. Install the toolchain

brew install openjdk # or: sudo apt install default-jdk

2. Save your code as Hello.java, then compile & run

javac Hello.java && java Hello

Official install docs ↗

progress by section

1Fundamentals0/25start →

2Strings In Depth0/18start →

Case and trimming Index a string Iterate characters Classify characters Strings are immutable Replace text Search a string split and join substring Zero-pad numbers StringBuilder Compare strings Format a string Iterate lines Pad and align Strip a prefix String and bytes Chars as a stream

3Values & References0/16start →

Aliasing Copy a list == vs equals Immutable strings null and NPE Primitive arguments are copied Value vs reference types final variables Unmodifiable collections Pass-by-value of a reference Reassigning a parameter is local Records have value equality Defensive copy Boxed Integer identity Optional over null Shallow copy shares nesting

4Slices & Arrays0/18Unlock →

Membership First and last Reverse in place Resize with copyOf Slice an array Fill Find an index Sort in place subList view Stream over a range Sort by a key Filter and map an array Slice a stream subList is backed Swap and bulk copy Binary search Fixed-size chunks Sliding windows

5Data structures0/18Unlock →

ArrayList basics HashMap basics HashSet dedup Immutable List.of List operations Iterate a map Arrays and 2D arrays Group with computeIfAbsent ArrayDeque as a queue ArrayDeque as a stack Set algebra Count with merge PriorityQueue (min-heap)Sequenced collections Sequenced map Max-heap with a comparator Records as data TreeMap keeps order

6Iterators0/28Unlock →

7Traits0/21Unlock →

8Generics0/20Unlock →

9Lifetimes0/18Unlock →

Object references Block scope Returning a reference try/finally cleanup Nulling a reference Static holds for the program Custom AutoCloseable Capturing a field Effectively-final capture A class holding a reference Deferred creation Shadowing in a nested scope try-with-resources Capturing a loop value Resources close in reverse WeakReference Cleaner Weak reference after GC

10Closures0/20Unlock →

11Pattern Matching0/19Unlock →

Switch on a String Switch on an enum Pattern matching for instanceof Switch expression Arrow switch statement Switch as an expression Block arm with yield Binding used in the condition The null pattern Handle present or absent Type patterns in switch Guarded patterns Record deconstruction Record pattern in an if Sealed types in switch Generic record pattern Nested record patterns Recursive sum over a tree Evaluate an expression tree

12Error Handling0/21Unlock →

13Option & Result0/20Unlock →

14Conversions0/15Unlock →

Double.parseDouble Integer.parseInt String.valueOf Array to List char and int String and char array toBinaryString toHexString Integer.valueOf Autoboxing Numeric casting List to array parseInt with a radix int[] to List<Integer>Stream to int array

15Collection Conversions0/19Unlock →

Array to List Keys to List List to array List to Set Map from entries Set to sorted List List to TreeSet Values to List distinct in a stream Entries to List Flatten nested lists Frequency count with merge List to Deque HashMap to TreeMap Unique, keeping order Frequency with counting groupingBy Invert a map Collectors.toMap

16Math0/14Unlock →

Math constants Float functions Integer arithmetic min, max, clamp Rounding Logarithms and exp abs difference and signum Trigonometry Floor div and mod GCD and LCM hypot and atan2 Primality test Factorial with BigInteger Mean, median, stdev

17Encoding0/22Unlock →

18Random0/20Unlock →

19Data I/O0/18Unlock →

Parse CSV into rows Tab-separated values Build a CSV string Select one column CSV into a number matrix Split off the header Whitespace-separated matrix Read a data file Serialize records to CSV Skip blanks and comments Print an aligned table Write CSV to a temp file Write a matrix as text Append a row Stream lines with BufferedReader Parse key=value config Round-trip write then read Drop unparseable rows

20Linear Algebra0/21Unlock →

21Text Processing0/16Unlock →

Repeat a string Word count Capitalize the first letter Count occurrences Replace text Reverse word order Strip whitespace Count vowels Zero-pad a number Pad and align Character frequency Longest word Number the lines Title case Truncate with an ellipsis Levenshtein distance

22Date & Time0/13Unlock →

Day of the week Leap year LocalDate LocalDateTime Add and subtract time Days between dates Duration arithmetic Format a date-time Seconds to HH:MM:SS Instant and epoch Parse a date Period between dates ZonedDateTime and ZoneId

23Memory0/13Unlock →

Array default values == vs equals Value equality with records References vs values Arrays are references Autoboxing pitfall Identity hash code Integer caching String interning Cloning a 2D array Deep copy Shallow copy WeakReference

24Modules0/20Unlock →

25Macros0/20Unlock →

26Testing0/20Unlock →

27Algorithms0/11Unlock →

Reverse a string Greatest common divisor Binary search Pseudo-random numbers Palindrome check Fibonacci with memoization Merge two sorted arrays Partition around a pivot Fisher-Yates shuffle Two sum Quicksort

