Overview
- 1Java is a statically-typed, object-oriented language created by Sun Microsystems in 1995 and now developed by Oracle under the OpenJDK project as free, open-source software.
- 2Code compiles to platform-independent bytecode that runs on the Java Virtual Machine (JVM), available for Linux, Windows, macOS, and virtually every cloud environment.
- 3Since 2017, Java has shipped a new feature release every six months, with a Long-Term Support (LTS) release every two years — versions 17, 21, and 25 are the LTS releases in active enterprise use in 2026.
- 4Java remains the dominant language for large-scale enterprise backends, Android app development, and big-data platforms such as Hadoop, Kafka, and Spark, which are themselves written in Java or run on the JVM.
- 5Popular JDK distributions include Oracle JDK, Eclipse Temurin (Adoptium), Amazon Corretto, and Azul Zulu, all built from the same OpenJDK source with different support and licensing terms.
Key Features in 21 / 25 LTS
Virtual Threads — lightweight JVM-managed threads (Project Loom) that let a single machine handle millions of concurrent tasks without the memory cost of OS threads.
Records — concise, immutable data carrier classes that auto-generate constructors, accessors, equals(), hashCode(), and toString().
Pattern Matching for switch — expressive, type-safe switch expressions that can deconstruct records and exhaustively check sealed type hierarchies.
Sealed Classes — restrict which classes can implement or extend a type, enabling safer domain modeling and exhaustive pattern matching.
Sequenced Collections — a unified API (getFirst(), getLast(), reversed()) across List, Set, and Map implementations with a defined encounter order.
Compact Object Headers — shrinks per-object memory overhead on the heap, cutting footprint and improving cache efficiency for object-heavy applications.
Strong Backward Compatibility — code written for Java 8 still compiles and runs on Java 21/25, a guarantee few platforms offer at this scale.
Use Cases
- →Enterprise backend systems and microservices for banking, insurance, and e-commerce platforms.
- →Android application development, where Java remains fully interoperable with Kotlin.
- →Big data and distributed systems infrastructure — Kafka, Hadoop, Spark, Elasticsearch, and Cassandra all run on the JVM.
- →High-throughput REST/gRPC APIs and cloud-native services built with Spring Boot, Quarkus, or Micronaut.
What's New in Java 21 and 25
- Java 21 (Sept 2023, LTS) finalized Virtual Threads (JEP 444), Record Patterns (JEP 440), Pattern Matching for switch (JEP 441), Sequenced Collections (JEP 431), and Generational ZGC (JEP 439) as a low-latency garbage collector.
- Java 25 (Sept 2025, LTS) is the current LTS release, finalizing Compact Object Headers to reduce per-object memory overhead and Scoped Values as a safer, faster alternative to ThreadLocal for sharing immutable data across virtual threads.
- Module Import Declarations let code import an entire module's exported packages with a single `import module java.base;` statement, simplifying multi-package imports.
- Flexible Constructor Bodies allow statements to run before the mandatory `super()`/`this()` call, making validation and argument pre-processing in constructors cleaner.
- A simplified "instance main method" syntax lets beginners write a runnable class without the full `public static void main(String[] args)` boilerplate, lowering the entry barrier for teaching Java.
- Structured Concurrency continues to mature as a preview API, treating a group of related tasks on virtual threads as a single unit of work with unified cancellation and error handling.
- Ahead-of-time class loading and command-line ergonomics, building on Project Leyden, continue reducing JVM startup and warmup time for cloud and serverless workloads.
Core Language & JVM
- Everything in Java is either a primitive type (int, long, double, boolean, etc.) or an object reference, and every class implicitly extends java.lang.Object.
- The JVM's automatic garbage collection manages heap memory; modern collectors like G1 (default) and Generational ZGC target sub-millisecond pause times even on multi-gigabyte heaps.
- Generics provide compile-time type safety for collections and APIs, though type erasure means generic type information isn't available at runtime.
- Checked exceptions force callers to handle or declare recoverable error conditions, a design choice unique among mainstream languages that remains debated.
- The Java Memory Model defines precise rules for how threads interact through shared memory, underpinning the `volatile`, `synchronized`, and `java.util.concurrent` APIs.
- Bytecode verification gives Java a strong sandboxing and code-integrity story, historically important for applets and still relevant for plugin architectures.
- Just-In-Time (JIT) compilation via HotSpot profiles running code and compiles hot paths to native machine code, often matching statically compiled languages for long-running server processes.
Concurrency & Virtual Threads
- Virtual threads (Project Loom) are cheap, JVM-scheduled threads — you can create millions of them, each mapped onto a small pool of OS carrier threads, without changing existing thread-based code.
