☕ EPAM Java Introduction: Full Course Solutions

"Don't be fooled by the word 'Basics' in the title."

My solutions for the [Beginner] Java Introduction course by EPAM, a structured 141-hour Java curriculum across 6 modules. Despite the name, this course goes surprisingly deep. It took a few months of consistent work alongside a full-time job, and it genuinely changed how I think about software design.

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📋 Course Overview

Module	Topic	Focus
01	Java Essentials	Language foundations
02	Java Basics	Specialized class types
03	Java Collections	Data structures
04	Java I/O Streams	File & data I/O
05	Lambdas & Streams	Functional programming
06	Exam Tasks	Applied assessments

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🗂️ Repository Structure

src/
├── module_1_java_essentials/        # Core syntax & OOP foundations
├── module_2_java_basics/            # Nested classes, generics, enums
├── module_3_collections/            # Data structures & algorithms
├── module_4_io_streams/             # File I/O, NIO.2, serialization
├── module_5_lambdas_and_streams/    # Functional programming
└── module_6_exam_tasks/             # Final exam solutions

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📚 What I Learned

Module 1: Java Essentials

The foundation. This module goes much further than basic syntax and forces you to understand how Java works at the memory level.

• First Programs & Data Types: primitives vs objects, type casting, how the JVM handles memory for different types
• Conditions & Loops: control flow, subtleties of switch, loop optimization
• Arrays: single and multi-dimensional, in-place algorithms (cycle swap, spiral fill, matrix transposition, local maxima removal)
• Classes: constructors, method overloading, encapsulation, the this keyword, object lifecycle
• OOP: inheritance chains, polymorphism in practice, method overriding vs overloading, what abstraction actually means
• Abstract Classes & Interfaces: when to use which, default methods, Comparable vs Comparator
• Working with Strings: String pool internals, StringBuilder vs StringBuffer, regex, path conversion logic
• Exceptions: checked vs unchecked, exception chaining, custom exception hierarchies, finally block behavior

Notable tasks: Battleship 8x8 (bit manipulation using long), Spiral matrix fill, DecrementingCarousel (stateful OOP design), Sprint Planning (full inheritance hierarchy with real business logic)

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Module 2: Java Basics

Where Java starts feeling like a real language. Each topic unlocks a new level of expressiveness.

• Nested Classes: inner, static nested, anonymous, and local classes with practical use cases for each
• Annotations: built-in annotations, retention policies, how the compiler uses metadata
• Generics: type erasure, bounded wildcards (<? extends T>, <? super T>), generic methods, why raw types are dangerous
• Enums: enums as full classes with fields and methods, EnumSet, EnumMap
• Wrapper Classes: autoboxing/unboxing pitfalls, Integer cache behavior, static utility methods
• Optional: null-safe programming, chaining with map, flatMap, orElseGet
• Code Documentation: Javadoc conventions, self-documenting code principles

Notable tasks: ArithmeticExpressions DSL (anonymous classes), ContactBook (multiple nested class types), Battleship8x8 (bit operations via wrapper class statics)

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Module 3: Java Collections

The most practically dense module. You don't just use collections here, you understand their internals, time complexity, and when each one is the right choice.

• Introduction: the Collections Framework design, Iterable vs Collection vs List, iterator pattern
• Lists & Queues: ArrayList vs LinkedList tradeoffs, custom SimpleArrayList from scratch (dynamic resizing at 75% load factor), custom DoublyLinkedList from scratch with head/tail pointers
• Sets: HashSet (hash buckets), LinkedHashSet (insertion order), TreeSet (red-black tree), EnumSet (bit vector)
• Maps: HashMap internals (hashing, collision resolution), TreeMap with custom Comparator, BooksCatalog (TreeMap + Comparable), StudentsGradebook with nested TreeMap<Student, HashMap<String, BigDecimal>>
• Legacy Collections: Vector, Stack, Hashtable, Properties and why they're thread-safe but slow
• Collection Algorithms: Collections.sort, Collections.shuffle, Collections.binarySearch, Big O across all data structures

Notable tasks: Custom ArrayList from scratch, DoublyLinkedList from scratch, BooksCatalog with Comparable, BirthJournal (immutable Map), NewPostOfficeStorage (aggregate pattern)

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Module 4: Java I/O Streams

One of the trickier modules. I/O in Java is layered: byte streams, character streams, buffered streams, object streams. Knowing when to wrap what took real practice.

• Byte & Character Streams: InputStream/OutputStream vs Reader/Writer, the decorator pattern in the stream hierarchy
• Other I/O Streams: BufferedReader, Scanner, DataInputStream, PrintWriter, stream chaining
• NIO.2: Path, Files, Paths, directory walking with Files.walk(), building a file tree with size calculation
• Serialization: Serializable, transient keyword, serialVersionUID, complex object graph serialization with the Order/Item/ItemCharacteristic hierarchy

Notable tasks: FileTree (NIO.2 directory traversal with tree-style output), Order serialization (polymorphic object graph), VehicleRentalAgency IO task

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Module 5: Lambdas & Streams

Where Java starts feeling modern. Functional programming flips how you think about data: instead of writing loops that mutate state, you describe transformations as a pipeline.

• Lambda Expressions: syntax variations, effectively final variables, method references (::)
• Functional Interfaces: Predicate, Consumer, Supplier, Function, BiFunction, UnaryOperator, composing with andThen, compose, negate
• Stream API: lazy evaluation, intermediate vs terminal operations, filter, map, flatMap, sorted, reduce, collect, groupingBy, partitioningBy
• Collectors: toList, toSet, toMap, groupingBy, joining, counting, averagingInt
• Optional with Streams: combining them for null-safe pipelines

Notable tasks: MovieQueries (Stream API over a movie/person domain), VehicleRentalAgency (full domain model with Sort/Find interfaces using only Streams + Lambdas), client ranking by order totals

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Module 6: Exam Tasks

Standalone assessed tasks that combine everything. Closer to real mini-projects with UML diagrams, proper interfaces, and full test suites.

• OOP Model Exam: full domain model with inheritance, encapsulation, proper equals/hashCode/toString
• Collections Exam: filtering, sorting, grouping across multiple collection types
• Streams Exam: VehicleRentalAgency with full Stream API
• IO Exam: VehicleRentalAgency with CSV file handling

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🛠️ Tech Stack

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💡 Things That Actually Stuck

A few things I didn't fully get until working through this:

The Collections Framework is a great example of OOP design. Understanding why ArrayList extends AbstractList which implements List which extends Collection which extends Iterable taught me more about interface design than any article I've read.

Generics are purely a compile-time feature. Type erasure means they don't exist at runtime. Once that clicked, wildcards stopped being confusing.

Streams are lazy. Nothing runs until a terminal operation is called. That's why you can chain 10 intermediate operations and still only make one pass through the data.

Serialization is fragile. Changing a class can break deserialization of previously serialized objects if you're not managing serialVersionUID. This is why most modern Java code uses JSON instead.

Anonymous classes are not just pre-lambda lambdas. They can hold state, implement multiple methods, and access the enclosing class, things a lambda can't do.

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📊 Stats

• Total hours: ~141
• Platform: EPAM Learn with Autocode automated testing
• Exercises: 60+ individual coding tasks
• Modules: 6, each with multiple sub-topics

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