28Sorting Algorithms0/14Unlock →

Arrays.sort Check if sorted Bubble sort Sort with a Comparator Insertion sort Reverse order Selection sort Counting sort Multi-key sort Merge sort Shell sort Heapsort Quicksort Radix sort

29Expression Parsing0/6Unlock →

Evaluate postfix (RPN)Tokenize an expression Evaluate infix Recursive-descent calculator Right-associative power Shunting-yard

30Idioms0/9Unlock →

Enhanced for var and the ternary Streams over loops Comparator.comparing String.join and Collectors Optional over null Records Switch expressions try-with-resources

31OOP & Traits0/12Unlock →

A class Encapsulation Composition Inheritance Overriding methods Records Static members Abstract classes Polymorphism Record deconstruction Calling super Flexible constructor bodies

32Trait Objects0/14Unlock →

Default method via a reference Factory returns an interface A list of interfaces Take an interface Shared reference An array of interfaces Downcasting A list of functions instanceof dispatch A command table Generic vs interface param Interface extends interface A handler registry Sealed interface

33Design Patterns0/14Unlock →

Prototype copy Enum singleton Strategy Adapter Builder Dependency injection Factory method Observer (pub/sub)Command Decorator Lazy initialization State machine Object pool Actor (message passing)

34Async0/3Unlock →

supplyAsync and thenApply thenCompose and thenCombine allOf, exceptionally, orTimeout

35File I/O0/6Unlock →

Read a file Write a file Read all lines Append with APPEND mode Read lines with BufferedReader Buffered writing

36Filesystem0/18Unlock →

Does a path exist?Copy a file Create nested directories Files vs directories Build a path List a directory Rename or move Change the extension File metadata Path components Remove a tree Sorted listing A temp directory Absolute path Filter by extension Last modified time Total size of a tree Walk the tree

37Networking0/17Unlock →

Parse the request line Route by path Classify an IP address A socket address Bind an ephemeral port Build an HTTP response Read a line from a stream Resolve a hostname Half-close a connection TCP client Read timeout UDP send and receive Build a datagram packet HTTP/3 client A thread per connection TCP echo round-trip Host a web page

38Concurrency0/11Unlock →

Start a thread Virtual thread per task Start a virtual thread synchronized method AtomicInteger ExecutorService and Future CountDownLatch ReentrantLock Scoped values Producer/consumer queue Structured concurrency

39Unsafe & FFI0/14Unlock →

System.arraycopy ByteBuffer put and get A raw char buffer Identity hash code Reverse the bytes Byte order Direct ByteBuffer Pack bytes into an int Read a private field Overwrite a private field Cleaner Reinterpret float bits VarHandle on an array VarHandle on a field

40Commands0/14Unlock →

Command-line arguments Enumerate the arguments Env var with a default Read an environment variable Current directory Iterate all env vars Exit with a code System properties Pass env to a child Run a command Spawn and wait Command exit status Read all of stdin which via PATH

41Array0/10Unlock →

Contains Duplicate Two Sum Best Time to Buy and Sell Stock Maximum Subarray Container With Most Water Find Minimum in Rotated Sorted Array Product of Array Except Self Maximum Product Subarray Search in Rotated Sorted Array 3Sum

42Binary0/5Unlock →

Missing Number Number of 1 Bits Counting Bits Reverse Bits Sum of Two Integers

43String0/10Unlock →

Valid Anagram Valid Palindrome Valid Parentheses Encode and Decode Strings Group Anagrams Longest Substring Without Repeating Characters Longest Repeating Character Replacement Longest Palindromic Substring Palindromic Substrings Minimum Window Substring

44Matrix0/4Unlock →

Rotate Image Set Matrix Zeroes Spiral Matrix Word Search

45Interval0/5Unlock →

Meeting Rooms Merge Intervals Insert Interval Non-overlapping Intervals Meeting Rooms II

46Linked List0/6Unlock →

Reverse Linked List Linked List Cycle Merge Two Sorted Lists Remove Nth Node From End Merge K Sorted Lists Reorder List

47Tree0/14Unlock →

Maximum Depth of Binary Tree Invert Binary Tree Same Tree Lowest Common Ancestor of a BST Binary Tree Level Order Traversal Subtree of Another Tree Implement Trie (Prefix Tree)Kth Smallest Element in a BST Validate Binary Search Tree Add and Search Word Construct Binary Tree from Preorder and Inorder Binary Tree Maximum Path Sum Serialize and Deserialize Binary Tree Word Search II

48Graph0/8Unlock →

Number of Connected Components Number of Islands Clone Graph Course Schedule Longest Consecutive Sequence Graph Valid Tree Alien Dictionary Pacific Atlantic Water Flow

49Heap0/3Unlock →

Kth Largest Element in an Array Top K Frequent Elements Find Median from Data Stream

50Dynamic Programming0/11Unlock →

Climbing Stairs House Robber Unique Paths Coin Change House Robber II Jump Game Combination Sum IV Decode Ways Longest Common Subsequence Longest Increasing Subsequence Word Break