Java is an object-oriented programming language designed at Sun Microsystems and released in 1995. Its grammar and rules are defined in the Java Language Specification, the authoritative document Oracle (which acquired Sun in 2010) maintains for what counts as valid Java code and how it should behave.
A central idea of Java is portability through compilation to an intermediate form. Oracle’s own Java tutorial explains that source code “written in plain text files ending with the .java extension” is “compiled into .class files by the javac compiler,” and that a class file “does not contain code that is native to your processor; it instead contains bytecodes, the machine language of the Java Virtual Machine.” Because the virtual machine is available on many operating systems, the tutorial notes that “the same .class files are capable of running on Microsoft Windows, the Solaris Operating System, Linux, or Mac OS.”
This design separated the language from any single processor or operating system. A programmer compiles a program once into bytecode, and any machine with a compatible Java runtime can run it, an idea Sun marketed as “write once, run anywhere.”
Java combined a familiar C-like syntax with object-oriented structure, automatic memory management through garbage collection, and a large standard library. Those qualities, together with strong portability, made it one of the dominant languages for enterprise and server software for decades after its release.