- Virtual threads make blocking I/O code scale like async code without rewriting to callbacks, futures, or reactive streams — `Thread.ofVirtual().start(task)` replaces most reactive plumbing for typical CRUD services.
- Structured Concurrency (preview) groups child tasks under a single scope so that cancellation, timeouts, and error propagation happen consistently instead of leaking async work.
- Scoped Values offer an immutable, inheritable alternative to ThreadLocal, purpose-built to work efficiently with the potentially millions of virtual threads an application can spawn.
- The traditional java.util.concurrent toolkit — ExecutorService, CompletableFuture, locks, and atomics — remains fully supported and interoperates with virtual threads.
- Servers like Tomcat and Jetty support running each incoming request on its own virtual thread, letting existing servlet-based applications gain Loom's scalability with minimal code changes.
Ecosystem & Build Tools
- Maven and Gradle are the two dominant build tools; Maven uses declarative XML (pom.xml) with a fixed lifecycle, while Gradle uses a Groovy/Kotlin DSL for more flexible, script-based builds.
- Maven Central is the primary public repository for Java libraries, hosting hundreds of thousands of artifacts consumed by both Maven and Gradle projects.
- JVM-based languages such as Kotlin, Scala, Clojure, and Groovy compile to the same bytecode and interoperate directly with Java classes and libraries.
- GraalVM Native Image compiles Java applications ahead-of-time into standalone native executables, cutting startup time and memory usage for containerized and serverless deployments.
- JUnit 5 and Mockito remain the standard tools for unit and integration testing; Testcontainers has become the de facto way to run real databases and services in CI.
- Jakarta EE (the successor to Java EE) standardizes enterprise APIs like servlets, JPA, and CDI, while frameworks like Quarkus and Micronaut target fast-startup, cloud-native Java outside the traditional application-server model.
Frequently Asked Questions
Is Java worth learning in 2026, and what do Java developers earn?▼
Yes — Java consistently ranks among the top backend languages in job postings and developer surveys, and it remains a primary language for Android development, enterprise backends, and big data platforms like Kafka and Spark. Because most large enterprises run Java systems that need years of maintenance, demand for experienced Java developers stays strong, and compensation for backend/Java roles is typically on par with or above general software engineering averages in most markets. Its long-term-support release model also means skills learned today stay relevant for years rather than becoming obsolete with the next release.
What is Java used for?▼
Java is used for enterprise backend systems and REST/gRPC APIs (often with Spring Boot), Android app development, big data platforms such as Hadoop, Kafka, and Spark, and large-scale distributed systems at banks, e-commerce companies, and cloud providers. Its combination of strong typing, mature tooling, and backward compatibility makes it a default choice for long-lived, mission-critical systems.
Java vs Python — which should I learn first?▼
Python has a gentler learning curve and is generally the faster path into scripting, data science, and quick prototypes, while Java's static typing and verbosity make it better preparation for large, team-maintained codebases and enterprise software engineering roles. If your goal is data science or automation, start with Python; if your goal is backend/enterprise development or Android, Java is the more directly useful skill. Many developers eventually learn both, since the languages serve different, complementary niches.
What is the difference between JDK, JRE, and JVM?▼
The JVM (Java Virtual Machine) executes compiled bytecode and is what makes Java "write once, run anywhere." The JRE (Java Runtime Environment) bundles the JVM with the core class libraries needed to run Java applications, but not to compile them. The JDK (Java Development Kit) includes the JRE plus the compiler (javac), debugger, and other development tools; since Java 11, standalone JRE downloads were discontinued, so developers now install the full JDK even just to run applications.
What are virtual threads in Java and why do they matter?▼
Virtual threads, finalized in Java 21 (JEP 444) as part of Project Loom, are lightweight threads managed by the JVM rather than the operating system, letting a single application run millions of them concurrently. They let developers write simple, blocking-style code (one thread per request) that scales like asynchronous code, removing much of the need for reactive frameworks or manual thread-pool tuning. This matters because it lets existing server code scale to far higher concurrency with little to no rewriting.
Which Java version should I use in production — 17, 21, or 25?▼
Java 21 is currently the most widely adopted LTS release, with mature tooling, library, and cloud-provider support, making it a safe default for most production systems. Java 25, the newest LTS (released September 2025), adds compact object headers and finalized Scoped Values, and is worth adopting for new projects where memory footprint and startup time matter, once your dependencies confirm support. Java 17 remains acceptable for legacy systems, but new projects should target 21 or 25 rather than a non-LTS release, since only LTS versions receive multi-year security patches.