Best Java IDEs for AI Coding, Testing and CI/CD (2026)
Compare IntelliJ IDEA, VS Code, Eclipse, Cursor and Devin Desktop for AI-assisted Java development in 2026. AI models, free tiers, pricing, testing and CI/CD support at a glance.
“Core Java” is Sun’s term, used to refer to Java SE, the standard edition and a set of related technologies, like the Java VM, CORBA, etc. This is mostly to differentiate from others like Java ME or Java EE. “Core Java” is Oracle’s definition and refers to subset of Java SE technologies.
The Core Java technologies and application programming interfaces (APIs) are the foundation of the Java Platform, Standard Edition (Java SE). They are used in all classes of Java programming, from desktop applications to Java EE applications.
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Tutorials
Decimal to Hexadecimal Conversion in Java With Examples
Convert decimal to hexadecimal by hand with worked examples, and in Java with Integer.toHexString(), String.format() and HexFormat. Also covers hex to decimal, negative numbers and invalid input.
Java Tuple: Records, Pair and the javatuples Library
Java has no built-in tuple type. Learn how to group values with a record, Map.entry(), Commons Lang Pair and Triple, and the javatuples classes Pair to Decade.
Module Import Declarations in Java 25: import module
A module import declaration such as import module java.base; imports every package a module exports with one line, and it is final in Java 25. Learn what it imports, how to fix “reference to List is ambiguous” with a single-type import, and how it works with java.se and compact source files.
NoSuchMethodError in Java: Causes, Diagnosis and Fix
java.lang.NoSuchMethodError means code compiled against one version of a class runs with another version that lacks the called method. Learn to read the message, find the wrong jar with mvn dependency:tree, -verbose:class and javap, and fix the conflict with a BOM, an exclusion and the Enforcer dependencyConvergence rule.
StackOverflowError in Java: Causes, Stack Trace and Fixes
A StackOverflowError means a thread made more nested calls than its stack can hold. Learn to read the repeating stack trace, fix missing base cases, deep recursion and object cycles in toString(), Lombok and Jackson, and when -Xss helps.
IllegalArgumentException in Java: Real Causes and Fixes
IllegalArgumentException is the unchecked exception a Java method throws when an argument has a value it does not accept. This post lists the JDK calls that throw it with their real Java 25 messages and the fix for each, the subclasses such as NumberFormatException, and when our own code should throw it instead of IllegalStateException or NullPointerException.
Java CompletableFuture Tutorial with Examples (Java 25)
A CompletableFuture is a Future that runs the next step when its result arrives, so no thread has to wait for it. This tutorial creates, chains and combines CompletableFuture tasks on virtual threads, handles errors and timeouts, and shows the mistakes that make async code hang or lose exceptions.
Java 25 Migration: Upgrade From Java 21 Step by Step
A Java 25 migration is mostly a build change. This step-by-step guide upgrades a Maven project from Java 21 to Java 25, shows the support dates, and fixes the real errors from Lombok, Mockito and JaCoCo on the way.
Java 25 Features (LTS): All 18 JEPs With Code Examples
Java 25 is the LTS release of September 2025 with 18 JEPs. This guide runs every one of them, from compact source files, module imports, flexible constructors and scoped values to compact object headers, AOT caches and the new JFR events, and shows the preview features with their –enable-preview flags. It ends with the features that became final between Java 21 and 25 and a migration checklist.
Java 26 Features: All 10 JDK 26 JEPs with Code Examples
Java 26 (JDK 26) shipped on 17 March 2026 as a non-LTS release with 10 JEPs. This overview lists every JEP with its status and shows short, runnable examples for HTTP/3, final field warnings, the AOT cache with ZGC and the preview APIs.
Java 27 Features: All 9 JEPs in JDK 27 with Examples
Java 27 (JDK 27) reached general availability on 15 September 2026 with nine JEPs. Four final JEPs change JVM defaults (G1 everywhere, compact object headers, post-quantum TLS, JFR redaction), and five continue as previews or incubator. A summary table, one example per feature and the removed options.
