Java is an object-oriented programming language created by Sun Microsystems and released in 1995, designed so that the same program runs on any system thanks to a virtual machine, the JVM.
Java code is compiled into an intermediate format (bytecode), which the JVM runs on Windows, Linux or macOS. Sun was acquired by Oracle in 2010, which has steered the language since, with a new release every six months. Open distributions such as OpenJDK are widely used.
Java underpins a great many critical systems: banks, insurers, telecoms, large e-commerce sites, public administrations. Its ecosystem is massive, with Spring as the dominant server-side framework. The JVM also runs other languages, such as Kotlin (widely used on Android) and Scala.
Its strengths: robustness, performance, mature tooling, and a huge pool of developers.
Why it matters when hiring
The Java talent pool is one of the largest, but very uneven: between a developer maintaining a legacy application and someone fluent in recent versions, Spring Boot, microservices and the cloud, the gap is considerable. Ask which versions they have worked on and how they design and test their code. Also tell Java and JavaScript apart: two unrelated languages, despite the name.