Java code explainers

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

java

@Service
public class PaymentGatewayClient {
 
    private static final Logger log = LoggerFactory.getLogger(PaymentGatewayClient.class);

Resilient payment calls with Spring Retry

retry backoff fault-tolerance

Intermediate 7 steps

java

public final class Debouncer<T> {
 
    private final ScheduledExecutorService scheduler =
            Executors.newSingleThreadScheduledExecutor(runnable -> {

How a debouncer collapses bursts in Java

debounce concurrency scheduling

Intermediate 9 steps

java

package com.example.logs;
 
import java.io.IOException;
import java.io.UncheckedIOException;

Counting HTTP statuses with Java streams

streams regex file-io

Intermediate 6 steps

java

@Service
public class GitHubClient {
 
    private final WebClient webClient;

Calling the GitHub API with Spring WebClient

reactive http-client dependency-injection

Intermediate 7 steps

java

public class ThumbnailProcessor {
 
    private static final int MAX_CONCURRENCY = 4;

Bounded parallel thumbnail rendering in Java

concurrency thread-pool futures

Intermediate 7 steps

java

public class SortedListMerger {
 
    public static int[] merge(int[] a, int[] b) {
        int[] result = new int[a.length + b.length];

Merging two sorted arrays in Java

two-pointers merging arrays

Beginner 6 steps

java

import java.util.ArrayDeque;
import java.util.Deque;
 
public final class RollingAverage {

A rolling average over a sliding window

sliding-window running-sum deque

Intermediate 7 steps

java

@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = StrongPasswordValidator.class)
@Documented

Building a custom @StrongPassword validator in Spring

bean-validation annotations regex

Intermediate 7 steps

java

@Component
public class JwtAuthenticationFilter extends OncePerRequestFilter {
 
    private final JwtTokenProvider tokenProvider;

How a JWT auth filter works in Spring

authentication jwt servlet-filter

Intermediate 8 steps