Java List Initialization in One Line: List.of and ArrayList
Initialize a Java List in one line with List.of() for an unmodifiable list, or new ArrayList<>(List.of(…)) when the list changes later. Compare Arrays.asList(), Stream.toList(), Collections and Guava by mutability and null support.
Unsupported Class File Major Version 69: Fix for Java 25
“Unsupported class file major version 69” means a JVM or bytecode tool older than Java 25 tried to read a Java 25 class file. Learn to read the class file version, map it to the Java release, and fix the error by upgrading the runtime, compiling with –release, or upgrading Gradle, Spring, JaCoCo, Mockito and Lombok.
Spring Boot Elasticsearch Configuration with Spring Data
Connect Spring Boot 4.1 to Elasticsearch 9 with Spring Data Elasticsearch 6.1 and the new Java client: run Elasticsearch in Docker, map documents, search with repositories and NativeQuery, and test with Testcontainers.
Kubernetes for Beginners: Hands-On Guide With Spring Boot
A hands-on Kubernetes guide for Java developers: deploy a Spring Boot API and PostgreSQL to a local kind cluster and learn pods, Deployments, Services, Ingress, ConfigMaps, Secrets, volumes and StatefulSets with real kubectl output.
Spring Boot: Import Excel File into Database (Apache POI)
Upload an .xlsx class roster to a Spring Boot endpoint, read it with Apache POI, validate every row with Bean Validation and save the valid rows in JDBC batches. Includes a JSON error report, duplicate checks, a SAX reader for large files with measured heap, upload limits and MockMvc tests.
Spring Boot Hibernate 7 Example with Spring Boot 4.1
Set up Hibernate 7.4 in a Spring Boot 4.1 application: the JPA starter, datasource and JPA properties, entities, Spring Data repositories, transactions, the Session API, SQL logging, tests on H2 and MySQL, and the Hibernate 7 upgrade changes.
Spring Boot Virtual Threads: Setup, Load Test and WebFlux
One property, spring.threads.virtual.enabled=true, moves Spring Boot requests, @Async methods and scheduled tasks to virtual threads. A weather API measured against platform threads and WebFlux, plus pinning and monitoring on Java 25.
Java Random Number Generators in Multithreaded Code
A shared Random is thread-safe but makes every thread fight over one seed. Learn which generator each thread should use, how to keep seeded results reproducible, and what a JMH benchmark measured on Java 25.
Angular UI Testing With Selenide and Selenium in Java
Angular UI testing in Java with Selenide and JUnit 5. Wait for spinners without explicit waits, handle timeouts and failures, and add Allure reports.
Generate PDF from HTML Template in Java with OpenHTMLtoPDF
Generate PDF from HTML template in Java with OpenHTMLtoPDF 1.1.97, Thymeleaf or FreeMarker, plus jsoup for broken HTML, page numbers, fonts and safety.
Compare Java Objects Without Getters: reflectionEquals()
Learn to compare Java objects without getters or equals() using EqualsBuilder.reflectionEquals(), excluded fields, recursive mode and AssertJ.
Serialize Object to String in Java: Base64 and JSON
Serialize object to String in Java with ObjectOutputStream and Base64 or with Jackson JSON, read it back, and avoid the deserialization security risk.
Java Executors.newVirtualThreadPerTaskExecutor() Example
Executors.newVirtualThreadPerTaskExecutor() runs each task in a new virtual thread. Learn close(), invokeAll, Semaphore limits, thread names and errors.
Filter a Map by Keys and Values Using Java Streams
Learn to filter a map by keys and values with Java streams, plus Predicate helpers, removeIf(), order-preserving toMap() and null-safe collecting.
Java Stream contains(), containsAny() and containsAll()
Java Stream contains, containsAny and containsAll checks with anyMatch() and a Set, with empty-input rules, single-use streams and collection alternatives.
Filter Nested Collections with Java Streams (flatMap, anyMatch)
Learn to filter nested collections with Java streams using flatMap(), mapMulti() and anyMatch(), trim inner lists without mutation and handle null lists.
