Java code explainers

java

import java.time.Instant;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

Converting timestamps across time zones in Java

date-time time-zones instant

Intermediate 6 steps

java

package com.example.payments.config;
 
import com.stripe.StripeClient;
import org.springframework.beans.factory.annotation.Value;

Swapping payment gateways by profile in Spring

dependency-injection profiles configuration

Intermediate 6 steps

java

public class EventBus {
 
    private final Map<Class<?>, List<Subscriber>> subscribers = new ConcurrentHashMap<>();
    private final Executor executor;

Building an annotation-driven EventBus in Java

publish-subscribe reflection annotations

Intermediate 7 steps

java

public class OrderSerializer {
 
    private final ObjectMapper mapper;

Configuring a reusable Jackson ObjectMapper

serialization json builder-pattern

Intermediate 8 steps

java

package com.example.maintenance;
 
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

How a scheduled cleanup job runs in Spring

scheduling cron transactions

Intermediate 6 steps

java

@Component
public class RateLimitingFilter extends OncePerRequestFilter {
 
    private static final int MAX_REQUESTS = 100;

How a rate-limiting filter works in Spring

rate limiting concurrency servlet filter

Advanced 8 steps

java

public List<Employee> parseEmployees(Path csvPath) throws IOException {
    List<Employee> employees = new ArrayList<>();
 
    try (BufferedReader reader = Files.newBufferedReader(csvPath, StandardCharsets.UTF_8)) {

Parsing a CSV into typed objects in Java

csv-parsing file-io try-with-resources

Intermediate 7 steps

java

public final class RetryExecutor {
 
    private final int maxAttempts;
    private final Duration initialDelay;

Exponential backoff retry in Java

retry exponential-backoff jitter

Intermediate 8 steps

java

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.locks.ReentrantReadWriteLock;

A thread-safe LRU cache in Java

lru-cache concurrency linkedhashmap

Intermediate 7 steps

java

public record ServerConfig(String host, int port, boolean tls, List<String> allowedOrigins) {
 
    public ServerConfig {
        Objects.requireNonNull(host, "host is required");

Validating config with a Java record

records validation immutability

Intermediate 9 steps

java

@RestController
@RequestMapping("/api/users")
public class UserController {

How a Spring REST controller maps users

rest-api dependency-injection dto-mapping

Intermediate 7 steps

java

import java.util.concurrent.atomic.AtomicInteger;
 
public final class RequestCounter {

A thread-safe request counter with AtomicInteger

concurrency atomics lock-free

Intermediate 6 steps

java

@Service
public class ReportGenerationService {
 
    private final ReportRepository reportRepository;

Async report generation with Spring @Async

async dependency-injection completablefuture

Intermediate 7 steps

java

package com.example.billing;
 
import java.math.BigDecimal;
import java.math.RoundingMode;

Locale-aware currency formatting in Java

internationalization rounding immutability

Intermediate 6 steps

java

@Service
public class ProductCatalogService {
 
    private final ProductRepository productRepository;

How Spring cache annotations keep data fresh

caching cache-invalidation dependency-injection

Intermediate 7 steps

java

@RestController
@RequestMapping("/api/users")
public class UserController {

Bean Validation in a Spring REST controller

validation rest-api exception-handling

Intermediate 9 steps

java

@Service
public class OrderService {
 
    private final OrderRepository orderRepository;

Decoupling side effects with Spring events

event-driven transactions decoupling

Intermediate 8 steps

java

public Map<Long, List<Order>> ordersByCustomer(List<Order> orders) {
    return orders.stream()
            .collect(Collectors.groupingBy(Order::getCustomerId));
}

Grouping streams with Java Collectors

streams grouping collectors

Intermediate 5 steps