.Java is a file extension used for Java source code files. These files contain code written in the Java programming language, which is widely used for building applications, from mobile apps to large-scale enterprise systems. Java is known for its portability, allowing developers to run their applications on any device that has a Java Virtual Machine (JVM). When a developer writes code in a .Java file, it must be compiled into bytecode using the Java compiler. This bytecode is then executed by the JVM, making Java a versatile choice for cross-platform development. The language was created by James Gosling and released by Sun Microsystems in 1995.