Oracle Java SE 21 case studies
Oracle Java SE 21 · Put your knowledge to work in context.
The Oracle Java SE 21 Developer credential - exam code 1Z0-830 - is Oracle's professional-level certification for programmers who build applications on the Java Platform, Standard Edition, version 21. It is aimed at working developers rather than beginners: candidates are expected to already write, compile, and debug Java code comfortably, and to understand the library APIs well enough to reason about how a given snippet will behave at runtime. The Oracle Java SE 21 case studies are 5 cases and about 2,978 words.
1The December Report Filed Under the Wrong Year
A payroll service writes a nightly CSV summary. Each LocalDate is formatted with DateTimeFormatter.ofPattern("YYYY-MM-dd") .
553 words, about 3 min2Fall-Through in an Order State Machine
An order service models state as enum Status { NEW, PAID, SHIPPED, REFUNDED, CLOSED } . One method uses a colon-form switch: case NEW logs "new" and then falls into case PAID 's audit call; case REFUNDED increments refundCount and then falls into case CLOSED 's closeCount++ before the break .
599 words, about 3 min3Ordering and Duplicates in a Parallel Grouping Pipeline
A log aggregator reads 200,000 lines and builds Map<String, List<Event grouped by tenant with Collectors.groupingBy(Event::tenant) . First, a de-duplication step using Collectors.toMap(Event::id, e - e) fails on retried events with IllegalStateException: Duplicate key .
554 words, about 3 min4Virtual Threads That Stop Scaling
A document service blocks about 200 ms per request on a remote parser. With a fixed pool of 200 platform threads and 5,000 concurrent requests, p95 latency is 4 s.
624 words, about 3 min5A 40-Minute Import and 1,900 Duplicate Rows
A nightly importer streams a 5,000-line CSV with Files.newBufferedReader and, for each line, concatenates an INSERT for a single Statement taken from a pooled connection configured with autoCommit=true , then calls executeUpdate . The job takes 40 minutes.
648 words, about 3 min