Oracle Java SE 21 study plan

Build and format LocalDate, LocalTime, LocalDateTime and ZonedDateTime values, converting between them and Instant

Write code that uses Period and Duration correctly and explain why one throws on LocalDate and the other does not

Format and parse dates with DateTimeFormatter patterns and localization, including custom patterns and predefined constants

Declare classes with encapsulation, inheritance, and polymorphism, and override equals, hashCode and toString consistently

Create and use interfaces, abstract classes, enums and records, and choose correctly between them for a given design

Declare, initialize and traverse one-dimensional and multi-dimensional arrays, including Arrays.asList, Arrays.sort with Comparator, and Arrays.equals

Select the right collection (List, Set, Map, Deque) for a scenario and justify the choice in a comment

Compare ArrayList, LinkedList, HashSet, TreeSet, HashMap and TreeMap on ordering, duplicates, and null handling

Iterate and mutate collections safely using Iterator, ListIterator, and the removeIf/forEach default methods

Use generics basics: bounded wildcards, <? extends T> vs <? super T>, and type inference in method calls

Write lambdas for the built-in functional interfaces (Function, BiFunction, Predicate, Consumer, Supplier, UnaryOperator) and use method references in all four forms

Build stream pipelines using filter, map, flatMap, distinct, sorted, peek, limit and skip

Reduce and collect results using reduce, Collectors.toList/toSet/toMap, groupingBy, partitioningBy, joining and counting, including downstream collectors

Compare Optional-returning terminal operations (findFirst, findAny, max, min) and handle empty results

Explain and demonstrate lazy evaluation, intermediate vs terminal operations, and why a stream cannot be reused

Create threads with Runnable and Callable, and submit work to an ExecutorService, shutting it down correctly

Synchronize shared mutable state with synchronized blocks, Lock/ReentrantLock, and atomic classes, and identify a lost-update race in given code

Use concurrent collections (ConcurrentHashMap, CopyOnWriteArrayList, BlockingQueue) and explain why they beat synchronized wrappers

Use parallel streams and ForkJoinPool while identifying when parallelism is unsafe or slower

Diagnose deadlock, starvation and visibility problems, and use volatile correctly

Write try/catch/finally and try-with-resources blocks, and predict output order when finally returns or throws

Distinguish checked from unchecked exceptions and create a custom exception with a cause chain

Use multi-catch and rethrow with improved type inference, and wrap exceptions without losing the original cause

Write if/else, switch statements and switch expressions with arrow labels, yield, and pattern matching for types

Write and control loops (for, enhanced for, while, do-while) including labeled break/continue, and use var correctly in local scope

Read and write text files with Files.newBufferedReader/Writer, readAllLines and lines(), and choose buffered vs unbuffered correctly

Use Path and Paths/Files for path resolution, normalization, relativization and directory traversal with Files.walk

Copy, move, delete and inspect files and directories with the Files utility methods, handling attributes and symbolic links

Serialize and deserialize objects with ObjectOutputStream/ObjectInputStream and explain serialVersionUID and transient

Use java.nio.file streams and InputStream/OutputStream with try-with-resources, and convert between byte and character streams

Connect to a database with DriverManager, close resources with try-with-resources, and read connection metadata

Execute Statement and PreparedStatement queries, bind parameters safely, and process ResultSet with correct column access and null handling

Perform INSERT/UPDATE/DELETE and read generated keys, and manage transactions with setAutoCommit, commit and rollback

Organize code into packages, compile with javac -d, package with jar, and run with java -cp including a multi-release or classpath ordering scenario

Describe the module system basics: module-info.java, requires, exports, and how to run a modular jar

Take three full-length timed practice exams and log every missed question by domain

Re-drill the two domains with the lowest practice-exam accuracy using targeted exercises

Re-read your own notes on stream collectors, concurrency primitives and exception ordering, and re-code one example for each from memory

Practice reading unfamiliar code and predicting output under time pressure without compiling