“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.
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.
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 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 (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 (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.
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” 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.
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.
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.
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.
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.
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.
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.
The error “java.lang.IllegalArgumentException: Failed to create folder” in Selenide typically happens when Selenide tries to save screenshots, page sources, or logs, but it cannot create the required directories due to permission issues or incorrect paths.
In-browser UI testing of Angular applications poses several challenges such as asynchronous operations and unpredictable loading screens with spinners or dimmers.
Selenide has an implicit capability to wait for the loading screen to disappear.
OpenHTMLToPDF library generates PDF from HTML and CSS content taken from templates such as HTML file, Freemarker and Thymeleaf templates.
As a best practice in Java, POJO classes or Data objects always declare the fields private and expose their access through the getter methods. On rare occasions, we may encounter situations when the member fields are private, and the class does not provide the public accessor methods. For example, the …
Learn to create an immutable or unmodifiable Map using the factory methods added in Java 9 and other versions (Java 8 to Java 21) with examples.
In Java, we can serialize an object to String in different ways such as converting to byte[] and thenBase64 encoded string, XML or JSON. Learn with examples.
Added in Java 21, the Executors.newVirtualThreadPerTaskExecutor() method returns an Executor that creates a new virtual thread for each task submitted to it.
Learn to filter a Map by keys or values or both, using the Java 8 Stream filter() and collect() methods with examples.
Learn to create a Java Stream utility class with contains(), containsAll() or containsAny() methods and learn to use them with examples.
Java Stream API provides excellent filtering capability on the elements in the Stream. Sometimes, the stream elements also contain the nested collections; we need to filter those elements in the nested collections. This Java tutorial discusses various approaches with examples to apply filtering on nested collections and collecting the filtered …
Stream mapMulti() is a specialized form of the map() method that transforms each element of a stream into one or multiple output elements or none at all.
Learn to use Java 8 LocalDate and TemporalAdjusters class to get the first and the last day of the current week, month or year with examples.
This short Java tutorial lists two ready-to-use snippets for invoking the default methods in an interface without implementing the interface in a class.
In Java, an identity string is a unique identifier associated with an object instance without calling the overridden toString() or hashCode() methods.
Since Java 8, the java.util.Base64 class provides convenient methods for encoding and decoding files such as text, image, JSON files.
Learn to initialize Java HashMap object in different ways such as empty map, pre-populated map, and collecting stream elements into the map.
In this Java tutorial, learn the different approaches for calculating the number of weeks between two dates using the Java Date and Time API.
In this Java tutorial, we will explore various methods to display the name of the day of the week, catering to different scenarios and requirements. Note that we should be careful of using the int values assigned to weekdays as they differ from method to method. 1. Display the Day …
In this Java date-time tutorial, we’ll explore how to split a date-time range into equal intervals. Conceptually, splitting a date-time range into equal intervals involves dividing the entire range into smaller, equally-sized sub-ranges. In short, we need to determine the start and end points of each sub-range. 1. Splitting Date-time …
Learn to implement and use a StopWatch in Java with a custom implementation for simple uses and Apache Commons StopWatch class for advanced uses.
Learn to calculate the elapsed time since midnight (in seconds or milliseconds) using Java 8 classes such as LocalTime, LocalDateTime and ZonedDateTime.
Learn to use new Java 8 APIs to obtain the current quarter, the quarter from a given date and the start and end date of a quarter.
Learn to get the Month information in either number or name, and to convert it into another using Java Date-time API and simple examples.
Since Java 16, we can use the additional parameter [B] to format a date-time object with the date-time information using the DateTimeFormatter class.
Beginning Java 18, we can use Math.divideExact() method that throws an ArithmeticException if the result overflows. Learn with example.
This Java tutorial explores what are absolute values, the overflow, and underflow problems of Math.abs() method, and how Java 15 Math.absExact() solves them.
This article delves into the differences between these ways and highlights how Java optimizes the bytecode to make the String concatenation faster.
Learn to apply String interpolation style formatting of Java text blocks by variable (or expression) substitutions. Also see why to avoid concatenation.
Writing a multiline string can be approached differently depending on the version of Java you are using. Since Java 15, text blocks are the preferred method.
Learn to capture and analyze thread dumps in Java either manually to using tools such as JStack, VisualVM, FastThread, TDA and JProfiler, etc.
This Java cheat sheet discussed the JDK jar utility for creating and extracting the JAR files. It mimics the Unix tape archive command, tar.
Learn to install OpenJDK Correto distributed by Amazon on different operating systems such as Linux, macOS, and Windows.
Using the TextTable class or String.format() methods, we can easily align the text in columns and print them in the console or any output file.
Learn to use Gson, Jackson, and Org.json libraries to convert a JSON array into an ArrayList of objects with easy-to-understand examples.
Unnamed patterns and variables represent the record components and local variables as an underscore (_) to denote that we’re not interested in them.
In Java 21, Scoped values are implicit method parameters and help in sharing data with virtual threads that can be created in millions.
In this Java tutorial, we’ll explore the Math.round(), Math.ceil() and Math.floor() methods in detail, understand their differences and discover their use cases.
HowToDoInJava provides tutorials and how-to guides on Java and related technologies.
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