Java code explainers

java

public List<String> collectTagsFromArticles(List<Article> articles) {
    return articles.stream()
        .flatMap(article -> article.getTags().stream())
        .map(String::trim)

Flattening nested data with Java streams

streams flatmap collectors

Intermediate 7 steps

java

@Component
@Order(20)
public class ProductSearchIndexRunner implements ApplicationRunner {

Rebuilding a search index at Spring startup

startup-hook batching streaming

Intermediate 8 steps

java

public final class FileChecksum {
 
    private static final char[] HEX = "0123456789abcdef".toCharArray();

Streaming a SHA-256 file checksum in Java

hashing streams resource-management

Intermediate 7 steps

java

@Service
public class OrderCheckoutService {
 
    private final OrderRepository orderRepository;

How @Transactional guards a Spring checkout

dependency-injection transactions atomicity

Intermediate 6 steps

java

@Service
public class OrderMetricsService {
 
    private final MeterRegistry registry;

Instrumenting orders with Micrometer in Spring

metrics micrometer observability

Intermediate 8 steps

java

@RestController
@RequestMapping("/api/files")
public class FileUploadController {

Handling secure file uploads in Spring

file-upload input-validation path-traversal

Intermediate 10 steps

java

package com.example.fs;
 
import java.io.IOException;
import java.nio.file.FileSystems;

Watching a directory with Java NIO

file-io event-loop resource-management

Intermediate 8 steps

java

public final class QueryParams {
 
    private final Map<String, List<String>> params;

Parsing and building URL query strings in Java

parsing multimap url-encoding

Intermediate 9 steps

java

public final class ShippingAddress {
    private final String recipient;
    private final String line1;
    private final String line2;

Building immutable objects with the Builder pattern

builder-pattern immutability validation

Intermediate 7 steps

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