Java Stream mapMulti() with Examples and flatMap Comparison
Learn Java Stream mapMulti() with examples, the type witness it needs, when it beats flatMap() and when it does not, primitive variants and a real case.
First and Last Day of Week, Month or Year in Java
Get the first and last day of week, month or year in Java with TemporalAdjusters, WeekFields for locales, YearMonth and quarter boundaries.
How to Call an Interface Default Method in Java (With Proxy)
Learn how to call an interface default method in Java, from Interface.super to proxies with InvocationHandler.invokeDefault() and MethodHandles.
Object Identity String in Java with Objects.toIdentityString()
Learn what an object identity string is in Java, how Objects.toIdentityString() and System.identityHashCode() build it, and what its hash part means.
Java Base64 Encode and Decode a File (Text, Image, Large Files)
Base64 encode and decode a file in Java with examples for text and image files, data URIs, streaming large files with wrap(), MIME and decoding errors.
Initialize HashMap in Java: Capacity, Load Factor, newHashMap
Learn to initialize HashMap in Java with Map.of, newHashMap and streams, and see how initial capacity and load factor decide when the map resizes.
orElse vs orElseGet in Java Optional (with Examples)
orElse vs orElseGet in Java explained with examples. Learn why orElse always computes its default, when orElseGet saves work and how to avoid side effects.
Calculate Number of Weeks Between Two Dates in Java
Calculate the weeks between two dates in Java with ChronoUnit.WEEKS.between(), get leftover days, round up started weeks and count calendar weeks.
Get the Day Name From a Date in Java (Localized)
Get the day name from a date in Java with getDisplayName() and TextStyle, print it in any language, list all names and parse names back.
Split a Date-Time Range into Equal Intervals in Java
Split a date-time range into equal intervals in Java, cut fixed-length chunks, or split by days, weeks, months and years with half-open ranges and DST.
Java StopWatch: Guava, Apache Commons, Spring and java.time
Java StopWatch examples with Guava Stopwatch, Commons Lang and Spring StopWatch, plus a small testable java.time version that records named laps.
Get Elapsed Time Since Midnight in Java (Seconds, Millis)
Get the elapsed time since midnight in Java in seconds or milliseconds with LocalTime, in any time zone, correct on daylight saving days.
Get the Quarter From a Date in Java, Start and End Dates
Learn to get the quarter from a date in Java with IsoFields, find quarter start and end dates, format Q3 labels and calculate fiscal quarters.
Convert Month Number to Name and Back in Java
Convert month number to name in Java with Month.getDisplayName(), and parse full or short month names back to numbers in any locale.
Java Date Format with Day Period: Pattern B Examples
Java date format with day period using pattern B in DateTimeFormatter (Java 16+). Print in the morning or at night, parse it and set custom ranges.
Java Math.divideExact(): Detect Integer Division Overflow
Learn how Math.divideExact() in Java detects the Integer.MIN_VALUE / -1 overflow, how it handles zero, and how it compares to floorDivExact().
Java Math.abs() vs Math.absExact(): Overflow Explained
Java Math.abs() vs Math.absExact() with examples. Why Math.abs(Integer.MIN_VALUE) is negative, when absExact() throws, and safe alternatives.
Java String Concatenation Performance: + vs StringBuilder
Java string concatenation performance on JDK 25 with JMH results, how + compiles since JEP 280, why += in loops is slow, and concat() vs StringBuilder.
Java Text Block Formatting with Variables and Placeholders
Java text block formatting with variables using formatted(), String.format(), MessageFormat and named placeholders, plus what happened to string templates.
Java Multiline String: Text Blocks and Older Options
Write a Java multiline string with text blocks, control indentation, line breaks and trailing spaces, and see the String.join() options for Java 8 to 14.
Java Thread Dump: Capture with jcmd, jstack and Analyze It
Capture a Java thread dump with jcmd, jstack, kill -3 or JSON for virtual threads, read every line and find deadlocks, CPU hogs and stuck pools.
Create and Extract a JAR File on Linux, macOS and Windows
Create and extract a JAR file with the jar command on Linux, macOS and Windows. Cheat sheet, runnable JARs, manifest, –dir and unzip options.